evening primrose, borage and blackcurrant have some commercial importance. Evening primrose oil contains about
10% GLA, and is widely used both as a nutraceutical and a
11-cis,14- cis-Eicosadienoic acid (20:2(n-6)) is a common
minor component of animal tissues. 8-cis,11-cis,14-cis-Eico-
satrienoic acid (dihomo-γ-linolenic acid or 20:3(n-6)) is the
immediate precursor of arachidonic acid, and of a family
of eicosanoids (PG1 prostaglandins). However, it does not
accumulate to a significant extent in animal tissue lipids,
and is typically about 1-2% of the phospholipid fatty acids.
arachidonic acid (5-cis,8-cis,11- cis,14-cis-eicosatetrae-
noic acid or 20:4(n-6)) is the most important metabolite of
linoleic acid in animal tissues, both in quantitative and bio-
logical terms. It is often the most abundant polyunsaturated
component of the phospholipids, and can com-
prise as much as 40% of the fatty acids of phos-
phatidylinositol. As such, it has an obvious role in
regulating the physical properties of membranes,
but the free acid is also involved in the mechanism
by which apoptosis is regulated (Fig 2).
Meat is the main dietary source in humans.
While arachidonate is present in all fish oils, polyunsaturated fatty acids of the (n-3) families tend to
be present in much larger amounts. Arachidonic
acid is frequently found as a constituent of mosses, liverworts, and ferns, but there appears to be only one definitive
report of its occurrence in a higher plant (Agathis robusta).
The fungus Mortierella alpina is a commercial source or
arachidonate via a fermentation process.
Several families of eicosanoids are derived from arachidonate, including prostaglandins (PG2 series), thrombox-anes, leukotrienes, and lipoxins, with phosphatidylinositol
being the primary source. These have an enormous range of
essential biological functions that are discussed in elsewhere
in these web pages. In addition, 2-arachidonoylglycerol and
anandamide N-arachidonoylethanolamine) have important
biological properties as endocannabinoids, although they
are minor lipids in tissues in quantitative terms.
4,7,10,13,16-Docosapentaenoic acid (22:5(n-6)) is usually a relatively minor component of animal lipids, but it is
the main C22 polyunsaturated fatty acid in the phospholipids
of testes. It can amount to 70% of the lysobisphosphatidic
acid in this tissue, for example. In this instance, C22 fatty
acids of the (n-3) family are present at relatively low levels,
in contrast to most other reproductive tissues.
Other fatty acids of the (n-6) family that are found in
animal tissues include 22:3(n-6) and 22:4(n-6). The last of
these, 7,10,13,16-docosatetraenoic or adrenic acid, is a
significant component of the phospholipids of the adrenal
glands and of testes. Tetra- and pentaenoic fatty acids of
the (n-6) family from C24 to C30 have been found in testes,
and even longer homologues occur in retina. Those in testes are known to be essential for male fertility and sperm
maturation. Very-long-chain fatty acids of this type were
first reported from human brain in patients with the rare
inherited disorder, Zellweger’s syndrome, but it is now established that such fatty acids with up to 38 carbon atoms and
with from 3 to 6 methylene-interrupted double bonds are
present at low levels the brain of normal young humans,
with 34:4(n-6) and 34:5(n-6) tending to predominate. The
function of these is not known.
The most highly unsaturated fatty acid of the (n-6)
family to have been characterized are 28:7(n-6) (4,7,10,13,
16,19,22-octacosaheptaenoate), which has been found
in the lipids of marine dinoflagellates and herring muscle,
and 4,7,10,13,16,19,22,25,28-tetratriacontanonaenoic acid
(34:9(n-6)) from the freshwater crustacean species
Bathy-nella natans. n
FIG. 2. Arachidonic acid