The enzymes for elongation and
desaturation are found in the microsomal
fraction of cells. The distribution of
enzyme activity varies among organs of
animals, with the greatest activity in the
liver and the adrenal glands and only
limited activity in tissues such as the heart,
kidneys and brain (Bézard et al., 1994).
One function of the essential
unsaturated fatty acids is in maintaining
the appropriate fluidity of cell membranes
(Sardesai, 1992). Because unsaturated fatty
acids have lower melting points than
saturated fatty acids, their presence in
membranes makes the membranes more
fluid. Changes in the fluidity of cell
membranes can affect the degree to which
integral membrane proteins such as
receptors are assembled into and diffuse
laterally in the membrane. Such changes
can also affect the activity of membrane-
associated enzymes, change the expression
and function of receptors and alter the
transport of molecules across the
membrane.
While the biochemical basis of the
requirement for linoleic acid and its
metabolites in animal metabolism has been
relatively well understood for some time,
functions of the -3 family of fatty acids
have been harder to delineate (Sardesai,
1992). Docosahexaenoic acid is a com-
ponent of lipids in the grey matter of the
cerebral cortex of the brain and of the
photoreceptor membranes of the rod outer
segment of the retina. As a result, depriva-
tion of linolenic acid in pregnant female
rats leads to impairments in cognitive func-
tion, learning and vision in the rat pups.
Thus, the presence of adequate linolenic
acid during fetal and neonatal develop-
ment is critical.
Arachidonic acid in cell membrane
phospholipids can be released in response
to various signals that activate phospho-
lipase activity. Arachidonic acid can then
become a substrate for conversion to the
eicosanoids of the so-called series 2 type
(prostaglandin E 2 , thromboxane A 2 ,
12-hydroxyeicosatetraenoic acid and
leukotriene B 4 ), which have strong pro-
inflammatory and pro-aggregatory actions.
In contrast, the series 3 products pro-
duced from eicosapentaenoic acid114 J.K. Drackley
ω-6 Series
LA 18:2 ω-6
ω-3 Series
GLA 18:3 ω-6
20:3 ω-6
AA 20:4 ω-6
22:4 ω-6
22:5 ω-6
∆-6-Desaturase
∆-5-Desaturase
Elongase
∆-4-Desaturase
Elongase
18:3 ω-3 ALA
18:4 ω-3
20:4 ω-3
20:5 ω-3 EPA
22:5 ω-3
22:6 ω-3 DHA
Fig. 5.4.Metabolism of essential fatty acids of the -6 (linoleic) and -3 (linolenic) series. LA, linoleic acid;
GLA, -linolenic acid; AA, arachidonic acid; ALA, -linolenic acid; EPA, eicosapentaenoic acid; DHA,
docosahexaenoic acid.