Download 1 BOZZA 5. DEFIN. 09.06.04 < NEUROMELANIN AND BIOLOGICAL

BOZZA 5. DEFIN. 09.06.04
< NEUROMELANIN AND BIOLOGICAL FUNCTION >
< SULLE NEUROMELANINE E LORO RUOLO BIOLOGICO >
Nota del socio ord. non res. Bruno J.R. Nicolaus [1,2]
Accademia Pontaniana, Napoli, June. 24.2004
KEYWORDS melanin, eumelanin, pheomelanin, allomelanin, neuromelanin, sepiomelanin,
eye, iris, eye fundus, ear, brainstem, substantia nigra, locus coeruleus, retinal epithelial cells,
RPE, melanocyte, melanosome, albinism, Parkinson, Alzheimer, deafness, tyrosinase,
monoaminoxidase, tyrosine, dopamine, serotonine, epinephrine, norepinephrine,
catecholamines, polyacetylene, polypirrole, polyindole, polyphenylene, acetyleneblack,
pyrroleblack, indoleblack, benzeneblack, epinephrineblack, serotonineblack, dopamineblack,
tyrosineblack, black particles, semiconductors, conducting polymers.
PREFACE
< Natura enim simplex est >
Isaac Newton, 1687
Modern biochemistry and molecular biology confirm Newton’s axiom, according to which, all
life’s systems are run on the same lines of simplicity, as physical phenomena. Melanin’s do comply
with this law, being sometime difficult to prove it, due to their rather strange physical and chemical
behaviour.
All over the planet, there are manifold variegated pigments playing major biological roles. The
black particles of the living world are synthesized under genetic and hormonal control in specific
cell systems (melanocytes and neuro epithelial cells) and are called Melanin’s (from Greek,
melanos = black). The melanins were currently grouped in four classes [3], updated as follows:
1) Eumelanins ( eu, good ): Black or brown amorphous particles formed in the chemical or
enzymatic oxidation of the amino acid DOPA; found in hair, feather, skin, eyes, melanoma,
etc.; mainly consisting of 5,6-dihydroxyindole units (DHI); derived biogenetically from
tyrosine.
2) Pheomelanins (faios, dusky): Sulphur or protein containing amorphous particles, obtained by
oxidation of CYSDOPA ( chiefly cystein-5-S-yl-dopa ); found in hair, fur, feather, etc.; often
accompanied by crystalline feochromes ( tricosiderin, tricochromes ); they contain the
dibenzothyazinone chromophore, firstly synthesized by Kaul [4].
1
3) Allomelanins: Black or brown amorphous, often nitrogen-free, particles, obtained by
oxidation of typical polyphenols ( catechols, 1,8-dihydroxynaphtalene ).They are widely spread
in fungi’s and in soil (humic acids).
4) Neuromelanins: Black or brown, amorphous particles found in the CNS of humans and
vertebrates. They are formed by enzymatic or chemical oxidation of catecholamines in various
substrates (tyrosine, dopamine, epinephrine, nor epinephrine, serotonine, dihydroxy-quinolines,
etc.).
Hybrids of these classes are formed by inclusion of foreign materials.
***
The melanin’s are derivatives of pyrrole, indole, or benzene and belong to three macro systems:
polypyrrole, polyindole and polyphenylene.
Black, red biline and porphyrins are namely pyrrole derivatives; epinephrine black and serotonine
black, two components of neuromelanin, are indole derivatives; graphite, fullerenes, aspergillin
from Aspergillum niger, humic acids, DOPA-black, again a component of neuromelanin, are
benzene derivatives.
Under proper conditions, these black pigment particles are able, to absorb and dissipate light and
sound, to bind organic compounds and ions, to store liquid and gases, to conduct electricity and to
transform light in electric energy. This wide range of heterogeneous physical and chemical
properties has been currently correlated to biological functions, which still deserve extensive
physiological confirmation, as free radical scavenging, detoxification from drugs and other harmful
substances, iron binding/ release, radiation shield, sound protection, etc. The true biological role of
substrate specific melanin’s is, accordingly, difficult to understand, suggesting they may just
represent biological garbage.
The term particle in melanin chemistry was introduced for the first time by Chedekel, recent
studies showing that they are organized in units and subunits, responsible for the physical and
biological behaviour [5].
Carbonaceous black materials were identified on meteorites, as well as in the clouds of interstellar
space by means of radio-spectroscopy [6].
The neuromelanins build a family of black particles, typical of the Central Nervous System (CNS)
and some other inner organs of humans and vertebrates. Their biological function is not always
evident or well documented and their mode of action remains in many cases obscure.
In this paper, we shall focus on neuromelanin’s, with the aim of interpreting their physical,
chemical and biological properties by an integrated approach.
INTRODUCTION
In ancient times already, the diversity of skin colour excited the attention and curiosity of our
ancestors and most classifications of human kind were based on this property. We still don’t fully
understand the true mechanism of skin tanning, why some individuals are more susceptible than
others to the harmful effects of sunlight and what agents actually protect exposed black skin from
photo damage.
In recent years, numerous, biological functions were postulated for these black pigments [7], the
photo protective action being mostly accepted [8].
Quality and quantity of skin pigmentation result from a complex interaction between biochemical
factors and skin colour is a genetic property strongly influenced by the environment, fluctuating
between the full white of Albinos, to the pale white of Caucasians up to the deep black of Africans.
2
It has been argued, that skin tanning is not a vital property and melanin not a vital pigment.
Heavily pigmented skin, however, is far less susceptible to actinic damage than lightly pigmented
one and good tanners or dark-skinned people are knowingly at much lower risk to burn and develop
skin cancer. Moreover, albinos who have faulty melanocytes in their skin, are at disadvantage
compared to normal pigmented individuals: They easily get burned, show a progressive
degeneration of sight, due to partial or total lack of melanin in skin and eye and are more prone to
skin cancer, while melanin formation in their brain stem appears to be normal [9].
***
In humans, melanin is found in the skin, retina, iris, and certain areas of the CNS. Melanin may
increase or decrease with age and greying of the hair occurs in all individuals irrespective of gender
and race. This age-related pigment reduction of the hair bulb is observed in the epidermis, in the
retinal pigment epithelial cells (RPE) and in other areas of the CNS, namely the substantia nigra
and locus coeruleus. An aged related increase in melanin can be found in the senile lentigo, which
occurs on exposed surfaces of the skin of past middle age individuals.
The granular dark brown pigment of the nigrostriatal neurons of the brain stem is chemically
similar but not identical to skin melanin, which is synthesized in the epidermal melanocytes, under
the influence of light, whereas neuromelanin is formed in the absence of visible light, in the
catecholaminergic neurons of the CNS, which are furthermore deprived of true melanocytes.
Different chemical structure, different biogenetic site, different biochemical pathway must
necessarily lead to different biological properties and functions [10].
Skin melanin (an indole derivative) acts as sun screen, while the biological activities of
neuromelanin ( a benzene derivative) can vary, according to the site of deposition and chemical
composition. Based upon the present state of the art, the substantia nigra melanin is involved in
movement coordination and protection of neurons against oxidative stress being moreover supposed
to be instrumental to neuronal death and Parkinson’s disease; in the eye, ocular melanin is devoted
to absorption of excessive light; in the ear, in deadening acoustic waves. Several attempts were
made, to correlate the biological activities of melanin’s with their property to conduct electricity
and to transform light into current. Convincing physiological evidence is, however, still missing.
***
The neuromelanins show biological functions, which are strictly related to chemical and particle
structure. There is not one single neuromelanin, but a family of chemically different black
pigments, exercising specific roles on different substrates, at different sites. Surprisingly enough,
amount and location of neuromelanin varies among animal species, being greatest in humans and
progressively lower in the lower species. In humans and primates, neuromelanin predominates in
the catecholaminergic neurons of the substantia nigra and locus coeruleus, were dopamine,
epinephrine and nor epinephrine are present in high concentration, raising the question, whether this
melanin is composed by one pigment only, or by a mixture of two or more.
