Download CHAPT 01 Introduction to Minerals

Chapter 1. Introduction to the Minerals
The word mineral has its derivation in the word “mine”, or more specifically, referring to
substances in the earth’s crust that can be obtained by “mining”. To a nutritionist, however,
minerals are essential components of life that play a multitude of functional roles in cells,
functions that their organic counterparts cannot duplicate. Minerals thus represent a special
class of food nutrients that have no parallels.
To appreciate the role of minerals in a living system, we need first consider the four
major elements of living matter: proteins, carbohydrates, fats, and nucleic acids. These four
fundamental compounds, in turn, take their foundation from only six elements in the chemist’s
Periodic Table: carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorous. As shown in Fig.
1.1, carbon, hydrogen and oxygen are the basic building elements of carbohydrates and fats.
With nitrogen and sulfur in the synthetic mix it is now possible to build proteins. Adding
phosphorous allows the construction of nucleic acids. Together, the six elements make up the
molecular compounds common to all living forms.
Carbon
Hydrogen
Oxygen
Carbohydrates
Fats
Nitrogen
Sulfur
Phosphorous
Proteins
Nucleic Acids
We may ask, however, will six basic elements suffice to give us the properties we
associate with life? For example, will the six build bone or allow blood cells to bind oxygen, or
permit enzymes to function as catalysts? Will any stimulate muscles to contract, propagate
nerve impulses, or maintain fluid balance. Evidently when we ask these questions we see that
six elements we denoted as essential to life fall short of the number actually needed to make
life happen. In essence the six can only give us the components of life, not life itself. To put
this into a more biological perspective, we associate life with components within systems that
determine movement, growth and development, turnover, energy production and utilization
and maintenance of internal homeostasis, what are called the attributes of a living system.
Presently, there are 27 elements deemed essential for animal life (Table 1.1). In addition
to the 6 mentioned above, which constitute the bulk of organic matter, 21 additional elements
are needed for living systems (Table 1.1). Note that because all elements do not make their
appearance in equal quantities, it is necessary to refer to those that occur in greater quantity as
“macrominerals” and those less conspicuous as “microminerals. As pointed out by
Underwood, only three essential elements namely iodine, tin and molybdenum have atomic
numbers above 34. Most occur in the range of 23-34 implying a selection process avoiding
heavier elements. Thus, in the final analysis, of the 27 elements that form the chemical
foundation of life, nearly 80 percent of these comprise the class we call minerals.
Macroelements
Metals
(cations)
Sodium
Potassium
Calcium
Magnesium
Non-metals
(anions)
Chlorine
Phosphorous
Sulfur
Micro- or Trace Elements
Metals
(cations)
Iron
Zinc
Copper
Manganese
Nickel
Cobalt
Chromium
Molybdenum
Tin
Non Metals
(anions)
Iodine
Fluorine
Vanadium
Selenium
Silicon
Boron
Table 1.1. Inorganic Elements of Life.
ORGANIC AND INORGANIC MOLECULES
Living systems are composed of two major classes of molecules, organic and inorganic.
Organic are present in a variety of shapes and sizes, from the small molecules such as carbon
dioxide to the highly complex proteins and nucleic acids. Common to all organic molecules is a
chemical foundation built around carbon. The carbonaceous molecules were given the
moniker “organic” in recognition of their highly “organized” architecture that was considered
unique to molecules derived from living organisms. Organic has, therefore, become a synonym
for life. Molecules without this architecture or void of carbon were thus classified as
“inorganic”, which carried the connotation of not having a design or structure of living
molecules. As a class most were small molecular weight complexes or ions composed mostly
of metal ions, metalloids and non-carbon complexes. In higher animals their presence was
seen in crystalline structures such as bones and teeth. Denoting minerals as “inorganic”,
therefore, carried the stigma of not being part of a living environment. Some, such as sodium
and chloride were even consider symbolizing an imprint of life’s early origins from the sea.
