Chap. 10B. Lipids
• Storage Lipids
• Structural Lipids in Membranes
• Lipids as Signals, Cofactors, and Pigments
• Working with Lipids
Fig. 10-4a. Fat droplets in human
adipose tissue cells.
Lipids as Signals, Cofactors, and Pigments
The two classes of lipids considered in the Chap. 10A file (storage
lipids and structural lipids) are major cellular components.
Membrane lipids make up 5% to 10% of the dry mass of most cells,
and storage lipids can make up more than 80% of the mass of an
adipocyte. With some exceptions (phosphatidylinositols and
sphingosine derivatives), these lipids play a passive role in the cell.
For example, lipid fuels are simply stored until oxidized by
enzymes, and membrane lipids mostly form impermeable barriers
around cells and cellular compartments. (Phosphatidylinositols and
sphingosine derivatives are also involved in several signal
transduction processes in cells). We now will discuss another group
of lipids, present in much smaller amounts, that have active roles in
cell physiology as metabolites and messengers.
Eicosanoids (I)
Eicosanoids (Fig. 10-18) are paracrine hormones, substances that
act only on cells near the point of hormone synthesis instead of
being transported in the blood to act on cells in distant tissues or
organs. These fatty acid derivatives have a variety of effects on
vertebrate tissues. They are involved in reproductive function; in
the inflammation, fever, and pain associated with injury or disease;
in the formation of blood clots and the regulation of blood pressure;
in gastric acid secretion; and in various other processes important in
human health or disease. All eicosanoids are derived from
arachidonic acid [20:4(∆5,8,11,14)], the 20-carbon polyunsaturated
fatty acid from which they take their general name (Greek eikosi,
“twenty”). There are three classes of eicosanoids: prostaglandins,
thromboxanes, and leukotrienes.
Eicosanoids (II)
Prostaglandins contain a five-carbon ring originating from the chain
of arachidonic acid (Fig. 10-18). Their name derives from the
prostate gland, the tissue from which they were first isolated.
Prostaglandins have a wide array of functions, including elevation of
body temperature (fever) and causing inflammation and pain.
Thromboxanes have a six-membered ring containing an ether. They
are produced by platelets (also called thrombocytes) and act in the
formation of blood clots and the reduction of blood flow to the site
of a clot. The nonsteroidal antiinflammatory drugs (NSAIDS), such
as aspirin and ibuprofen, inhibit the COX enzyme, which catalyzes
an early step in the pathway from arachidonate to prostaglandins
and thromboxanes. Leukotrienes, first found in leukocytes, contain
three conjugated double bonds. Leukotriene D4, derived from
leukotriene A4, induces contraction of the smooth muscle lining the
airways of the lung. Overproduction of leukotrienes occurs in asthma
and anaphylactic shock. The steroid drug prednisone, for example,
inhibits the synthesis of prostaglandins, thromboxanes, and
leukotrienes by blocking the release of arachidonic acid from
membrane lipids by phospholipase A2.
Steroid Hormones (I)
Steroids are oxidized derivatives of sterols. They have the sterol
nucleus but lack the alkyl chain attached to ring D of cholesterol,
and they are more polar than cholesterol (Fig. 10-19). Steroid
hormones are transported through the bloodstream on protein
carriers from their sites of synthesis to target tissues, where they
enter cells. On entering cells, they move to the nucleus where they
bind to specific receptor proteins
that bind DNA and modulate gene
expression and thus metabolism.
The major groups of steroid
hormones are the male and
female sex hormones (e.g.,
testosterone and ß-estradiol),
and the adrenal steroids
produced by the adrenal cortex
(e.g., cortisol and aldosterone).
Cortisol mediates the stress
response and glucose metabolism,
while aldosterone regulates salt
excretion by the kidney.
Steroid Hormones (II)
The synthetic steroids,
prednisone and prednisolone have
potent antiinflammatory activity.
As noted above they inhibit
synthesis of prostaglandins,
thromboxanes, and leukotrienes.
Prednisone and prednisolone
mimic the natural
antiinflammatory activity of
cortisol and are prescribed for
asthma and rheumatoid arthritis,
among other disorders. Lastly,
the plant steroid, brassinolide, is
a potent growth regulator that
increases the rate of stem
elongation and affects the
orientation of cellulose
microfibrils in the cell wall during
growth.