Dopamine brain concentration is age related, with a marked decline in senescence (onset of
senescence: cattle 15-20 years; dog 10-15 years; humans 60-80 years; mouse 2-3 years; rhesus
monkey 20-30 years; rat 2-3 years; rabbit 4-6 years).
During aging, a dark brown pigment called lipofuscin (age pigment) accumulates throughout the
body in the lysosomes .These intracellular deposits, which in the past were often confused with
melanin, are the end product of the natural cell turnover, representing biological garbage
[11,12].Fig.1
3
Depigmentation of the substantia nigra is a constant feature of Parkinson’s disease and is caused by
greatly reduced dopamine concentrations ( to about one-tenth of normal ), as consequence of the
concomitant death of dopaminergic neurons, either as a result of virus infection, exposure to
exogenous toxins or other unknown causes. Fig.2
4
5
CONJUGATED ORGANIC POLYMERS
Chemically, natural and synthetic melanin’s are substituted derivatives of polyacetylene, containing
in their backbone the polyacetylene spine. They are stable free radicals (radical polarones), holding
positive charges balanced by counter anions, exhibit typical broad IR and EPR spectra and behave
like electrical conductors, in the doped state. The unpaired electrons and the positive charges
distributed along the conjugated polyacetylene chain are responsible for their conductivity and
colour. Fig.3
The most synthetic electrically conducting polymers are black in the doped state. In the un-doped,
semi conducting state, their colour can vary from clear to red, depending on the forbidden electron
energy band gap. The colours change according to the length and the identity of the polymer chain,
whereby the longer the chain, the darker the polymer.
Here below, we shall review some properties of synthetic conjugated organic polymers, a class of
intriguing compounds, to which melanin’s are chemically related.
***
Polymeric materials in the form of wood, bone, skin and fibres have been used since very ancient
times. During history, man found out how to modify in a proper way all these materials, as well as
to manufacture new ones. The main target of the synthetic polymers has been focused so far, on
mechanical strength, processing, barrier properties, electrical insulation, etc. Electrical conductivity
represent the new frontier of conjugated polymers, materials able of being doped to states of high
electrical conductivity.
In 1964, W.A.Little proposed a chemical structure for a consistent ideal organic superconductor,
composed by a polyunsaturated chain, called <spine>, substituted in some points by heterocyclic
structures (often resonance hybrids) having cation centres and counteranion. Even if the target of
finding a room temperature superconductor has not been realized, Little’s hypothesis has been very
fruitful, allowing the synthesis of numerous organic compounds, which proved to be
semiconductors in the doped state [13].
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In 1977, Heeger, MacDarmid and Shirakawa succeeded in doping polyacetylene to a relatively
high electrical conductivity, of approx. 10+3 Scm-1, although various kinds of conducting polymers
had been described for almost one century. The chemistry of conjugated polymers, produced
thereafter interesting practical developments in the areas of electronics (LED and transistors),
integrated circuit technology ( electro-resistant materials for high spatial resolution ) and high
technology materials ( aircraft construction materials and high-strength fibres ).
This new class of synthetic materials combines the electronic and optical properties of
semiconductors and metals with the outstanding mechanical properties and processing advantages
of synthetic polymers [14].
The best known synthetic conducting polymers are listed in Fig. 4.
7
***
Polyacetylene is the simplest organic polymer. Its repeating unit consists of two carbon and two
hydrogen atoms with a carbon-carbon double bond − (CH=CH)n−, simply denoted as (CH)n.
In 1958, Natta et al. for the first time converted acetylene into a polymer, using Ziegler-Natta
catalysts, to open the triple bond of acetylene. The discovery, that polyacetylene films can be doped
with a variety of agents, increasing conductivity up to 105 Scm-1, started the era of conducing
polymers (Chiang et al.).
Polyacetylene itself is an inhomogeneous mixture of non-perfect planar polymeric chains, with
varying conjugation lengths, linked through structural kinks (sp3 carbon atoms ), not allowing to
establish a linear relationship between chemical, structural and electronic properties. Undoped
polyacetylene is silver grey, with a forbidden energy gap of about 1.7 eV. In the doped state, it
looks black, becoming electrically conducting.
Polyacetylene occurs in the two possible cis and trans configurations, whereby the cis/trans isomers
content can be varied, according to the polymerisation conditions. The conductivity of
polyacetylene films is decreasing with film thickness and is increasing with in-plane alignment of
the conjugated chains. Once made, polyacetylene is insoluble, infusible and unstable to air:
properties which make it less attractive, indeed.
***
Polypyrrole shows a relatively good stability, high conductivity and easy of chemical or
electrochemical synthesis, the last one producing free standing highly conductive films. The
conductivity of polypyrrole depends on the kinds of solvent and oxidant, as well as on the reaction
temperature, concentration of oxidants and the ratio oxidant and pyrrole monomer. The rate
determining step in the polymerisation process is represented by the formation of the radical cation
of the pyrrole monomer, formed by oxidant, each polymerisation step ( dimer → trimer → oligomer
→ polymer ) having a different oxidation potential. The macromolecular chain structure of
polypyrrole (PPY) is known as α,α′-linked pyrroles and recalls that of graphite, the details of the
chain architecture being still lacking, because of the insolubility of the polymer.
Polypyrrole has a linear and planar chain structure and is a good semiconductor in the doped state.
The structure was confirmed by the isolation of pyrrole acids after oxidation with KMnO4 . Pyrrole
black gives a strong EPR signal and contains cationic centres with counteranions.
***
.
Industrial conductive polymers are usually a physical mixture of a non- conductive polymer with a
conductive material, such as a metal or carbon powder, distributed throughout the material. An
organic polymer that has the electrical, electronic, magnetic and optical properties of a metal, while
retaining the mechanical properties, processibility, etc., commonly associated with a conventional
polymer, is called an intrinsically conducting polymer (ICP), more commonly known as a
synthetic metal. Its properties are intrinsic to a doped form of the polymer. During the doping
process, an organic polymer, having a low conductivity (10 -10/10 -5 S cm -1 ) is converted to a
conducting polymer ( 1-10 +4 S cm-1 ). The addition of small non stoichiometric quantities of
chemical species results in dramatic reversible changes in the electronic, magnetic, electrical,
optical and structural properties.
8
THE NEUROMELANINS
In men and other vertebrates, neuromelanin is found in the brain stem, the eye and the ear. In the
brain, two mesencephalic areas, the substantia nigra and the locus coeruleus, are reach in melanin
in form of granules located in the catecholaminergic neurons, surrounded by a double membrane.
The same neurons, account for 80% of the dopamine in the brain and also contain remarkable
concentrations of nor epinephrine. The black dark brown neuromelanin makes the pigmented brain
regions appear black. Oxidative degradation studies of neuromelanin, from human substantia nigra
(SNM) in comparison to model melanin, showed that the pigment is a mixed-type indole polymer,
consisting of benzothiazine and dopamine units in equal amounts. It is chemically different from
that of skin melanin formed in the melanocytes, when stimulated by sunlight [3,15].
Electron paramagnetic resonance spectroscopy (EPR) was showing, that SNM is an atypical
melanin, structurally different from synthetic dopamine melanin. Both pigments include, however,
an aromatic multilayer graphite-like three dimensional backbone, substituted by aliphatic chains
[16].
The graphite-like conformation is common to all synthetic and natural melanin’s, building the base
to their bioelectric behaviour. Structurally and functionally, neuromelanin appears to be a more
complex pigment, than synthetic dopamine melanin, simply formed via dopamine self oxidation
[17].
