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Steroids and Lipids
2 .1
Chapter 2
Introduction
Steroids form
a group of structurally related compounds which are widely
distributed in animals and plants. Included in steroids are Sterols, vitamin D, bile
acids, a number of sex hormones, the adrenal cortex hormones, some carcinogenic
hydrocarbons, certain sapogenins etc. Sterols are classified into zoo sterols,
phytosterols and mycosterols. The sterols that are obtained from animal sources are
often referred to as zoo sterols. Cholesterol, cholestenol and coprastenol are some
animal sterols. Ergosterol and stigma sterol are principal plant sterols or
phytosterols. Sterols which are obtained from yeast and fungi are referred to as
mycosterols. This classification, however, is not rigid, since some sterols are
obtained from more than one of these groups.
Lipids may be regarded as organic substances relatively insoluble in water,
soluble in organic solvents (alcohol, ether etc.), actually or potentially related to
fatty acids and utilized by the living cells. Lipids are a heterogeneous group of
compounds like triacylglycerols, phospholipids, cholesterol, vitamins (A, D, E and
K) steroid hormones and prostaglandins. As such, lipids are a heterogeneous group
of compounds and, therefore, it is rather difficult to define them precisely. Unlike
the polysaccharides, proteins and nucleic acids, lipids are not polymers. They are
mostly small molecules.
2. 2. Description of steroids
The structure of steroids is based on the 1, 2-cyclopentenophenanthrene
skeleton1. All steroids give, among other products, Diel’s hydrocarbon (3’- methyl1, 2- cyclopentanophenanthrene, molecular formula C18H16) when distilled with
selenium. One major steroid, sterol occurs in animal and plant oils and fats. They are
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Chapter 2
crystalline compounds and contain an alcoholic group. They occur as free or as
esters of higher fatty acids and are isolated from unsaponifiable portion of oils and
fats. A large and important class of terpene-based lipids is the steroids. This
molecular family, whose members affect an amazing array of cellular functions, is
based on a common structural motif of three six-member rings and one five-member
ring all fused together. Cholesterol is the most common steroid in animals and
the precursor for all other animal steroids. The numbering system for cholesterol
applies to all such molecules. Many steroids contain methyl groups at positions 10
and 13 and an 8- to 10- carbon alkyl side chain at position 17. The polyprenyl nature
of this compound is particularly evident in the side chain. Many steroids contain
oxygen at C-3, either a hydroxyl group in sterols or a carbonyl group in other
steroids. It is also noted that the carbons at positions 10 and 13 and the alkyl group
at position 17 are nearly always oriented on the same side of the steroid nucleus, the
β – orientation. Alkyl groups that extend from the other side of the steroid backbone
are α - orientation.
Steroids derived from cholesterol in animal include five families of
hormones (the androgens, estrogens, progestins, glucocorticoids and mineral
corticoids) and bile acids. Androgens such as testosterone and estrogens such as
estuarial, mediate the development of sexual characteristics and sexual function in
animals. The progestins, such as progesterone, participate in the control of the
menstrual cycle and pregnancy. Glucocorticoids (cortical, for example) participate in
the control of carbohydrate, protein, and lipid metabolism, whereas the mineral
corticoids regulate salt (Na, K, and Cl) balances in tissues. The bile acid (including
cholic and dcoxycholic acid) are detergent molecules secreted in bile from the
gallbladder that assist in the adsorption of dietary lipids in the intestine.
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Sterols are structural lipids present in the membranes of most eukaryotic
cells. Biological lipids are a chemically diverse group of compounds, the common
and defining feature of which is their insolubility in water. Lipids are the principal
form of stored energy. Sterols and phospholipids are the major structural lipids of
biological membranes. Other lipids play crucial roles as enzyme cofactors (vitamin
K), electron carriers, light-absorbing pigments (carotene, retinal), and hydrophobic
anchors, emulsifying agents (bile salts), hormones and intracellular messengers.
The triterpene squalene is the precursor of the steroids. There are many
biologically interesting steroids and sterols. Some of them are ergo sterol (yeast), 7dehydrocholesterol or provitamin D, cholic acid (bile), estradiol, testosterone,
cortisone, digitogenin etc. Their characteristic structure is the steroid nucleus
consisting of four fused rings, three with six carbon atoms and one with five.