Other Plant Signaling Lipids
Plants produce thousands of different lipophilic compounds, volatile
substances used to attract pollinators, to repel herbivores, to
attract organisms that defend the plant against herbivores, and to
communicate with other plants. Jasmonate, for example (see Fig.
12-33) derived from -linolenic acid [18:3(∆9,12,15)] in membrane
lipids, triggers a plant’s defense systems in response to insectinflicted damage. The methyl ester of jasmonate is responsible for
the characteristic fragrance of jasmine oil, which is used in the
perfume industry. Many plant volatiles are derived from fatty acids,
or from compounds made by the condensation of five-carbon
isoprene units (see below). These include geraniol (the characteristic
scent of geraniums), ß-pinene (pine trees), limonene (limes),
menthol, and carvone (spearmint), to name but a few.
Vitamin D
The fat-soluble vitamins, A, D, E, and K, are all derived from
isoprene units. Vitamins A and D are precursors of hormones.
Vitamin D3, also called cholecalciferol, is normally formed in the
skin from 7-dehydrocholesterol in a photochemical reaction driven
by UV light absorption (Fig. 10-20). Vitamin D3 itself is not
biologically active, but is converted by enzymes in the liver and
kidney to 1,25-dihydroxyvitamin D3 (calcitriol). Calcitriol is a
hormone that regulates calcium uptake in the intestine and calcium
levels in kidney and bone. The deficiency of vitamin D leads to
defective bone formation and the disease rickets, for which
administration of vitamin D produces a dramatic cure. Vitamin D2
(ergocalciferol) is a commercial product formed by UV irradiation of
ergosterol from yeast, which resembles vitamin D3. It is further
processed to a calcitriol-like active hormone. Vitamin D2 is added
to milk and butter as a dietary supplement. Calcitriol regulates
gene expressing by interacting with specific nuclear receptor
proteins.
Vitamin A
Vitamin A1 (retinol) derivatives function as a hormone and as the
visual pigment of the vertebrate eye (Fig. 10-21). The vitamin A
derivative, retinoic acid, is a hormone that regulates gene expression
by binding to nuclear receptor proteins. Retinoic acid is required for
the development of epithelial tissues, including the skin. It also is
the active ingredient in the drug tretinoin (Retin-A) used in the
treatment of acne and wrinkled skin. Retinal, another vitamin A
derivative, is the pigment that initiates the response of rod and
cone cells of the retina to light, producing a neuronal signal to the
brain. Good sources of vitamin A are fish liver oils, liver, eggs,
whole milk, and butter. In vertebrates, ß-carotene, the pigment
that gives carrots, sweet
potatoes, and other yellow
vegetables their characteristic
color, can be enzymatically
converted to vitamin A1 (Fig. 1021). Deficiency of vitamin A
leads to dryness of the skin,
eyes, and mucous membranes;
retarded development and
growth; and night blindness, an
early symptom commonly used in
diagnosing a vitamin A deficiency.
Other Isoprenoids (I)
Vitamin E is the collective name for a group of closely related lipids
called tocopherols, all of which contain a substituted aromatic ring
and a long isoprenoid side chain (Fig. 10-22a). As hydrophobic
molecules, tocopherols associate with cell membranes, lipid
deposits, and lipoproteins in the blood where they react with and
destroy oxygen radicals and other free radicals that would
otherwise react with and damage unsaturated fatty acids in
membrane lipids. Good sources of vitamin E include vegetable oils,
eggs, and wheat germ. Laboratory animals fed vitamin E-depleted
diets develop scaly skin, muscular weakness and wasting, and
sterility. In humans, vitamin E deficiency is rare, and the principal
symptom is fragile red blood cells.
Other Isoprenoids (II)
Vitamin K (Fig. 10-22b) is a cofactor that is required for the
synthesis of prothrombin, a blood protein essential in blood
coagulation. Vitamin K deficiency slows blood clotting and therefore
can be fatal. Vitamin K1 (phylloquinone) is present in green leafy
vegetables. A related molecule that is biologically active, vitamin K2
(menaquinone) is formed by bacteria living in the intestine of
vertebrates. The drug warfarin (Fig. 10-22c) is a synthetic
compound that inhibits the synthesis of active prothrombin by
competing with vitamin K for binding to the enzyme that modifies
prothrombin. Warfarin is an important anticoagulant drug which is
used to treat patients prone to thromboses. It also is used as a rat
poison, causing death by internal bleeding.