Recent comparative degradation studies on SNM of normal individuals post mortem and synthetic
melanin’s confirm that it derives from dopamine with a 25% incorporation of cysteine in the form
of benzothiazine structures. Preventing dopamine and cysdopa to accumulate in the brain, was
interpreted in terms of an hypothetical cellular detoxification mechanism [18].
***
During the course of life, SNM accumulates in the dopaminergic neurons, decreasing during
senescence, accompanied by a decline in the number of neurons and in synthesised dopamine. The
neurons with the highest amounts of black pigment and the smallest quantities of reduced
glutathione (GSH) are the first to degenerate, leading to the axiom neuromelanin accumulation =
neuronal death. Fig 5.
9
Fig. 5 PARKINSON’S DISEASE: A) SN-MELANIN AS NEURON KILLER
B) DEPIGMENTATION AS CAUSE OF PARKINSON’S MOTORIAL SYMPTOMS
According to this view, SNM would behave like a waste product, whose pathologic accumulation
takes precious dopamine away from the life cycle, thus becoming a neuron killer. Whilst the
epidermis’ melanocytes and melanosomes are conceived for the production and distribution of
melanin from tyrosine, the brain lacks such a system, as well as tyrosinase, and is not exposed to
solar radiation [19,20]. These facts enforced the hypothesis, according to which extracellular
neuromelanin, both in normal and pathological conditions ( e.g. Parkinson’s disease), would
originate at random from dopamine by radicalic accidents, catalyzed by iron (Fe/ H2O2) and
monoamine oxidase (MAO), both largely present in the substrata.
Certain drugs and chemicals are bound to melanin and retained in pigment cells for long periods of
time. This specific retention can cause adverse effects in the skin, eye, inner ear and pigmented
nerve cells of the cerebral substantia nigra [21].
10
The ‘neuromelanin = biological garbage hypothesis neglects aspects, worth of closer
consideration:
-
Both skin and eye melanin are definitely instrumental to light absorption and protection,
confirming that, melanogenesis plays a fundamental non casual, genetically controlled
biological role;
-
The structural similarity and Newton’s simplicity axiom suggest that, SNM carries out
some related biological function;
-
SNM is formed in the brainstem at a very early age, being continuously regenerated during
life, whereby the basal nuclei are knowingly involved in modulating and transmitting
nervous impulses. SNM presence at this strategic site is unlikely to be fortuitous, suggesting
some more fundamental biological role;
-
SNM is absent or significantly scarce in two life conditions, in which movement
coordination is either not efficient (newborn babies) or strongly compromised ( Parkinson’s
patients );
-
The traditional neuronal theory provides a convincing model for the transmission of nervous
impulses. It is still unclear, however, if in the strongly pigmented basal nuclei, the
information is transmitted by dopamine only, or if SNM also plays a role;
-
Assuming that SNM is not biological trash, let us suggest, the bioelectric properties of SNM
to be somehow involved in this process.
As a conclusive remark, we can say, that the biological functions of neuromelanins are specific to
site of action and substrate, while the mechanisms of action remain rather obscure.
The bioelectric properties of melanin’s in general and SNM in particular, deserve accordingly
further attention in future studies.
ALBINISM AND EYE MELANIN (OCULAR MELANIN)
In primates, melanocytes and neuro epithelial cells are responsible for melanin synthesis
(melanogenesis).
Melanocytes originate from neural crest and in Caucasians, melanin appears in the skin and iris
stroma around 6 weeks after delivery ( for this reason, the newborns show similarly blue eyes ).
The Retinal Pigment Epithelium (RPE) originates, on the contrary, from neuroepithelial cells,
which develop from the outer layer of the optical vesicle. RPE pigmentation starts in human
embryos early and can be detected at the 7-week stage already.
Morphological melanogenesis in both type of cells shows similarities, the mammalian RPE cells
build melanin, however, only during the foetal and perinatal period and there is no evidence of
melanosome formation after birth.
11
The RPE is strategically located between the rod and cone photoreceptors and the vascular bed of
the choriocapillaris. It is believed to exercise various fundamental functions, such as metabolism of
photoreceptors and vitamin A, monitoring of ion gradients, building up the blood-retina barrier and
providing the transport from blood to retina and back, supporting the vegetative functions of the
rods and cones, recycling the bleached visual pigments.
The RPE contains the brown black pigment melanin, which has photoactive properties, without
being involved in visual phototransduction. Ocular melanin behaves like a broadband optical
absorber and is generally thought to protect ocular tissues against excess light, reducing intraocular
light scatter and thereby increasing the contrast of visual images formed on the retina. Retinal
mitosis is monitored by DOPA, a melanin precursor present in the RPE. Absence or low levels of
DOPA and melanin result in retinal deficiency and a failure of the rod number of up to 30% of the
normal levels [22]. It remains obscure, how DOPA can influence the retinal development in such a
profound way.
According to some authors, melanin is supposed to participate in tissue protection, sequestering
heavy metals and trapping free radicals produced by photochemical aggression, the light-activated
melanosomes being responsible for photochemical reactions within the RPE cells. Besides reactive
oxygen species and other radicals, ocular melanin binds numerous pharmaceuticals, a process which
can result in ocular toxicity. Particularly dangerous are known photo sensitizers, able to induce
pigmentary retinopathies and to affect the quality of vision [23].
During aging and under the action of oxidative stress, the melanin content in RPE cells decreases
remarkably, raising the question on how these age related changes are influencing the photo
protective function [24].
Melanin plays a role in the development of the fovea and routing of optic nerves and its deficiency
in the RPE is associated with age-related macula degeneration, the leading cause of blindness [25].
As lipofuscin accumulates in the aging eye, it might contribute to the oxidative stress, starting
peroxidation of fatty acids even in the absence of light. In vitro data on cultured RPE cells confirm
that melanin acts as an effective antioxidant, protecting RPE cells from excessive lipofuscin
formation [26], perhaps interacting with transition metals [27].
The lipofuscin found in the RPE cells is an heterogeneous material composed of a mixture of lipids,
proteins and different fluorescent compounds, among which retinol derivatives. RPE-lipofuscin
differs from that of other body tissues, because it originates from the modified residues of the not
completely metabolised photoreceptor outer segments .Based upon the actual scientific evidence,
ocular lipofuscin, the second most prominent RPE pigment, can be considered biological trash
[28].
Within the RPE cells, melanin is packaged in spherical to ellipsoid shaped melanosomes of
different size depending on the species, surrounded by a coat of protein like material, called melano
proteins. At the direct experimental observation using scanning tunnelling microscopy (STM) and
small angle X-ray scattering (SAXS), natural and synthetic melanin reveal a graphitic-like layer
structure, organized in fine granules with a particle size, ranging from the nanometre to the micron.
According to the most recent biophysical and biochemical studies, melanin chemistry is strongly
dependent upon the state of aggregation, the binding of heavy metals and the presence or absence
of oxidants [5,29].
***
Albinism is the most severe genetically determined disorder in ocular pigmentation, characterized
in man by congenital hypopigmentation of eyes, skin and hair (oculocutaneous albinism ) or
apparently limited to the eyes ( ocular albinism ). Individuals affected with albinism have little or
no pigment in their eyes, skin or hair, because they have inherited genes that do not make the
normal amount of melanin. In USA, one out of 17000 inhabitants suffers of some type of albinism,
independently from gender and race and often people do not recognize their condition. Examining
12
this pathology thoroughly, helpful clues about the function of ocular melanin in the vision process
become evident. Currently, 20 different types of oculocutaneous and 4 of ocular albinism have
been identified [30].