Cholesterol, the major sterol in animal tissues is amphipathic, with a polar head
group (the hydroxyl group at C-3) and a non-polar hydrocarbon body (the steroid
nucleus and the hydrocarbon side chain at C-17). Similar sterols are found in other
eukaryotes; stigmasterol in plants and ergosterol in fungi. The sterols of all
eukaryotes are synthesized from the simple 5 carbon isoprene unit. The isoprene
units, (given in fig.2.1) that are key intermediates in the pathway from acetate to
CH3 ─ COO─
Acetate
Isoprene unit
CH3
│
CH2 ═ C ─ CH ═ CH2
Figure.2.1. Structure of acetate and isoprene unit
(structure of isoprene unit formed from 3 acetate groups)
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Chapter 2
the sterol, are precursors to many other natural lipids and the mechanism by which
isoprene units are polymerized are similar in all of these pathways.
In addition to their roles as membrane constituents, the sterols serve as
precursors for a variety of products with specific biological activities. Steroid
hormones, for example, are potent biological signals that regulate gene expression.
Bile acids are polar derivatives of cholesterol which act as detergents in the intestine
emulsifying dietary fats to make them more readily accessible to digestive lipase.
2. 3. Structure of Sterols
Steroid means sterol-like. Sterols (Greek, stereos is solid, plus “ol”, the
generic ending for alcohols) are solid alcohols having 27-29 carbons. The larger
designation steroid covers those compounds containing the parent nucleus consisting
of the A, B, C and D rings as shown in figure 2.2. The steroid nucleus is almost
planar and is relatively rigid. The stereochemistry of steroids is due to the ring
juncture and to the constituents located at various positions of the rings 2. The
groups below the plane are called α-oriented and those above the plane β-oriented.
Figure 2.2. Steroid nucleus
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Steroids are the compounds containing a cyclic steroid nucleus (or ring)
namely cyclopentanoperhydrophenanthrene (CPPP). It consists of a phenanthrene
nucleus (rings A, B and C) to which a cyclopentane ring (D) is attached. The main
atomic sites of substitution in the steroid ring system are carbon-3 of ring A, carbon
11 of ring C and carbon 17 of ring D (figure 2.2). The fused rings do not allow
rotation about C-C bonds. The structure and numbering of CPPP are shown in
Fig.2.2. The steroid nucleus represents saturated carbons, unless specifically shown
as double bonds. The methyl side chains attached to carbons 10 and 13 are shown as
single bonds. At carbon 17, steroids usually contain a side chain. There are several
steroids in the biological system. These include cholesterol, bile acids, vitamin D,
sex hormones, adrenocortical hormones, sitosterols, cardiac glycosides and
alkaloids. If the steroid contains one or more hydroxyl groups it is commonly known
as sterol.
2.4.
Lipids
Lipids (Greek: lipos-fat) are of great importance to the body as the chief
concentrated storage form of energy, besides their role in cellular structure and
various other biochemical functions. As such, lipids are a heterogeneous group of
compounds and, therefore, it is rather difficult to define them precisely. Lipids may
be regarded as organic substances relatively insoluble in water, soluble in organic
solvents (alcohol, ether etc) actually or potentially related to fatty acids and utilized
by the living cells. Unlike the polysaccharides, proteins and nucleic acids, lipids are
not polymers. They are mostly small molecules. The structure of lipids is depicted in
fig.2.3.
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Chapter 2
Fig.2.3.
Structure of lipids
2.4.1 Classification of Lipids
In the lipids are included the neutral lipids (fats and oils), the
glycerophosphatides, the sphingolipids, and the isoprenoid lipids. The latter include
a number of substances related by having a common structural unit namely isoprene.
Lipids are broadly classified into Simple lipids, Complex lipids, Derived lipids,
Neutral lipids and miscellaneous lipids.
 Simple lipids
Simple lipids are esters of fatty acids with alcohols. These are mainly of two
types- triacylglycerols and waxes. Triacylglycerols (Fats and oils) are esters of fatty
acids with glycerol. The difference between fat and oil is only physical. Thus, oil is
a liquid while fat is a solid at room temperature. Waxes are esters of fatty acids
(usually long chain) with alcohols other than glycerol. These alcohols may be
aliphatic or alicyclic. Ceryl alcohol is most commonly found in waxes.
Complex lipids
Esters of fatty acids with alcohols containing additional groups such as
phosphate, nitrogenous base, carob-hydrate, protein etc. are called Complex (or
compound) lipids. They are further divided:
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a) Phospholipids: These are Lipids containing phosphoric acid and frequently a
nitrogenous base. This is in addition to alcohol and fatty acids.
