Lipids I:
Fatty Acids and Eicosanoids
Lipids (or fats) are a heterogeneous group of organic com-
pounds defined by their solubility in nonpolar solvents
such as chloroform, ether, and benzene and by their poor
solubility in water.
Lipids may be polar or nonpolar (amphipathic). Po-
lar lipids have limited solubility in water because they
are amphipathic, i.e., they possess hydrophilic and hydro-
phobic regions in the same molecule. Major polar lipids
fatty acids, cholesterol, glycerophosphatides,
Very short chain fatty acids and
ketone bodies are readily soluble in water. Nonpolar lipids
serve principally as storage and transport forms of lipid
and include
(also called triglycerides)
cholesteryl esters.
Lipids have numerous functions including the follow-
ing: thermal insulation, energy storage (as triacylglyc-
erol), metabolic fuels, membrane components (phospho-
lipids and cholesterol; Chapter 10), hormones (steroids
and vitamin D metabolites; Chapters 32 and 37, re-
spectively), precursors of
(discussed on
p. 391) and
(vitamins A, C, D, E, and K;
Chapters 36-38), emulsifying agents in the digestion and
absorption of lipids (bile acids; Chapters 12 and 19),
and surfactants in the alveolar membrane (phosphatidyl-
choline; Chapter 19). The metabolism of fatty acids (satu-
rated and unsaturated) is discussed in this chapter. The
metabolism of phospholipids, glycosphingolipids, and
cholesterol is considered in Chapter 19.
Fatty acids that contain no carbon-carbon double bonds
are known as saturated and those with carbon-carbon dou-
ble bonds as unsaturated. Fatty acids that contain an even
number of carbon atoms and are acyclic, unbranched, non-
hydroxylated, and monocarboxylic make up the largest
group. The most abundant saturated fatty acids in-animals
are palmitic and stearic acids (Table 18-1). The melting
point of fatty acids rises with increase in chain length,
the even-numbered saturated fatty acids having higher
melting points than the odd-numbered. Among the even-
numbered, the presence of cis double bonds lowers the
melting point significantly. Free fatty acids at physiolog-
ical pH are ionized (pK ~4.85) and exist only in small
quantities; in plasma, they typically are bound to albumin.
They are usually present as esters or amides.
Digestion and absorption of lipids are discussed in
Chapter 12. The Western diet contains about 40% fat,
mostly as triacylglycerol (100-150 g/day). Triacylglyc-
erols packaged as chylomicrons in the intestinal epithelial
cell are delivered to the blood circulation via the lymphatic
system and are hydrolyzed to glycerol and fatty acids by
endothelial lipoprotein lipase. Fatty acids are taken up
by the cells of the tissue where the hydrolysis occurs,
whereas glycerol is metabolized in the liver and kidney
(Chapter 15). Another means of triacylglycerol transport
is very-low-density lipoprotein (VLDL), which is synthe-
sized in the liver. Its triacylglycerol is also hydrolyzed by
endothelial lipoprotein lipase. The metabolism of plasma
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