Persons with cutaneous albinism (partial) develop the disorder through autosomal recessive
inheritance. They show the classic signs of oculocutaneous albinism at birth but, being capable of
synthesizing small amounts of tyrosinase produced melanin, their body pigmentation is increasing
until age 6, developing varied skin and iris colours, in direct ratio to the melanin amount actually
produced. Caucasians are affected more by the lack of pigment than are dark-skinned persons.
Pigmentation of the eye ground ( fundus) in cutaneous albinism is near normal, with far less evident
nystagmus (< dancing eyes >) and visual intolerance to light being substantially reduced, in
comparison with patients having oculocutaneous albinism. Transillumination of the irises is also
more difficult in cutaneous albinism due to the presence of more melanin. Modest skin tanning from
the sun’s rays is also possible in Tyr-pos (Tyrosinase-positive)patients, because of the small
amounts of melanin present.
Oculocutaneous albinism involves primarily the eyes, hair and skin. It exhibits a total lack of
pigment, resulting in white hair, brows and lashes and skin colour ranging from pale to silky white,
when compared with others of the same ethnic or racial background. The irises are pale blue in
colour and are translucent, which renders them able to be totally trans illuminated, due to
inadequate melanization. The fundi are also light pale in colour and vision problems are invariably
present ( moderate to severe nystagmus, moderate to high astigmatism, photophobia, strabismus and
marked low vision) [31].
Ocular albinism involves primarily the eyes, while skin and hair show normal or near-normal
colouration. Four types of ocular albinism have been described so far, all of them giving Tyr-pos
hair bulb tests. The lack of pigment in the eye is causing various vision problems (reduced visual
acuity; nystagmus; strabismus; sensitivity to bright light and glare). The colour of the iris and the
eye may vary from blue to green or even brown and sometimes darkens with age. The main
problem is found in the fovea, the small area of the retina which affords acute vision, which does
not develop completely, presumably because of the lack of the melanin pigment, which is needed
for prenatal growth of the intraocular fine structures and normal pattern of nerves routing from the
back of the eye to the brain. Animal in vivo experiments confirm in fact that, the albino retina is
stressed by abnormal levels of proliferation, which are not found in normally pigmented animals
[32].
Normally, the RPE act as a sink for incoming exceeding light. In case of a dysplastic retinal
pigment epithelium, illumination scatters freely within the eye, with photophobia being a marked
subjective complaint. Moreover, the photophobia is intensified by stray light waves, that enter the
eye through the hypo pigmented irises and the incomplete development of the macula helps
explaining the oscillating nystagmus, as the patient’s eyes continually seek out the clearest possible
image [33].
***
The RPE is a non-dividing biological system with a very complex and intensive metabolism. Its
main aim consist in digesting large amounts of their own structural components, as well as
membranous material belonging to the adiacent rods and cones. In case of imperfect metabolism,
harmful residues are going to accumulate within the epithelial cells, interfering with the organ
function. This aspect might be additionally influenced by lipofuscin accumulation.
Elemental analysis of the sulphur content of melanin from bovine hair, iris, choroids and RPE
reveal that in an individual animal ocular melanin has different chemical structure from hair
melanin [34]. This fact reveals unexpected implications in the biosynthesis of ocular and skin
melanin.
13
RPE melanin was repeatedly stated to protect the ocular tissues, removing from the cytoplasm
redox - active heavy metals, free radicals and a variety of harmful chemical molecules, drugs
included ( molecular sieve model ).
It was argued, melanin would, moreover, behave like an ion pump, binding and discharging cations,
like iron, at request (exchange resin model ).
These mechanisms of action are far from being convincing. It is difficult to figure out in fact, how
this very heterogenous absorbed material can be washed out, regenerating and recycling the original
pigment substrate, without damaging it.
Numerous scientific papers describe how easy melanin absorbs and binds foreign compounds. No
paper explains, however, how to regenerate the original melanin, either in vivo or in vitro. In our
body, there are many physiological mechanisms in force, aiming at binding and releasing active
compounds at given times, conditions and in given quantities. All these reactions are reversible, by
nature. As an example, the iron metabolism is monitored by ferritin, which provides iron binding
and releasing at low energy consumption. The melanin ion-absorption-releasing system appears to
be, on the contrary, not sufficiently flexible to avoid a molecular suicide.
Washing out organic molecules absorbed by melanin, is not easier, either. It is well known, that
synthetic and natural melanin bind and strongly retain complex organic molecules for long periods
of time. Cocaine, amphetamines, chloropromazine, tranquillizers, arsenic and many other drugs are
examples of substances which can be easily traced in the hairs of users or abusers, even very long
time after intake. In these cases, binding looks like an irreversible process and the best way to make
free a compound absorbed is to destroy the melanin structure by oxidative degradation, a catabolic
mechanism well known in nature.
According to Finey-Burns [35] and Sarna [36] < Since the pigmented cells in the eye are mostly
non- dividing and practically no melanin renewal is known to occur, the ocular pigmentation that
has been once constituted is for life. This is an important biological difference between the skin
melanin and the eye melanin. While the skin melanin is being constantly synthesized by the
epidermal melanocytes, transferred to keratinocytes, digested by lysosomes, and extruded from the
body by shedding of dead keratinocytes at the level of stratum corneum, melanin in the RPE, and
probably in other pigmented eye tissues , shows very little, if any turnover. Thus the biological
consequences of any structural modification that may occur in eye melanin as a result of
environmental insults or simply of aging are potentially much more severe than those of skin
melanin modification >.
***
Ocular melanin ( RPE, iris and choroids ) and skin melanin are synthesized in different cells (RPE
or respectively epidermal melanocytes) from DOPA by tyrosinase dependent reaction mechanisms.
Brain stem melanin (SNM) is formed, on the contrary, in the substantia nigra from
CYSDOPA/DOPA, via a biochemical mechanism which is not dependent from tyrosinase. Its
formation starts early in the RPE tissues of human embryos and can be detected at a 7-week stage
already. Pathologic deficiency in eye ground pigmentation (albinism) causes visual disturbances, of
some time severe entity. Different genes control skin and eye pigmentation: There are individuals
affected by cutaneous albinism, showing white skin, normally pigmented eye ground (fundus) and
normal vision. There also individuals affected by ocular albinism, showing normally pigmented
skin, pale eye grounds and moderate to severe visual impairment.Low DOPA- and melanin-levels
result in retinal deficiency and a significant decrease of the rod number. Ocular melanin plays a role
in the development of the fovea and routing of optic nerves from the back of the eye to the brain.
Its deficiency is, moreover, associated with age-related macula degeneration.
The physiology and pathology of individuals, affected by the various forms of albinism, confirm
that ocular melanin acts as a light screen, absorbing excessive radiation and reducing scattering in
the eye, without being involved in phototransduction and nervous conduction.
14
It is not clear, however, how the light absorbed is dissipated by ocular melanin, whether it is
transformed into heat ( the most likely way ), or other forms of energy. In laboratory experiments,
natural and synthetic melanin’s behave in the doped state like electric conductors, whereas
physiological data in support, are still lacking. More studies are needed to check if the electric
conducting properties of ocular melanin might be involved in dissipating the excessive light
absorbed by the RPE, which knowingly amounts to approx.80% of the total solar energy absorbed
by the eye.
It was moreover suggested, that ocular melanin protects the eye tissues, binding heavy metals and
trapping oxygenated free radicals.
This hypothesis does not represent a validated protection mechanism of the eye tissues, deserving
further physiological confirmation in vivo. Binding of ions and molecules by the melanin particles,
as detected in laboratory models so far, should be considered, in our view, as fortuitous molecular
accidents of low biological relevance.
For skin and eye melanin, photo absorption represent the common denominator of their
biological function. Ocular melanin acts as light screen, absorbing excessive radiation and
reducing scattering in the eye. It is influencing, moreover, the prenatal growth of the
intraocular fine structure and normal nerves pattern, by unknown mechanisms.