(i) Glycerophospholipids: These phospho-lipids contain glycerol as the
alcohol e.g., lecithin,
cephalin.
(ii) Sphingophospholipids: Sphingosine is the alcohol in this group of
Phospho-lipids e.g., sphingomyelin.
b)
Glycolipids: These lipids contain a fatty acid, carbohydrate and nitrogenous
base. The alcohol is Sphingosine; hence they are also called as glycosphingolipids.
Glycerol and phosphate are absent e.g., cerebrosides, gangliosides.
c) Lipoproteins: These are macromolecular complexes of lipids with proteins.
d) Other complex lipids: Sulfolipids, amino-lipids and lipopolysaccharides are
among the other complex lipids.
 Derived lipids:
These are the derivatives obtained on the hydrolysis of group I and group 2
lipids which possess the characteristics of lipids. These include glycerol and other
alcohols, fatty acids, mono- and diacylglycerols, lipid soluble vitamins, steroid
hormones, hydrocarbons and ketone bodies.
 Miscellaneous lipids:
These include a large number of compounds possessing lie characteristics of
lipids e.g., carotenoids, squalene, hydrocarbons such as pentacosane, ternenes etc

Neutral lipids:
The lipids which are uncharged are referred to as neutral lipids. These are
mono-, di-, and triacylglycerols, cholesterol and cholesteryl esters.
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2.4.1.1. Discussion on lipids
Fats and oils belong to the neutral lipids, which are fatty acid esters of
glycerol, the fatty acids being generally those with long chains. The term ‘fat’ is
used in a general way, but it should be reserved for the neutral lipids that are solid at
ambient temperature; oils are liquids.
The acyl groups can be all the same as can differ from one another.
Triglycerides are hydrolyzed by acids and alkalis into glycerol and fatty acids or
alkali salts of fatty acids, respectively. The latter are known as soaps. In most
organisms there are enzymes that catalyze the hydrolysis of fats and oils. The acids
from fats and oils are long, straight-chain acids; some are unsaturated.
The most commonly found fatty acids are stearic acid with 18 carbons) and
its unsaturated analogue, oleic acid. It is interesting that double bonds occur
frequently in the middle of the molecule. In oleic acid, for example, the double bond
is between C 9 and C10.
The melting point of unsaturated fatty acids is lower than the melting point
of corresponding saturated acids. By way of an example, oleic acid – with one
double bond is a liquid at ambient temperature, whereas stearic acid, the saturated
analogue, is solid.
The solubility of fatty acids in water and in organic solvents is dictated by
the length of the hydrocarbon chain, often called ‘the head’. The head, which is
hydrophilic (water-liking), is responsible for the solubility of the acid in water; as
the length of the tail increases, solubility in water decreases. Thus acids with up to
seven carbons are soluble in water, but those with longer chains are increasingly
insoluble. Because the hydrocarbon chain is soluble in lipid solvents, it is called
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lipophiloc (lipid liking). An interesting consequence of this arrangement – a water
soluble head and water – insoluble tail – is the formation of spread monolayer when
long-chain fatty acids come in contact with water surfaces.
2.4.1.2. Triglycerides
Fats and oils are mainly mixed triglycerides, those having more than one
type of fatty acid. The fats being solid at ambient temperature are made up largely of
saturated fatty acids; as the number of double bonds in a triglyceride increase, the
melting point decreases. In the oils, for example, there is a predominance of
triglycerides with unsaturated fatty acids.
Generally, the properties of the triglyceride are an expression of their fatty
acid composition. Two fatty acids are quite frequently found in triglycerides:
palmitic acid and oleic acid. Short chain and very long chain fatty acids are also
found but less frequently. The type of acids making up the triglycerides is usually
related to the source. In animals there is a predominance of saturated triglycerides,
where as in plants there is a relative abundance of unsaturated triglycerides.
The fats that are synthesized by the organism serve as supporting material for
internal organs in animals; they also accumulate in certain regions of the body and
serve as food reserves in addition to their structural role.
The melting point of unsaturated fatty acids is lower than the melting point of
corresponding saturated acids. By way of an example, oleic acid – with one double
bond is a liquid at ambient temperature, whereas stearic acid, the saturated analogue,
is solid.
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The solubility of fatty acids in water and in organic solvents is dictated by
the length of the hydrocarbon chain, often called the head. The head, which is
hydrophilic (water-liking), is responsible for the solubility of the acid in water; as
the length of the tail increases, solubility in water decreases. Thus acids with up to
seven carbons are soluble in water, but those with longer chains are increasingly
insoluble. Because the hydrocarbon chain is soluble in lipid solvents, it is called
lipophiloc (lipid liking).