Other Isoprenoids (III)
Ubiquinone (also called coenzyme Q) and plastoquinone (Fig. 1022d,e) are isoprenoids that function as lipophilic electron carriers
in oxidation-reduction reactions used for ATP synthesis in
mitochondria and chloroplasts, respectively. Both molecules can
carry either one or two electrons and either one or two protons
(see Fig. 19-3). Dolichols (Fig. 10-22f) carry the sugar units that
are added to glycoproteins and glycolipids during their synthesis.
Hydrophobic dolichol molecules are anchored to the membrane
where these sugar-transfer reactions take place.
Natural Pigments
Many natural pigments are lipidic conjugated dienes (Fig. 10-23).
Conjugated dienes have carbon chains with alternating single and
double bonds. Because this structural arrangement allows the
delocalization of electrons, the compounds can be excited by lowenergy (visible) light, giving them colors that are visible to humans
and other animals. Subtle differences in the chemistry of these
compounds produce pigments of strikingly different colors. Birds
acquire the pigments that color their feathers red or yellow by
eating plant materials that contain carotenoid pigments, such as
canthaxanthin and zeaxanthin. The differences in pigmentation
between male and female birds are the result of differences in
intestinal uptake and processing of carotenoids. Like sterols,
steroids, dolichols, fat soluble vitamins, ubiquinone, and
plastoquinone, these pigments are synthesized from five-carbon
isoprene derivatives.
Polyketides
Polyketides are diverse natural products with potent biological
activities. They are lipids and are made via biosynthetic pathways
via reactions (Claisen condensations) similar to those used for
synthesis of fatty acids. Polyketides are secondary metabolites,
compounds that are not central to an organism’s metabolism, but
that serve some subsidiary function that gives their producers an
advantage in some ecological niche. Some polyketides used in
medicine are shown in Fig. 10-24. Erythromycin is an antibiotic,
amphotericin B is an antifungal, and lovastatin is an inhibitor of
cholesterol synthesis prescribed to decrease one’s risk of
cardiovascular disease.
Lipid Methods (I)
Because lipids are insoluble in water, their extraction and subsequent
fractionation require the use of organic solvents and some techniques
not commonly used in the purification of water-soluble molecules
such as proteins and carbohydrates. In general, complex mixtures of
lipids are separated by differences in polarity or solubility in
nonpolar solvents. An overview of methods used to isolate and
identify lipids is presented in Fig. 10-25.
Neutral lipids (triacylglycerols, waxes,
pigments, etc.) are readily extracted from
tissues with ethyl ether, chloroform, or
benzene, solvents that do not permit lipid
clustering driven by hydrophobic
interactions. Membrane lipids are more
effectively extracted by more polar organic
solvents, such as ethanol or methanol, which
reduce the hydrophobic interactions between
lipid molecules while also weakening the
hydrogen bonds and electrostatic
interactions that bind membrane lipids to
membrane proteins. A commonly used
extractant is chloroform, methanol, and
water (Fig. 10-25a). After extraction, the
lipids remain in the denser chloroform
phase, while proteins and sugars partition
into the upper methanol/water layer.
Lipid Methods (II)
Major classes of extracted lipids in the
chloroform phase may first be separated by
thin-layer chromatography, or by adsorption
chromatography (Fig. 10-25b). In thin-layer
chromatography, lipids are carried up a
silica gel-coated plate by a rising solvent
front. Less polar lipids travel farther up the
plate than do more polar or charged lipids.
Lipid bands can be visualized by a number of
stains, such as iodine vapor, which binds
reversibly to double bonds in unsaturated
fatty acids. The region of the silica gel
containing the lipids can be scraped from
the plates, the lipid eluted in organic
solvent, and mass spectrometry or other
methods can be used to identify it and its
component fatty acids (Fig. 10-26, not
covered). Adsorption chromatography on
columns of silica gel, through which solvents
of increasing polarity are passed, can also
be used to fractionate lipids. Closely related
lipid species such as phosphatidylcholine and
phosphatidylinositol, can be separated by
these techniques.
Lipid Methods (III)
As an alternative to the above methods
of analysis, a “shotgun” approach can be
used in which a sample of extracted,
unfractionated lipids is directly
subjected to high-resolution mass
spectrometry of different types and
under different conditions to determine
the total composition of all the lipids:
the lipidome (Fig. 10-25c). The lipidome
of a cell or tissue changes during
differentiation, diseases such as cancer,
and during drug treatment.