PARKINSON’S DISEASE AND SUBSTANTIA NIGRA- MELANIN ( SNM )
Neuromelanin (NM) is primarily found in the cathecolaminergic neurons of the human substantia
nigra (SN) and locus coeruleus (LC).
Extensive degradation studies, carried out on SNM of animal and human origin, as well as
comparative studies on synthetic melanin models, suggest a mixed-type structure, mostly derived
from dopamine with 25% incorporation of cysteine in the form of benzothiazine units (pheo- and
eumelanin hybrid ), [3,15-18].
X-ray diffraction studies confirm a multilayer graphite-like three dimensional structure common to
all other natural melanin’s [37-38].
It was not established so far, if nor epinephrine is also involved in the synthesis of SNM. LCneuromelanin (LCM) is believed to derive at least partially from nor epinephrine. Fig.6.
15
***
The first description of this serious neurological illness in scientific literature dates back to 1817
and was made by James Parkinson, who called it < Paralysis Agitans >.
Doctors still display different views on the disease’s historical origins: some claim it is not a
modern-age illness but rather a plague that has always caused distress to mankind, as reported in old
Ayurvedic texts [39]. The disease, as a matter of fact, is ubiquitously widespread amongst the
elderly populations of all countries and amongst the different ethnic groups and socio-economic
classes. Furthermore, its distribution does not seem to depend on demographical, climatic, dietary,
socio-cultural elements or any factor related to industrialisation. Its prevalence does not vary from
country to country, with the exception of China, Japan and Africa, where it is slightly lower [40].
Other researchers, on the other hand, believe what is written in the old Chinese and Indian texts not
to refer to Parkinson’s disease proper, but rather to mere tremors that are common to several
pathologies. The disease’s explosion-like diffusion, thus, can be linked to industrialisation, and
especially to the presence of an exogenous neurotoxin in the environment and/or to a substantial
change in dietary habits. This hypothesis is corroborated by the consequences of the substitution of
whole meal with refined white flour in Great Britain in the early 1800s, which led to the loss of the
antioxidant vitamin pool [41].
The damage caused by different kinds of herbicidal pesticides and other chemicals, which can cause
diseases similar to PD, gives us further evidence that the neurotoxic hypothesis is valuable .
The aetiopathogenesis of Parkinson’s disease is still controversial. This disorder is largely
widespread and develops in a most insidious way in elderly age, causing progressive movement
impairment. This process follows four clearly-defined stages: tremors, bradykinesia, rigidity and
postural instability [42].
Parkinson’s disease, after Alzheimer’s disease, is the chronic degenerative neurological disease
affecting the largest number of people worldwide. It is estimated that 1.5 million people are
affected in the USA, 80,000 in France, 100,000 in England and 120,000 in Italy. In the USA 50,000
new cases are reported every year.
It is exceptional for patients to develop the disease before they reach the age of 30. In rare occasions
the illness may occur before 40, but 80% of cases concern people between 40 and 70 years of age.
After the age of 80 the disease no longer occurs. This data must be taken carefully, as often PD is
not diagnosed at all or is mistakenly diagnosed in patients affected by other neurological
pathologies.
Our current knowledge does not allow us to prove any link between PD and other pathologies, and
the role of risk factors, such as brain traumas, low-vegetable-income diets and hypertension, should
16
be investigated further. It is interesting to note that the number of smokers affected by PD is
extremely low. Such phenomenon has been interpreted as the consequence of nicotine’s alleged
protective effect on the dopaminergic system, namely on the substantia nigra and striata.
The symptomatology consists in the derangement of basal ganglions’ functions, with a reduced
activity of dopaminergic components and deficiency of the neurotransmitter dopamine, as well as of
the black pigment neuromelanin.
It has been hypothesised that the disease is triggered by a slow and progressive lack of dopamine,
but there is still controversy on what leads the brains’ dopaminergic cells – located in the substantia
nigra zona compacta (SNZC)- to gradually reduce their dopamine production, or on what brings
this substance to be transformed into inactive products.
The substantia nigra is connected to the striatum at the brain’s base through nerve fibres, whose
ends secrete the neurotransmitter dopamine. The latter helps the striatum in controlling movements.
The progressive death of substantia nigra cells is followed by a reduction in dopamine availability.
Normally, dopamine is stocked in inert form in vesicles. When secreted, it is metabolised by
monoamine oxidase (MAO), or is subjected to auto oxidation processes, if need be. In both cases
hydrogen peroxide (H2O2) is formed, which can have a cytotoxic effect if not neutralised by
catalase, reduced glutathione (GSH) and glutathione peroxidase (GPO).
The pathological variation of the different biochemical parameters suggests that during PD the
substantia nigra is stressed by oxygenated free radicals [43], which leads to the degeneration of
approximately 80% of dopaminergic neurons in the substantia nigra striata, before clinical
symptoms arise [44].
As a matter of fact, two elements can be noted: the increase in water peroxide concentration (the
basis of monoamine oxidase B activity), iron, ferritin, neuromelanin-connected iron, lipofuscin, the
peroxidation of membrane lipids and a decrease in oxygen peroxide destruction systems such as
reduced glutathione (GSH), glutathione peroxidase (GPO), the I complex (NADH co-enzyme Qreductase), mitochondrial electron transport activity and calcium chelating proteins.
As the disease progresses, the iron concentration in the substantia nigra increases, while glutathione
concentration diminishes. It is common knowledge that Fenton’s reagent ionic iron together with
oxygen peroxide (H2O2) produces the hydroxyl radical (•OH), which destroys reduced glutathione
(GSH) and has a cytotoxic action, as do all radicals.
Post-mortem histopathological evidence in PD patients shows that this degenerative process goes
hand in hand with: a rise in the malonic dialdehyde and lipid hydroperoxides (which are an index of
lipid peroxidation) basal concentration; a decrease in reduced glutathione (GSH) levels; a boost in
the mitochondrial super oxide dismutase (SOD) activity; an alteration in the iron metabolism
17
together with a contraction of ferritin amounts and an increase in the number of ferric/ferrous ions;
enhanced neuronal phagocytary activity in the microglia.
These data support the hypothesis
according to which “oxidative stress” plays a role in the substantia nigra striata degenerative
processes observed in PD.
Such hypothesis has theoretical grounds, as the central nervous system is especially vulnerable to
any kind of oxidative aggression, in particular by oxygenated free radicals. The brain contains large
amounts of easily oxidisable polyunsaturated fat acids. Through blood, it receives and uses a
quantity of oxygen (O2) disproportionate to other organs. On the other hand, it lacks anti oxidative
protection mechanisms, which are profusely diffused in other body parts [20]. For instance, in
some areas of the brain high ferritin concentrations are to be found, though with low reduced
glutathione (GSH) and glutathione peroxidase levels and the complete absence of catalase.
Ferritin levels in the brain of patients with PD shrink, especially in the substantia nigra (SNZC),
whilst free iron ions multiply becoming available for possible oxidation reactions. Dopamine plays
a crucial role as a neurotransmitter and the onset of PD is linked to its decline in the SNZC.
In the meantime, catecholamines such as dopamine are oxidised, and H2O2 is released through a
radicalic process leading to the formation of neuromelanin. Neuromelanin, in the presence of
H2O2, can react with Fenton’s reagent and release more hydroxyl radicals •OH. The latter, if not
neutralised, can pursue the peroxidation of membrane lipids, which eventually brings to a halt the
mitochondrial electron transport system. It has been recently proved that during dopamine oxidation
significant amounts of 6-hydroxidopamine form, together with its quinone, Topamine-quinone
(TQ). Both are highly cytotoxic.