An interesting consequence of this arrangement – a water soluble head and
water – insoluble tail – is the formation of spread monolayer when long-chain fatty
acids come in contact with water surfaces.
2.4.1.3. Waxes
Waxes are water-insoluble, solid esters of higher fatty acids with long-chain
monohydroxylic fatty alcohols or with sterols. They are soft and pliable when warm
but hard when cold. Waxes are found as protective coatings on skin, fur, and
feathers, on leaves and fruits of higher plants, and on the exoskeleton of many
insects. The major components of beeswax are palmitic acid esters of long-chain
fatty alcohols with 26 to 34 carbon atoms. Lanolin, or wool fat, is a mixture of fatty
acid esters of the sterols lanosterol and agnosterol. Ceryl alcohol is most commonly
found in waxes. The stored fat of marine animal has a high wax component which
forms an energy reserve.
2.4.1.4. Role of phospholipids and sphingolipids
Phospholipids are highly dispersed in the cell, in contrast to the fats, which
are found as deposits of food reserve. As structural cell components the
phospholipids are intimately bound to other substances such as protein as
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carbohydrates. This is manifested in the difficulty with which phospholipids are
extracted from cells with solvents such as ether, in contrast to the fats which are
quite easily extracted by ether. Phospholipids are closely associated with membranes
in the cell. The most popular model of membranes is that proposed for the myelin
sheath covering some nerve fibers. The membrane is visualized as a sandwich of
oriented protein molecules holding the phospholipids molecules, which mentioned
previously, have a tendency to form monolayer. The phospholipids with polar and
non polar groups can orient themselves with their polar heads toward the polar
protein layers, leaving the non polar tails free to extend perpendicularly to the axis
of the protein layers.
2.4.1.5. Isoprenoid lipids
All the compounds classified under this heading are chemically related to the
unsaturated hydrocarbon, isoprene. In the list of isoprenoid lipids are included a
large number of biologically important substances: cholesterol, bile acids, several
vitamins, sex hormones, carotene and many more.
Isoprenoid lipids are chemically unrelated to the fats and phosphatides, but
they have the same solubility properties. When living material is extracted with a
mixture of ether and alcohol, for example, the extract obtained is a mixture of all
lipids originally present. The fats and phosphatides can be hydrolyzed with alkali to
yield water-soluble substances, whereas the water-insoluble isoprenoid lipids remain
unchanged. They can be extracted from the mixture with ether. Because they do not
undergo alkali hydrolysis, they are often called the non saponifiable fraction.
Saponification is the name given to the alkali hydrolysis of fats.
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2.4.1.6. Steroids
Steroids are biogenetically related to the unsaturated aliphatic hydrocarbon
squalene, which is an isoprenoid. Chemically steroids are derivatives of perhydro-1,
2-cyclopentanophenanthrane, which is a saturated phenanthrene system with an
additional five member ring, commonly called sterane, as in figure 2.4. A more
direct relationship is envisaged between some steroids and other related cyclic
hydrocarbons; for example, cholesterol is regarded as a derivative of cholestane.
Figure.2.4.
Phenanthrene and sterane structure
Isomerism resulting from the type of ring fusion is nearly impossible to
represent except by means of models, but it is important to be acquainted with the
notation frequently used to symbolize the different isomers. For example, when ring
A joins ring B, two possible forms can result: a transform or a cis form. Thus the
hydrogen’s or substituents can be on opposite sides (trans) or on the same side (cis)
of a plane. The accepted convention is that, if hydrogen or any substituent is
regarded above the plane (above this page, for example), it is represented by a solid
line joining it to the carbon and designated . If it is below the plane of the page, a
dash is used and designated . In steroid chemistry two references are often used:
5 androstane and 5 – androstane and is shown in figure 2.5.
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Chapter 2
figure.2.5. 5 androstane and 5 androstane.
Other substituents are referred to the methyl group: if they are on the same
plane they are , and if they are on opposite sides of the plane they are . In the
above example, the –0H group joined to the ring by a solid line is cis to the methyl
group and both are above the plane of the page: the designation therefore is 3-OH.
In the 3 isomer the OH is joined by a dash, indicating that it is oriented trans with
respect to the methyl group.
