Chap. 10B. Lipids

1
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.
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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.
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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
204(?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.
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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.
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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.
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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.
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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 183(?9,12,15) in
membrane lipids, triggers a plants defense
systems in response to insect-inflicted 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.
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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.
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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.
10-21). 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.
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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.
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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.
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Other Isoprenoids (III)
Ubiquinone (also called coenzyme Q) and
plastoquinone (Fig. 10-22d,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.
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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 low-energy (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.
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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 organisms 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 ones risk of
cardiovascular disease.
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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.
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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.
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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.