Melanin can capture consistent amounts of iron, leading to the formation of iron-melanin, a
complex that was said to be able to easily release activated iron, considered to be more neurotoxic
than normal free iron. Significant data proving that neuromelanin is really able to release iron easily
and quick, are however still lacking. The neuromelanin binding affinity towards iron and other
cations is in fact rather high, raising doubts about this mechanism of action being workable in vivo.
Such oxidative processes are catalysed by several metals and especially iron, whose highest
concentration in the brain is to be found in the substantia nigra. In physiological conditions, iron is
present in negligible amounts in its free state because it is stocked in ferritin and hemosiderin in an
inactive form. However, it becomes readily available according to physiologic (and pathologic)
needs. Suffice it to say that a mole of ferritin contains 4,500 atoms of iron, an amount which, even
if only partially released, reacting with hydrogen peroxide H2O2, as in Fenton’s reagent, could
generate a significant number of hydroxyl radicals •OH. It should be stressed, moreover, that the
binding affinities of ferritin and neuromelanin for iron and other heavy cations are not comparable.
18
Ferritin/iron- ferritin represent an almost ideal reversible system, able to easily and quickly bind and
release iron, at request. The neuromelanin/iron- neuromelanin complex is, on the contrary, much
too slow to satisfy the needs of our body’s requests.
It has been shown that 6-hydroxydopamine can release iron from ferritin, triggering a chain of
reactions that will lead to the formation of hydroxyl radicals •OH, extremely cytotoxic even for
dopaminergic neurons. •OH radicals have a short average life span. Thus, they are likely to act
close to where they formed.
This hypothesis is confirmed by the fact that the iron chelator desferroxamine – which can break the
oxidative chain – displays some activity in Parkinson’s and Alzheimer patients, as clinically
observed.
Post-mortem analysis of the human brain have established, that oxidative stress and iron content are
enhanced in association with neuronal death. A possible consequence of an overloading of
neuromelanin with redox-active elements is an increased contribution to the oxidative stress and
intraneuronal damage in patients with Parkinson’s disease [45].
Quantification of the total iron content in iron-loaded neuromelanin and synthetic melanin
demonstrated that the iron-binding capacity of neuromelanin is 10-fold greater than that of the
model melanin. These findings were interpreted as a confirmation of the hypothesis, that
neuromelanin may act as an endogenous iron-binding molecule in dopaminergic neurons, whereby
an increased iron in the CNS is associated with increased indices of oxidative stress [46].
In later studies neuromelanin was identified as a genuine melanin with a strong chelating ability for
iron and affinity for several organic compounds. The affinity for a variety of inorganic and organic
toxins appears to be consistent with the postulated protective function for neuromelanin. Moreover,
the neuronal accumulation of neuromelanin during aging and the link between its synthesis and a
high cytosolic concentration of catechols suggest a protective role [47]. The fact that all these
complexes are de facto irreversible, is not taken however into proper consideration by the authors.
In a recent study, the concentration of iron, ferritin and neuromelanin in substantia nigra from
normal subjects, aged between 1 and 90 years, dissected post mortem, was measured.. The iron
levels in substantia nigra were 20 ng/mg in the first year of age, had increased to 200 ng/mg by the
fourth decade and remained stable until the eighty decade of life. L-Ferritin also showed an
increasing trend during life, although the concentrations were approx. 50% less than that of HFerritin at the age point. Neuromelanin was not detectable during the first year, increased
continuously to 3500 ng/mg in the 80th year. Based upon these data, it was concluded that
neuromelanin is the major iron storage in substantia nigra neurones in normal individuals [48].
Because ferritin and neuromelanin strongly differ in their relative iron binding affinities, this
finding cannot mean, in our view, that ferritin and neuromelanin are bio equivalent iron storage
systems.
Our viewpoint is in fact confirmed by another paper, stating that the overloading of neuromelanin
with iron and other metals may trigger inflammatory and degenerative processes, aggravating
underlying pathologic conditions [49].
As shown in numerous papers, Parkinson’s disease is associated with a significant increase in iron
in the degenerating substantia nigra and is measurable in living PD patients and in post-mortem
brain. This increase, however, occurs only in the advanced stages of the disease, suggesting that this
phenomena may be secondary, rather than a primary initiating event, an hypothesis also supported
by evidence from animal experiments [50]. According to these findings, the role of iron as key
element in the pathogenesis of Parkinson, appears to be significantly diminished.
19
Nitrogen monoxide NO has been found capable of releasing iron from ferritin, contributing to the
formation of highly reactive oxygenated radicals [51]. This finding deserves further attention in
view of the intriguing function of nitrogen monoxide on numerous biological functions.
Several pharmacological agents, such as neuroleptics of the phenothiazine groups and
butyrophenons or reserpine, can induce syndromes that are similar to Parkinson’s. Both interfere
with the dopamine mechanism: the former block postsynaptic dopaminergic receptors, the latter use
reserpine to remove the available dopamine from presynaptic neurons.
Therefore, it can be reasonably argued that at the root of Parkinson’s disease there is a degenerative
process of dopaminergic neurons caused by the aggression of the oxygenated radicals that formed in
situ, with iron as a catalyser.
A person’s neuromelanin concentration in the substantia nigra varies with age. It is virtually nil
during early infancy, when the newborn does not have full control over his/her locomotorial
functions. The neuromelanin concentration reaches its peak in adult age, declines over the years and
is at its lowest in the elderly. Its decrease goes hand in hand with the natural degradation of
locomotorial coordination and reflex response.
Considering this phenomenology, it can be suspected that the substantia nigra plays some role in
nervous transmission, consistently with the electro conductivity of the SNM particle. It must be
highlighted that post mortem examination of patients with Parkinson’s disease in advanced stages
reveals severe depletion of the black pigment in the substantia nigra.
Neuromelanin is produced constantly by dopaminergic neurons through the oxidation of dopamine
and its derivatives. It is subsequently metabolised through oxidative degradation.
It can therefore be claimed that, in normal physiological conditions, a homeostatic balance
dopamine/neuromelanin is established.
A low dopamine concentration means smaller amounts of neuromelanin and consequently an
impoverished and in its electric functions deranged substantia nigra
On the other hand, an excessive formation of neuromelanin, through an abnormal dopamine
oxidation, leads to a lack of this neurotransmitter, which in turn upsets dopaminergic functions and
eventually kills the neuron. Fig.7
20
21
The dopamine/neuromelanin dynamic balance is critical for the system to perform correctly.
However, it is also quite unstable as its components are sensitive to and react to oxidation.
Yet, it must be said that doubts have arisen about the definitive aetiopathogenesis and about
whether the factors that trigger the degenerative process leading to Parkinson’s disease are of
endogenous or exogenous origin [52-53].
Microglial inflammation augments the progression of Parkinson’s disease due to the effects of
human SNM on the release of toxic mediators and the underlying signalling pathways from rat
microglia in vitro, confirming the supposed crucial role of SNM in the pathogenesis of Parkinson’s
disease, by augmentation of microglial inflammation [54].
Finally, it was recently suggested that the SNM deposition in the basal nuclei represents an
antioxidant mechanism, by trapping toxic quinines and semiquinones, as well as protecting neurons
from accumulating damage over years [55].
Defective sequestration of dopamine into vesicles, leading to the generation of reactive oxygen
species in the cytoplasm, is a key event in the demise of dopaminergic neurons in Parkinson’s
disease and might represent a common pathway that underlies both genetic and sporadic forms of
the disorder. Increased levels of cytoplasm dopamine in nigral neurons in PD patients might result
in dopamine oxidation and generation of reactive oxygen species, that can damage and eventually
kill these neurons. Dopamine induced oxidative stress, impaired synaptic vesicle function and
misfolding of α-synuclein, due to mutations or oxidative damage to this protein, might be
components in a self-perpetuating vicious cycle that might lead to the demise of dopaminergic
neurons [56].