2.4.1.7. Sterols
Sterols
are
hydroxylated
derivatives
of
the
perhydro-1,
2-
cyclopentanophenanthrene nucleus. They possess a hydroxyl group at C3, a side
chain at C17 and a double bond, mostly at C5. They occur in plants, animals, and
microorganisms, the best known animal sterol is cholesterol. The structure of
cholesterol was given previously without considering isomerism. Because there is a
double bond at C5, the isomerism resulting from the fusion of rings A and B is
abolished. The position of the hydroxyl group at C3, however, is cis to the angular
methyl group at C10; cholesterol is a 3 - hydroxysteroid, and so are all sterols so
far found in nature.
When cholesterol is reduced, it gives rise to two isomeric structures:
cholestanol and coprostanol. Both retain the 3 - configuration of the hydroxyl
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Chapter 2
group, but in cholestanol rings A and B are fused in the trans manner in coprostanol
in the cis manner (figure 2.6).
Figure.2.6. cholestanol and coprostanol
Rings A and B are fused cis as fused Trans, not only in the products just
discussed but in other steroids. Rings B and C and C and D are in all cases fused
Trans. Cholesterol is synthesized by the cell from simple molecules and serves as a
precursor for the biosynthesis of bile acids, sex hormones, adrenocarticoids, and
vitamin D. plants do not have cholesterol; instead of cholesterol they have a variety
of closely related sterols, the phytosterols (figure 2.7). Best known phytosterols are
stigmasterol, from soy bean oil; -sitosterol, from wheat germ; and ergosterol from
yeast.
Figure.2.7.Plant sterols
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Chapter 2
Ergosterol is converted by irradiation with ultraviolet light into calciferal or
vitamin D2 (figure 2.8).
Figure.2.8 Formation of vitamin D
2.4.1.8. Bile Acids
Bile contains, among its many components, the bile salts, which are amides
derived from bile acids and the amino acids glycine and taurine. The bile acids
(figure 2.9) possess hydroxyl groups in positions 3, 7 or 12 of the steroid ring
system and are -oriented. The A and B rings are cis fused. The side chain at C17 is
invariably a five carbon chain.
Figure.2.9. Bile acids
The amides, taurocholic and glycocholic acids, are excellent emulsifying
agents, as they possess a non polar structure and a charged side chain. Their main
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Chapter 2
role in the organism is to emulsify fats during digestion and facilitate their
absorption and enzymatic breakdown.
2.4.1.9. Sex Hormones
Two chemically different types are distinguished: the estrogens (figure 2.10),
which have some aromatic character, and progesterone and androgens, which do not.
The estrogens and progesterone are commonly known as female sex hormones and
the androgens as male sex hormones. Of the estrogens, estradiol is produced in the
ovaries and estrone and estrid formed as a result of enzymatic transformations of
estradiol:
Figure.2.10 Estrogens
The hydroxyl group at C3 has phenolic character: it is weakly acidic and can
form salts. A consequence of the aromatization of ring A is the disappearance of the
angular methyl group at C10. Progesterone, which is also produced in the ovaries, is
related more closely to the androgens (figure 2.11).
Figure.2.11. Androgens
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Testosterone is produced in the testes and biochemically transformed to
androsterone, which is the product usually found in urine.
2.4.1.10. Corticosteroids
The adrenal cortex of most animal produces a large variety of steroids, but
only 7 of the 30 compounds isolated are related to the function of the adrenal cortex.
One characteristic of corticosteroids (figure 2.12) is that all have 21 carbon atoms
and invariably C3 is oxidized to a ketone. Another distinguishing feature is the
hydroxyl or keto group at C11.
Figure.2. 12. Corticosteroids
The functions of corticosteroids are quite varied. They stimulate formation
and deposition of glycogen, particularly in the liver; they induce the retention of
sodium ions, thus maintaining the proper balance of electrolytes in the organism;
and they are anti-inflammatory. Not all of the seven corticosteroids possess all there
qualities nor do they possess a given quality to the same degree.
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Chapter 2
2.4.2. Functions of lipids
Lipids perform several important functions
 They are the concentrated fuel reserve of the body.
 Lipids are the constituents of membrane structure and regulate the
membrane permeability (phospholipids and cholesterol).
 They serve as a source of fat soluble vitamins (A, D, E and K).
 Lipids are important as cellular metabolic regulators (steroid
hormones and prostaglandins).
 Lipids protect the internal organs, serve as insulating materials and
give shape and smooth appearance to the body.
 As compounds of the inner mitochondrial membranes, lipids
(phospholipids) participate in electron transport chain.
A very large number of lipids have been shown to be derived from the
polymerization of Δ3-isopentyl pyrophosphate which, in turn, is formed from
acetate.