A HISTORICAL PERSPECTIVE EARLY STUDIES
Interest in biological minerals began more than 150 years ago and was prompted by the
discovery of metal-containing compounds in blood, pigments and fluids of animals. Most
reports were dismissed, however, because minerals were considered unimportant or having no
recognized biological function. To some, however, the discovery of minerals meant something
worthy of pursuit. Iron in blood was present in great quantities and an illness state was marked
by reduction in blood iron. In 1847 a protein in snail blood suspected of transporting oxygen in
that species was found to contain copper. Significant amounts of copper, up to 7% by weight,
and thus beyond simple contamination, were later discovered in turacin, a pigment extracted
from the feathers of a West African bird, the Turaco. This finding brought increased interest in
metal ions as the potentially significant to life when some twenty years earlier they were
considered a curiosity. As early as 1933 Shoh’s epic publication titled “Mineral Metabolism”,
concluded that only copper, manganese and iodine could clearly be classified as essential.
Shoh further specified these as “essential trace elements”, using the term “trace”to highlight
their very low concentration. In time minerals were regarded as essential to all living
organisms, not just animals but also plants and microorganisms. Table 1.2 list a series of
seminal discoveries that were made in the early history of mineral investigations.
Early studies on minerals lacked the technology to quantify minerals. Gross mineral
content in a food source relied on weighing the ash residue after burning off the organic
material with nitric acid. Indeed, classifying some minerals as “trace”, a term still used today,
betrayed an inability to determine the precise quantity of a particular mineral in a tissue or
fluid, but instead inferred a presence of barely detectable amounts. Even quantifying
macrominerals such as sodium, potassium, calcium in food stuff depended on gravimetric
procedures such as precipitating the ions from solution and weighing the precipitate.
Colorimetric analysis lacked the instrumentation to measure absorption and relied on visually
comparing the unknown with a series of standards. Measurements that accurately determined
the amounted present, especially in the micromineral category, had to await the advent of
newer more sensitive analytical instrumentation. Atomic absorption which is the method of
choice today for quantifying metal ions did not make an appearance until the 1940’s.
In the biological sphere, much of the early work focused on nutritional significance, but
even these studies suffered from low technology. Investigating essentiality of mineral
elements required formulating purified diets that had all the growth factors save the mineral
under study. Early diets were crude and lacked vitamins and other essential factors which
confounded the results of omitting the minerals. Because many of the vitamins were not
available in pure crystalline form, investigators had to rely on extracts of vegetables or yeast
cells as their source, which unavoidably brought the mineral of interest back into the diet.
These were some of the handicaps that early investigators had to overcome. The final barrier
to recognizing the essentiality of minerals fell when disease state or physiological impairments
brought on by a mineral deficiency could be reversed by reintroducing the missing mineral.
This not only put minerals in a therapeutic perspective, but also linked the mineral with the
impaired state.
BIOMINERALS
Minerals that act in a biological capacity are called biominerals. To be so designated a
mineral must have a critical function associated with its presence. Linking a mineral with a
disease or metabolic impairment thus underscored importance. For example, preventing
anemia with iron or the formation of a goiter with iodine salts made it apparent why these
minerals were required by the system. Linking minerals to the action of enzymes and growth
factors puts minerals in the realm of irreplaceable components of living systems.
Year
Discovery
1828
Friedrich Wohler, synthesized urea by heating ammonium cyanate. The
experiment showed that life chemicals could be synthesized in a test tube.
1832
Frodisch found that people who suffered from chlorosis, a greenish color of the
skin, had a lower blood iron content than health individuals.
1834
William Prout reported that stomach juices contained hydrochloric acid.
1840
Boussigault (French), and von Liebig (German) collaborated in recognizing the
importance of minerals. Wild animals would walk many miles to salt licks.
Bone was composed of calcium and phosphorous.
1847
Harless reported the discovery of copper in the blood of snails
1850
Boussigault observed that salt deposits which could be used to treat a goiter
contained iodine as the active component.
1850
Chatin, a French botanist, correlated the incidence of goiter with the iodine
content of soils, waters and foods in the environment
1855
Henneberg and Stohman, at the Weende Experiment Station in Germany
conducted the first systematic fractionation of animal feed, a process known
today as “proximate analysis”. Minerals were determined by ash content.
