Fatty acid nomenclature by a9342032


									                                                                                                 SAMT    DEEL 67   20 APRIL 1985     633

Fatty acid nomenclature
A short review

B. C. DAVIDSON,                    R. C. CANTRILL

                                                                      The structural system
                                                                      Fatty acids are identified solely by carbon number and number
   The nomenclature of the chemical structure of fatty                of unsaturations (double bonds), e.g. C20:4. This method fails
   acids can be confusing and should be standardized.                 to differentiate between fatty acids which have the same chain
   Criteria for a suitable terminology are proposed, and              length and degree of unsaturation but differ in the position of
   the recommendation is made that use of the 'n' arid                the double bonds or their stereo-isometric configuration.
   'w' systems comes closest to meeting these criteria.

   S Afr Med] 1985; 67: 633-634.                                      The on' and 'w' systems
                                                                      There are two slightly different homologous series classification
                                                                      systems, the 'n' and 'w' systems. The two methods are very
                                                                      similar in that they relate to the position of the first double
The terminology used to describe fany acid structure is               bond from the methyl terminal carbon and give a clear
occasionally confusing, especially to the non-specialist. An          indication of the stereo-isometric species concerned, e.g. eis
adequate system of terminology would specify: (I) carbon              C20:4 n- 6 and eis C20:4w6 • We feel that the two systems are
chain length; (il) number of unsaturations; (iil) stereo con-         almost completely interchangeable, but the 'w' system does
figuration of the unsaturations; and (iv) the position of the         have the advantage of indicating, implicitly and without prior
first unsaturation in relation to the methyl terminal carbon          knowledge of convention, the end from which the carbon
atom. Several conventions have been used, and at least four of        chain is being counted, i.e. the carbon atom furthest (w) from
them are common in different elementary and specialist texts.         the a-carbon at the carboxyl terminal.
The purpose of this article is to compare these systems and              The identification of stereo-isomers is important in
provide a guide to their uses and interconversion.                    differentiating the beneficial eis-polyunsaturated fatty acids
                                                                      from the potentially harmful crans-isomers. Only the cis-
                                                                      polyunsaturates (cis-PUFAs) exhibit the ability to ameliorate
The trivial system                                                    essential fatty acid (EFA) deficiency symptoms. I-3 The crans-
                                                                      polyunsaturates (crans-PUFAs) interfere with the main enzymes
Each fatty acid has a name which conveys no structural                involved in the metabolism ofEFAs.4 Trans-PUFAs, saturated
information, e.g. arachidonic acid. Confusion between                 and eis- or crans-mono-unsaturated fatty acids stimulate high
arachidonic acid and arachidic acid is easy, while the structures     blood cholesterol levels, a factor predisposing to the develop-
are markedly different (i.e. 20 carbons with four double bonds        ment of hypertension, atherosclerosis and heart disease. 5- 7 The
and 20 carbons with no double bonds respectively). Stereo-            cis-PUFAs, on the contrary, have been shown to reduce blood
isomers of the same basic structure have different names, e.g.        cholesterollevels,s-Io and to distinguish between cis and crans
oleic and elaidic acids (both 18 carbons with one double bond,        is therefore very important in any system of nomenclature.
but cis and crans respectively); this gives the impression of             Given the above, is it really important from a practical point
large differences in structure.                                       of view rather than the esoteric academic position to distinguish
                                                                      between eis C18:3 w3 (alpha-linolenic acid) and cis CI8:3 w6
                                                                      (gamma-linolenic acid)? The answer obviously must be yes.
The systematic method                                                 The two examples carry in their names the most important
                                                                      distinction between them, w3 and w6 (or n-3 and n-6). Plants
The fatty acid is named purely on the basis of the number of          are capable of insertmg double bonds into fatty acids of 18-
carbon atoms and the number (if any) of the unsaturations and         carbon chain length in both the w3 and w6 positions. Animals
their positions relative to the carboxyl groups, e.g. 5,8,11,14-      lack the enzymes necessary for this process 11 and are therefore
eicosatetraenoic acid. There is no possibility of mistaken identity   dependent on plant or other animal sources for their cis-
when this system is used; however, naming from the carboxyl           PUFAs, hence the term 'essential fatty acids' (Fig. 1). It will
group makes the recognition of related compounds of different         be seen that the inability to make w3 and w6 fatty acids will
chain length and degree of unsaturation difficult, since the          also of necessity rule out the interconversion of these fany
elongation always occurs at the carboxyl end. The relative            acids by animals, thus producing two distiner families of fatty
positions of the double bonds will appear to be different, while      acids. One series is derived from the parent w3, eis CI8:3 w3 '
in fact they remain the same relative to the methyl terminal          and the other from the w6 parent, eis CI8:2 w6 • These two
group.                                                .               families show distinctly different properties in cellular and
                                                                       physiological processes.
                                                                          The w6 series give rise to the 1 and 2 series of prostaglandins,
                                                                       prostacyclins, leukotrienes and thromboxanes (eicosanoids),
Department of Medical Biochemistry, University of the                  while the w3 series can produce the separate and metabolically
Witwatersrand, Johannesburg                                            completely different 3-series of eicosanoidsl 2 (Fig. 2). An
B. C. DAVIDSON, M.l. BIOL.                                             imbalance between the available metabolic pool of the two
                                                                       precursors can have significant biochemical and physiological
634         SAMJ     VOLUME 67        20 APRIL 1985

            Plants only                      Animals and plants          C18:2 w6 to cis C20:4 w6 • This enzyme is much more active with
                                                                         the w3 substrate cis C18:3 w3' to such an extent as not to be
                                                                         rate-limiting; 18 this may reflect a selective mechanism whereby
                          6                  12        15
                                                                  COOH   the much lower dietary levels of w3 farty acids are preferentially
                                                                         utilized. 19
                                                                            Recent research has shown the cis-PUFAs to be of major
From CIS:O (Stearic acid)                   H3C~COOH
                                                                         potential importance in the control and alleviation of a sur-
Animals can make cisCIS:l w9 (Oleic acid)   H3C~COOH                     prisingly large number of the currently most important problem
But not cisCIS:2 w6 (Linoleic acid)         H3C~COOH                     areas in medicine. Dietary EFAs have been shown to produce
Or cisCIS:3 w3 (a Linolenic acid)           H3C~COOH                     significant effects in cardiovascular disease, alcoholism, cyto-
                                                                         toxicity relating to carcinoma, premenstrual tension and hyper-
Fig. 1. Formation of unsaturated fatty acids from saturated
                                                                         tension, among others. 20 - 27 The ability not only to distinguish
precursors. Animals can desaturate fatty acids in positions higher       clearly between farty acids but at the same time to be able to
than w9.                                                                 appreciate the relationships between members of a homologous
                                                                         series is therefore vitally important. The only systems of
                                                                         nomenclature which have both these properties are the 'w' and
                                                                         'n' systems.


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