Figure 2.13 illustrates the compounds derived from Δ3-isopentyl
pyrophosphate. Isopentyl pyrophosphate which is derived form acetyl-CoA, is the
activated building-block precursor of many important bimolecular that contain
isoprene units. They include vitamins A, E and K; the corticoids; rubber; guttapercha; the phytol side chain of chlorophyll; many essential oils, such as the fragrant
principles of lemon oil, eucalyptus, and musk; as well as the hydrocarbons found in
turpentine.
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Chapter 2
Figure 2.13. Biocompounds derived from isopentenyl pyrophosphate
Fatty acids are carboxylic acids with hydrocarbon side chain. They are
the simplest form of lipids. Fatty acids mainly occur in the etherified form as major
constituents of various lipids. They are also present as free (unesterified) fatty acids.
Fatty acids of animal origin are much simpler in structure in contrast to those of
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Chapter 2
plant origin which often contain groups such as epoxy, keto, hydroxyl and cyclopentane rings. Most of the fatty acids that
occur in natural lipids are of even carbons (usually 14C-20C). This is due to the fact
that biosynthesis of fatty acids mainly occurs with the sequential addition of 2
carbon units. Palmitic acid (16C) and stearic acid (18C) are the most common.
Among the odd chain fatty acids, propionic acid (3C) and valeric acid (5C) are well
known.
Saturated fatty acids do not contain double bonds, while unsaturated fatty
acids contain one or more double bonds. Both saturated and unsaturated fatty acids
almost equally occur in the natural lipids. Fatty acids with one double bond are
known as monounsaturated and those with 2 or more double bonds are collectively
known as poly unsaturated fatty ands (PUFA). Lipids and steroids play an important
role in the transport of cholesterol in human body. Cholesterol is present in the
plasma lipoproteins in two forms. About 70-75% of it is in esterified form with long
chain fatty acids. And about 25-30% as free cholesterol. This form of cholesterol
readily exchanges between different lipoproteins and also with the cell membranes.
High density lipoproteins (HDL) and the enzyme lecithin-cholesterol acyl
transferase (LCAT) are responsible for the transport and elimination of cholesterol
from the body. In healthy individuals, the total plasma cholesterol is in the range of
150-250 mg/dl. In the new born, it is less than 100mg/dl and rises to about 150mg/dl
within a year. Women have relatively lower plasma cholesterol which is attributed to
the hormones -- estrogens.
Cholesterol level increases with increasing age and
also during pregnancy. Plasma cholesterol is associated with different lipoprotein
fractions. Cholesterol biosynthesis is controlled by the rate limiting enzyme HMG
CoA reductase, at the beginning of the pathway 4-11.
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HMG CoA reductase is found in association with endoplasmic reticulum and
is subjected to different metabolic controls. The end product cholesterol controls its
own synthesis by a feedback mechanism. Increase in the cellular concentration of
cholesterol reduces the synthesis of the enzyme HMG CoA reductase. These
discussions show that a thorough understanding of lipids is necessary for controlling
the growth of cholesterol crystals.
2.5. Bibliography
1.
Fieser ,L.M.and M.Fieser, “Steroids”, Reinhold Publishing Corporation,
New York (1959)
2.
Deuel, J.,R.,H.J., “The lipids I and II”, Interscience Publishers, Inc., New
York,(1951)
3.
I .L .Finar, “Organic Chemistry. Vol. II” , Chapter11;Longman Group (1975)
4.
Edited by Fine Gold ., “Cholesterol in Membrane Models” , CRC Press ,
Boca Raton , FL (1993)
5.
Voet. D. and Voet. J .G ., “Biochemistry” 2nd Edn , John Wiley , New
York(1995)
6.
Lange’s Handbook of Chemistry; pg 239
7.
Flynn, G. L., Y. Shah, S. Prakongpan, K. H. Kwan., A.F. Hoffmann.,
“J.Pharm . Sci”., 68: 1090(1979)
8.
Manjiro, N.,T. Mintsinovu, S. Ukari, A. Takaki, O. Chiyo, “Lipids” 23:439
(1988)
9.
Burton, W. K., Cabrera, N. and Frank, F. C. “Phil. Trans. Roy.
Soc.”, 243:299(1951)
10. David L. Nelson, Michael M. Cox, “Lehninger Principles of Biochemistry”,
3rd Edn (2001)
11.
Satyanarayana. U, “Biochemistry”, Books and allied (P) ltd., (2002)
*****
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