1869
Raulin identified zinc as factor needed for the growth of the bacterium
Aspergillus niger. Church found that copper made up 7% of the weight of
turacin, a pigment isolated from the feathers of the West African Turaco.
1898
Abderhalen introduced a milk-protein, iron free diet to study anemia
1927
Hart, Steenbock, Waddel and Elvehjem showed that the full recovery of anemia
in an iron-deficient rat required copper as well as iron.
1932
McCollum and coworkers showed magnesium to be essential for growth of
laboratories animals
1934
Todd and Elvehjem showed that zinc was essential to animal health
1952
Holmberg and Laurell reported the isolation of two globulins in human serum,
one containing iron they called transferrin and the other copper was call
ceruloplasmin to recognize its heavenly blue color.
Table i.1. Important discoveries in the history of mineral research
As noted above, biominerals are further classified by quantity as belonging to the
macro- and micro subclasses. Macrominerals are major constituents of blood and body fluids
as well the skeleton of vertebrates, the latter consisting almost entirely of calcium and
phosphorous. Most minerals, however, are metal ions such as Ca2+, Mg2+, K+, Na+, or complexes
such as NH4+, HPO4= , and SO4=. In contrast to the macro subclass of minerals, the
microminerals are several orders of magnitude lower in concentration and are so named
because of their scarcity in cells. The micro or trace minerals do not contribute appreciably to
body mass or structure, but participate as cofactors for enzymes or regulators of cellular
events. Metal ions make up the largest contingent in this class, especially metals of the first
transition series of element, Mn2+, Fe2+, Cu2+ and Zn2+ to name a few. Each has been considered
essential to practically all species of plan and animal life. Also included are non-metals such as
selenium (Se), vanadium (V) and boron (B). Because their scarcity precludes movement by
diffusion, microminerals are commonly found bound to proteins, which are either the targets
of their action or transporters for relocation. A further subset, the “ultratrace”, designates a
group below micro in quantity. Included are tin (Sn), Nickel (Ni), arsenic (As), and silicon (Si).
Although their concentration may be small, ultratrace represents a class of minerals suspected
of having nutritional value in that a complete omission from the diet could retard growth and
development. The final category is the toxic minerals. As the name implies, this class is
composed of anti-nutrients that are a threat to normal function. Cadmium (Cd), mercury (Hg),
lead (Pb), aluminum (Al), and arsenic (As) are the most notable members in the toxic mineral
category. All have the property of harming the system while redeeming no useful nutritional
value. Its important to note, however, that whether a mineral is toxic or beneficial will depend
on its concentration in the food source or site of action internally. Arsenic (As), for example, is
beneficial at low concentrations but deadly when in excess or upon prolonged exposure.
Naturally, this is a question of concentration and duration, since any mineral given in high
concentrations or over time has the potential to be toxic.
MINERAL COMPLEXES
All biologically important minerals exist as either complexes or free ions. Most
common are the simple biocomplexes such as those formed when an inorganic group attaches
to a sugar or fatty acid molecule. Phosphate (-HPO4=) is one of the most ubiquitous biomineral
complexes. Nitrogen give rise to ammonia or ammonium ion (-NH4+) nitrate (NO3-) and nitrites
(NO2-). Sulfur is more commonly present as sulfate (-SO4=) and vanadium and boron as
vanadate (VO4-) and borate (BO4), respectively.. A more highly complex arrangement of
atoms is represented by hydroxyapatite, which is a crystalline form of calcium and phosphorous
in bone. Heme, a complex that forms when iron engages porphyrin, is the iron complex in the
protein hemoglobin and the cytochromes, a family of iron-containing proteins that transfer
electrons. Minerals bound to organic complexes are a basic structural motif of biological
systems. As you may recall from biochemistry, phosphate-bound sugars are common
metabolites in biochemical pathways.