The discovery of Parkinson’s disease gene has led to the hypothesis, that misfolding of proteins and
dysfunction of the ubiquitin-proteasome pathway are pivotal to PD pathogenesis [57].
As for the aetiopathogenesis of Parkinson’s disease, it seems more and more likely, that oxygenated
radicals attack the dopamine-neuromelanin pump, thus breaking the homeostatic balance and
leading to the degeneration and death of dopaminergic neurons. When enough neurons have
irreparably degenerated, the first symptoms arise and the disease follows it course, until it reaches
its fatal end. Extracellular
neuromelanin itself, which is linked to highly reactive iron
(neuromelanin-Fe3+ complex), may act as a catalyst in the formation of hydroxyl radicals •OH that,
in turn, can attack other dopamine molecules.
It therefore seems reasonable to assume that the neuro degenerative process, once started, proceeds
by auto catalysis, until an unlikely event (radical scavenger, antioxidant) breaks the chain [58].
The concentration of neuromelanin in substantia nigra pars compacta (SNPC) has been measured in
male and female normal subjects at different ages in the range of 1-97 years old and in SNPC of PD
patients. These data confirm a continuous neuromelanin accumulation during life and severe
depletion during Parkinson’s disease [59].
22
The incidence of Parkinson’s disease increases with age, as the body’s defences weaken. The
greater the vulnerability to radical aggression, the less the body can fight back through appropriate
enzymatic mechanisms. These facts are in line with what has been put forward.
***
Enzymatic reactions use to follow fixed patterns, aimed at synthesising structures, with definite
biological properties, suitable for specific functions.
The new compounds are not an end in
themselves: they return into the life cycle thanks to appropriate, and sometimes simple, structure
modifications. The picture is more complex when it comes to radical reactions, forming materials
that are hard and slow to biodegrade and apparently have no biological purpose.
At first glance it may seem as though nature got stuck in a cul-de-sac, during Parkinson’s disease.
Whilst it is commonly accepted that oxidative stress of the dopamine-neuromelanin system, plays a
role in PD’s pathogenesis, there is disagreement on the causal relationship between the two issues:
which one is the cause and which one the effect.
We are more and more inclined to believe, that there is no true difference, as both are cause and
effect in an endless vicious circle.
SN-melanin is chemically different from the skin, eye and hair pigment, its biosynthesis being not
dependent from tyrosinase. As a matter of fact, individuals suffering of albinism show a normally
pigmented substantia nigra. There is no experimental evidence, however, that this pathology did
not modify in some way the chemical composition of their SNM.
As already mentioned, SNM is a pheo/eumelanin hybrid, derived from DOPA/CYSDOPA and is
involved in Parkinson’s disease.
CONCLUSIVE REMARKS
Melanin’s are chemical derivatives of either pyrrole, indole or benzene, their structure being
closely related to polypyrrole, polyindole and polyphenylene. They are composed of amorphous
black micro particles, organized in units and subunits of different shape and size and this physical
state is instrumental to their biochemical and biological behaviour. Under proper conditions,
melanin’s are able to absorb and dissipate light and sound, to bind metals and organic compounds,
to store liquids and gases, to conduct electrical current if doped and to transform light into electric
23
energy. There is large scientific evidence according to which, melanin’s play fundamental
biological functions through mechanisms, which are still poorly understood. This situation is best
exemplified by the axiom: <Melanin’s : Biological Jewel, Biochemical Puzzle >.
Skin melanin, a DHI-derivative synthesized in the epidermal melanocytes under the influence of
solar radiation, acts as sun screen, protecting skin from the harmful effects of excessive solar
radiation. The biochemical mechanisms, involved in skin tanning and dissipation of radiation are
not fully understood, so far.
Ocular melanin, synthesized in the retinal pigment epithelium (RPE) of human embryos before
birth, acts as a broad band optical absorber, protecting the eye ground (fundus) and ocular tissues
from excess light, reducing intraocular light scatter and thereby increasing the contrast of the visual
images formed on the retina. Ocular melanin also plays some unexplained role in the prenatal
development of the fovea and routing of optic nerves, its deficiency being associated with agerelated macula degeneration, the leading cause of blindness. The formation of ocular melanin in the
RPE is tyrosinase dependent and there are various genes, controlling eye and skin pigmentation (
cutaneous, oculocutaneous, ocular albinism ).
Skin melanin protects the skin from excessive solar exposure, while ocular melanin protects the
eyes from radiation hazards. Both pigments are therefore deeply implied in a fundamental
biological function: photo protection. We still don’t fully understand, how light is dissipated by
skin and eye, whether it is just transformed into heat or electrical current. In both cases, however,
the physical asset of the melanin particles is instrumental to capture the light beams and to avoid
them to escape from the black melanin cage (black body model).
SNM or Substantia nigra melanin, is chemically similar, but not identical to skin melanin. SNM is
formed after birth in the catecholaminergic neurons of the brain stem, in the absence of visible light,
its synthesis being not tyrosinase dependent. SNM is a pheo/eumelanin hybrid derived from
DOPA/CYSDOPA, apparently involved in movement coordination and neurons protection from
oxidative stress. Extra cellular SNM and SNM –iron complex are furthermore supposed, to be the
cause of neuronal death and Parkinson’s disease.
There are substantial differences between chemical composition,
pathway and biological function of ocular and SNM.
biogenetic site, biochemical
Skin and ocular melanin act as photo
absorbers, SNM does not. Dopamine deficiency means less available SNM; both aspects, less
SNM and less dopamine, are correlated with the movement impairments typical of Parkinson’s
24
disease. We don’t know however what is the cause and what the effect. Is PD caused by dopamine
shortage, as consequence of viral aggression or other exogenous causes, or is PD causing the
dopamine shortage, responsible for the motorial symptoms?
In case of skin and eye, photo absorption provides a convincing explanation for the biological
function, which can be easily correlated to two physical properties, black colour and particle
structure ( so called < melanin cage > or < black body >model ).
In case of SNM, binding of iron, heavy metals, free radicals and harmful chemicals, was claimed to
be part of hypothetical detoxification and protection mechanisms. This hypothesis is based,
however, on in vitro-experiments only, being unable to provide significant evidence of a convincing
physiological function. Binding of iron or other heavy metals, scavenging of free radicals or
harmful organic compounds and drugs, cannot be accepted, in our view, as a validated
physiological process, representing rather fortuitous molecular accidents of low physiological
relevance.
SN- depigmentation is accompanied by movements and motor coordination impairment, indicating
that SNM might be involved in the process of transmitting and modulating nervous impulses.
Moreover, SNM is absent or significantly scarce ( pale substantia nigra ) in two life conditions, in
which movement coordination is either not efficient (newborns) or strongly compromised
(Parkinson’s patients).
Based upon these facts, we suggest to consider the possibility of doped SNM acting as bioelectric
conductor in the basal ganglia ( nucleus caudatus, putamen, globus pallidus, substantia nigra pars
compacta and reticulata, nucleus hypothalamicus ).
***
The neuromelanins (brain, eye, ear ) are a family of chemically different black particles exercising
specific biological roles on different substrates, at different sites. Despite the different chemical
structure, all melanin’s share some common
physical property, as particle structure, photo
absorption (black colour), electro conductivity, binding of ions and chemicals, etc.
The biological functions of chemically different melanin’s are sometime similar, being determined
by a common physical property. In case of skin and eye, photo absorption appears to be the proper
common denominator.
In case of brainstem, where photo absorption plays no role and < detoxification mechanisms >
have no physiological relevance, the electro conductivity represent the physical property (common
25
denominator), able to explain the biological function of SNM, if any, by a unitary scientific
approach. Respecting, moreover, Newton’s simplicity law: < Natura enim simplex est >.
The bio electric properties of SNM deserve further attention and investigation.
