the lipids of all higher organisms contain
appreciable quantities of polyunsaturated
fatty acids (PuFa) with methylene-interrupted
double bonds, i.e. with two or more double
bonds of the cis-configuration separated by
a single methylene group. the term ‘
homo-allylic’ is occasionally used to describe this
In higher plants, the number of double bonds in fatty acids
only rarely exceeds three, but in algae and animals there can
be up to six. Two principal families of polyunsaturated fatty
acids occur in nature that are derived biosynthetically from
linoleic (9-cis,12-cis-octadecadienoic) and α-linolenic (9-cis,12-
cis,15- cis-octadecatrienoic) acids (Fig. 1).
In the shorthand nomenclature, these are designated
9c,12c-18:2 and 9c,12c,15c-18:3 respectively. The number
before the colon specifies the number of carbon atoms, and
that after the colon, the number of double bonds. The position of the terminal double bond can be denoted in the form
(n-x), where n is the chain-length of the fatty acid and x is the
number of carbon atoms from the last double bond, assuming
that all the other double bonds are methylene-interrupted.
Thus linoleate and α-linolenate are 18:2(n-6) and 18:3(n-3),
respectively (18:2ω6 and 18:3ω3 in the older literature).
Both of the parent fatty acids can be synthesized in plants,
but not in animal tissues, and they
are therefore essential dietary components. Polyunsaturated fatty acids
can be found in most lipid classes,
but they are especially important as
constituents of the phospholipids,
where they appear to confer distinctive properties to the membranes, in
particular by decreasing their rigidity. The exception is the sphingolip-ids, where they are rarely detected
in other than trace amounts.
The (n-6) family
Linoleic acid is a ubiquitous component of plant lipids, and
of all the seed oils of commercial importance. For example,
corn, sunflower, and soybean oils usually contain over 50%
of linoleate, and safflower oil contains up to 75%. Although
all the linoleate in animal tissues must be acquired from the
diet, it is usually the most abundant dienoic fatty acid in mammals (and in most lipid classes) typically at levels of 15 to 25%,
although it can amount to as much as 75% of the total fatty
acids of heart cardiolipin. It is also a significant component of
fish oils, although fatty acids of the (n-3) family tend to predominate in this instance.
Analogues of linoleic acid with trans-double bonds are
occasionally found in seed oils. For example, 9c,12t-18:2 is
reported from Dimorphotheca and Crepis species, and 9t,12t-
18:2 is found in Chilopsis linearis.
The remaining members of the (n-6) family of fatty acids
are synthesized from linoleate in animal and plant tissues
by a sequence of elongation and desaturation reactions as
described below. The intermediates can function as essential
fatty acids also. Shorter-chain components may be produced
by alpha or beta-oxidation.
γ-Linolenic acid (‘GLA’ or 6-cis,9- cis,12-cis
-octadecatrienoic acid or 18:3(n-6)) is usually a minor component of animal
tissues in quantitative terms (< 1%), as it is rapidly converted to
higher metabolites. It is found in a few seed oils, and those of
Lipid Snippets is a regular Inform column that features select content
from The AOCS Lipid Library ( http://lipidlibrary.aocs.org/).
the (n-6) Family of
Polyunsaturated Fatty acids
FIG. 1. Linoleic and α-linolenic acids
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