Perhaps the more familiar organic binder of minerals are the proteins. Metal ion binds
covalently or electrostatically to amino acids in the protein’s structure. In some instance a
biological function is initiated or suppressed by the binding. The protein acts as a ligand in the
sense that it traps the metal on the surface through multiple bonding. Thus, key amino acid
that act as ligands in a metal-binding protein have to be arranged in a select manner to tether
metal ions. Enzymes that require metal ions for catalytic action show no activity without the
metal bound. The enzyme is basically a functionless protein without the metal ion. Other
examples include membrane proteins that initiate signals in response to a hormone or
extracellular factors. Phosphate derived from ATP becomes attached to these proteins
putting them in the proper confirmation to propagate the signal inward. Besides proteins,
nucleic acids also form complexes with metals ions. Familiar examples include zinc and
magnesium engaging DNA and RNA, in the control of replication and transcription,
respectively. Today there is a growing interest in metal-ions involvement in the regulation of
cellular events
NUTRITIONAL PERSPECTIVE
The nutritional perspective on minerals aims is to distinguish those essential for life from
those whose role is dubious or dangerous. Setting nutritional standards for macrominerals is
less of a challenge due to their omnipresence in tissues and fluids. The challenge comes with
establishing criteria for the microminerals or trace minerals. Tissue and fluid levels of
microminerals are in the micromolar range. , Davies has suggested that for a trace element to
considered essential, it must:
1. be present in living tissues and fluids
2. have some connection with the function of a biological system or component
3. show a reduction in that function with a deficiency
4. be able to restore the function when added back to the system
These four basic criteria have set the ground work for nutritional investigations of minerals. All
four must be met in order to consider a mineral (or any other nutrient) essential for life. What
applies to one species should be applicable to others and any decisions regarding essentiality
should be based on the evidence obtained from numerous studies in different laboratories.
Determining essentiality is therefore a blend of nutrition and biochemistry, the former linking
essentiality to what is present in the food stuff and the latter identifying the key events at the
molecular level of function.
Of the four, the first criterion is perhaps the weakest. Merely finding a mineral in a
system does not suffice to indicate importance, especially life-threatening importance. The
second builds a stronger case because it is based on the interaction with the system
components, which suggest compatibility and necessity. The third is to recognize that
importance can best be established by observing the consequences of omitting the mineral
from the diet. The focus is on the impairments suffered by the system. The fourth criterion
works closely with the third in pinning down the mineral as the sole factor causing the impaired
condition. This criterion has the premise that pathologies wrought by omitting an essential
component are reversible and selective. This puts the focus stronger on the candidate if the
system of component regains normal function is response to the return of the putative missing
mineral.
SUMMARY
Minerals are the most abundant variety of life’s compounds. Although by quantity
minerals may regarded as minor, by sheer numbers and types they supersede organic
components in functions and presence. Other than calcium and phosphorous which appear as
major components of the skeleton, minerals tend to be more in the background of
biomolecules and represent a type of “hidden nutrition”. While their presence may be subtle,
the functions they perform are enormously important to cell and system functions. When
dealing with minerals, however, it is important to take note where they occur, the form they
are in, the vital physiological functions that require their presence. Chemical and physical
properties become the major factors to consider when rationalizing suitability and importance.
Solubility, ionization, binding interactions, all must be taken into account when associating a
functional role with a mineral’s property. Toxic minerals can interfere with normal metabolic
events or cause key components to falter. Establishing essentiality if a goal of components in
food is an important goal in nutrition. The importance of chemical properties in a mineral’s
function and selection will be discussed in subsequent chapters.
E. PROBLEMS
1. For each of the following, determine if it belongs to the class of Macro- (A) or Micro- (I)
minerals.
a.
b.
c.
d.
sodium
phosphorous
manganese
magnesium
e. silicon
f. calcium
g. cobalt
h. potassium
i. selenium
j. chromium
k. nickel
l. molybdenum
2. Based on the definition, offer two reasons why carbon dioxide (CO2) is not considered a
mineral. Why iron metal is not a mineral. Why water is not a mineral.
2. In question 1, distinguish between metal ions and non-metal ions.
3. What three chemical elements comprise the structure of glucose, lactose, and oleic acid.
4. There is a controversy as to whether arsenic should be considered a mineral with nutritional
value. Acquaint yourself with the controversy by reviewing papers on the Pubmed web site.
5. Name a non-organic nitrogen or sulfur compound in a living system. What are their
functions?
6. In the list of important early discoveries of minerals (Table 1.2), how does the work of
Abderhalen link with the studies of Hart et al?