LITERATURE AND NOTES
[1] Bruno J.R. Nicolaus < Malattie Molecolari >, Accademia Pontaniana, Napoli, Quaderno 34
(2002); Bruno J.R. Nicolaus < Health and molecular diseases-Speculating on the global spreading
of molecular diseases :How should mankind react?>, Accademia Pontaniana, Napoli, vol LII
(2003), 285-293; www.brunonic.org/highlights
[2] For further information, please contact: [email protected]
[3] Rodolfo A. Nicolaus and M. Piattelli <Structure of melanin’s and melanogenesis > J. Pol.
Sci.(1962) 58,1133; Rodolfo A. Nicolaus < Melanin’s > (1968) Hermann, Paris; Rodolfo A.
Nicolaus <Melanin’s>> Methodicum Chimicum ( 1978 ) Vol.11, Part 3,190 Ed. Korte and Goto,
AP. Tokyo; Giuseppe Prota <Melanin’s and Melanogenesis > (1992) Academic Press, Inc.; R.A.
Nicolaus and G. Parisi < The nature of animal blacks > Atti Accademia Pontaniana
(2000),Vol.XLIX, 197-233.; Rodolfo A. Nicolaus, A. Bolognese, B. Nicolaus < The Pigment Cell
and its Biogenesis> Atti Accademia Pontaniana ( 2001) Vol. L , 227-243.
www.tightrope.it/nicolaus/index.htm
[4] B.L. Kaul < Studies on heterocyclic colouring matters: δ-2,2’-bi(2H-1,4)-benzothiazinones >
Helvetica Chim. Acta (1974) 57,2664;
H.D. Martin <The function of natural colorants: The Biochromes > CHIMIA (1995) 49, 45-68.
[5] L. Zeise, B.L. Murr, M.R. Chedekel < Melanin standard method : particle description >
Pigment Cell Research (1992), 5,133-142 ; Pigment Cell Research Suppl.2,(1992), 48-53;
Nofsinger J.B., Forest S.E., Eibest L.M., Gold K.A. and Simon J.D < Probing the Building Blocks
of Eumelanins Using Scanning Electron Microscopy> Pigment Cell Research (2000) 13: 179-184;
Y. Liu, J.D. Simon < Isolation and biophysical studies of natural eumelanin’s : applications of
imaging technologies and ultra fast spectroscopy > Pigment Cell Research, (2003), 16,608-618;
26
16,72-80; 16,606-618 ; C.M.R. Clancy, J.D. Simon < Ultra structural organization of eumelanin
from Sepia officinalis measured by Atomic Force Microscopy (AFM) > Biochemistry, (2001), 40,
133-53 ; J. Phys. Chem. B, (2000), 104, 7871.
[6] Bruno J.R. Nicolaus and Rodolfo A. Nicolaus <Lo scrigno oscuro della vita-Riflessioni sul
ruolo chimico biologico della materia nera interstellare e sulla comparsa della vita nell’universo >
Atti Accademia Pontaniana,Napoli vol XLVIII (1999), 355-380
[7] F F Barr, J S Saloma, MJ Buchele < Melanin: the organizing molecule > Med. Hypothesis
(1986) 11:155-161.
[8] N Kollias, R M Sayre, L Zeise, M R Chedekel <NewTrends in Photobiology -Photoprotection
by Melanin > J.Photochem.Photobiol.B:Biol. (1991) 9:135-160.
[9] The melanocytes of avian and mammals consist of two anatomically distinct parts, the one
belonging to the basal layer of the epidermis and the other to the hair bulbs.
Melanocytes are also found at various internal sites, such as the uveal tract of the eye, the inner eye,
the leptomeninges and the oral mucosa. The retinal pigment epithelium (RPE) is often included
among these extracutaneous melanin cells, although it does not originate from the neural crest, but
from the outer wall of the optic cup. R.D.Adams (1987) < Neurocutaneous diseases > in
<Dermatology in General Medicine > (T.B.Fitzpatrick, A.Z.Eisen, K. Wolff, I.M. Freedberg and
K. Austen, Edts.).
[10] Bruno J.R. Nicolaus < Speculating on the Band Colours in Nature > Atti Accademia
Pontaniana, Napoli, Vol. XLV (1996), 366-385; Bruno J.R. Nicolaus and Rodolfo A. Nicolaus
<Melanins, Biological Garbage or Jewel > Scientific communication presented at the 8th.Meeting
of the European Society for Pigment Cell Research Sept. 23-26 (1998), Prague; Rodolfo A.
Nicolaus, A. Bolognese, B. Nicolaus < The Pigment Cell and its Biogenesis > Atti Accademia
Pontaniana, Napoli Vol. L (2001), 225-243
[11] Assessing melanins in organic tissues and fluids is not an easy task , due the intrinsic chemical
properties of this class of compounds. The EPR signal (paramagnetism) represent, whenever
applicable, a specific analytical tool for the detection of melanins, whereas any statement based on
colour and reactivity towards silver salts alone, may be misleading. There are various statements
published in the international press, which should be reconsidered, in view of this fact.
[12] Eduardo Porta <Pigments in aging: an overview> Ann. N.Y. Acad. Sci. 959: 57-65 (2002).
[13] W.A.Little < Possibility of synthesizing an organic superconductor > Phys. Rev. 134, A1417
(1964).
[14] W.R.Salaneck, I.Lundstroem, B.Ranby, Editors < Conjugated Polymers and related materialsThe interconnection of chemical and electronic structure > Proceedings of the eighty-first Nobel
Symposium; Oxford University Press (1993); A.J.Heeger < Conjugated Polymers: The
interconnection of chemical and electronic structure >; H.Shirakawa, Y-X.Zhang, K.Akagi <
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hydroxylase and then transported to the brain, by the large neutral amino acid transporter. Once in
the brain, tyrosine is converted to Dihydroxy-phenylalanine (DOPA) by the tyrosine hydroxylase
enzyme using oxygen, iron and tetrahydrobiopterin (THB) as co-factors. DOPA is converted to
dopamine by the aromatic amino acid decarboxylase using pyridoxal phosphate (PLP) as a cofactor. Both dopamine and norepinephrine are catabolized by a two-step process, involving the
enzymes mono amino oxidase (MAO) and catechol-O-methyltransferase (COMT).
.
[20] In contrast to the 6-20% of other organs, almost 50% of dry brain weight is made out of lipids.
These lipids are building, as myelin or membranes, the basic structure of nervous cells, whereas the
free fatty acids and triglycerids, are the main components of other organs fats. Besides the peculiar
functional role, this fact is outlining the unique chemical composition of the brain.
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some individuals with albinism have reddish or violet eyes, most have blue, some hazel or brown
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the retina and abnormal patterns of nerve connections between the eye and the brain. While most
people with albinism have very light skin and hair, not all do.
[31] In past years, the most common types were usually referred to as ty-positive and ty-negativ,
where ty- stands for tyrosinase. These terms evolved from a laboratory test in which a few hairs are
plucked from the scalp of a person with albinism and their roots incubated in a physiological
solution of tyrosine. Ty-pos hair roots turn black in the test , signifying they are building melanin,
whereas Ty-neg do not, indicating they are not synthesizing melanin.These patients are going to
experience, therefore, more difficulty with vision. On the contrary, those with Ty-pos albinism still
have slight pigmentation and shall experience less severe vision difficulties. Nowadays, more
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whereby the amount of uveal pigmentation developed is directly proportional to the degree of visual
functioning. Transillumination of the retina follows insufficient development of the RPE, as well as
poor uveal pigmentation. Thus, the consequent blond fundus appearance is often accompanied by
broad areas of hypopigmentation and the well visible vessel system, which furnishes blood to the
retina.
NOAH-The National Organization for Albinism and Hypopigmentation: www.albinism.org
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