SAMT DEEL 67 20 APRIL 1985 633 Fatty acid nomenclature A short review B. C. DAVIDSON, R. C. CANTRILL The structural system Summary 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 R. C. CANTRILL, B.SC. HONS, PH.D. 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. REFERENCES I. Mead JF, Fulco AJ. The unsaturated and polyunsaturated fany acids in health and disease. Springfield, Ill: Charles C Thomas, 1976. 2. Bun R, Buss DH, Kirk RS. Fany acids and sterols in the British diet. Proe Nutr Soe 1982; 42: 7lA. 3. Bun GO. Significance of the essential fany acids. Fed Proe 1942; 1: 224-233. 4. Cook HW. The influence of trans acids on desaturation and elongation of fany acids. Lipids 1981; 16: 920-926. 5. Holman RT, Aaes-Jorgensen E. Effects of trans fany acid isomers upon essential fany acid deficiency in rats. Proe Soe Exp Bioi Med 1956; 93: 175-179. 6. Sinciair H. Diet,ary fats and coronary hean disease. Lancer 1980; i: 441-445. 7. Kummerow FA. Nutrition imbalance and angiotoxins as dietary risk factors in coronary hean disease. Am] Clin Nutr 1979; 32: 58-83. 8. Man JW, Morris IN. Changing the national diet to reduce coronary hean disease. Lancer 1982; i: 1465. 9. Peufer JJ, Holman RT. Essential fany acids, diabetes and cholesterol. Arch Biochem Biophys 1955; 57: 520-521. 10. Horrobin DF, Manku MS. Haw do polyunsaturared fany acids lower plasma cholesterol levels? Lipids 1983; 18: 558-562. 11. Crawford MA, Sinciair AJ. Lipids, malnutrition and the developing brain. CIBA Found Symp 1972; pp. 267 - 292. 12. Crawford MA. Background to essential fany acids and their prostanoid derivatives. Br Med Bull 1983; 39: 210-213. 13. Rivers JPW, Crawford MA. Maternal nutrition and the sex ratio at birth. Nature 1974; 252: 297-298. Fig. 2. Enzymatic desaturation of EFAs to give eicosanoid pre- 14. Rivers JPW, Davidson BC. Linolenic acid deprivation in mice. Proe Nutr cursors. Soe 1974; 33: 48A. 15. Greenberg SM, Calbert CE, Savage EE, Devel HJ. The interrelation of linoleate and linolenate in supplying rhe essential fany acid requirement in effects on the individual. IJ - 17 In the metabolism of the EFAs the rat.] Nutr 1950; 41: 507-521. 16. Pudelkewicz C, Seufert J, Holman RT. Requirements of the female rat for to produce eicosanoids the major products from cis C18:2 w6 linoleic and linolenic acids.] Nutr 1968; 64: 138. (linoleic acid) are cis C20:3 w6 (dihomogamma-linolenic acid) 17. Aaes-Jorgensen E, Lepak EE, Hayes HW, Holman RT. Essential fany acid and cis C20:4 w6 (arachidonic acid), and those from cis C18:3 w3 deficiency: II: In adult rats.] Nutr 1958; 66: 245-259. 18. Kunau W-H, HOlman RT, eds. Polyunsaturated Fatty Acids (Proceedings of (alpha-linolenic acid) are cis C20:S w3 (eicosapentaenoic acid) rhe American Oil Chemists Sociery, Champaign, 1977). and cis C22:6 w3 (docosahexaenoic acid). The relationships 19. McCance RA, Widdowson EM. The Composition of Foods. 4th ed. London: HMSO, 1978. between the series members is fairly clear using the 'w' system 20. Enig MG, Munn RG, Keeney M. Dietary far and cancer trends. Fed Proe (or the 'n' system). However, if the systematic nomenclature 1978; 37: 2215-2220. 21. Davidson BC, Cantrill RC, Katzeff I, Booyens J. On the role of poly- were used, cis C18:2 w6 would be identified as cis-9,12- unsaturated fany acids in cytotoxiciry. S Afr Cancer Bull 1983; 27: 153. octadecadienoic acid, while the product cis C20:4 w6 would 22. Dippenaar N, Booyens J, Fabbri D, KalZeff 1. The reversibiliry of cancer: become cis-S,8,11,14-eicosatetraenoic acid - the double bonds evidence that malignancy in melanoma cells is gamma-linolenic acid deficiency-dependent. S Afr Med] 1982; 62: 505-509. would thus appear to shift position, making the relationship 23. Leary WP, Robinson KM, Booyens J, Dippenaar N. Some effects of much more difficult to visualize. The systematic identification gamma-linolenic acid on cultivated human oesophageal carcinoma cells. S Afr Med] 1982; 62: 681-683. of the w3 series is more confusing - cis C18:3 w3 becomes cis- 24. Horrobin DF. A biochemical basis far alcoholism and alcohol-induced 9,12,lS-octadecatrienoic acid, and the products cis C20:S w3 and damage inciuding the fetal alcohol syndrome and cinhosis: interference with essential fany acid and prostaglandin metabolism. Med Hypocheses 1980; 6: cis C22:6 w3 become cis-S,8, 11,14,17-eicosapentaenoic acid and 929-942. cis-4,7, 10,13,16,19-docosahexaenoic acid respectively. 25. Lloyd-Still JD, Johnson SB, Holman RT. Essential fany acid status in Enzyme specificity has been shown to vary between the two cystic fibrosis and the effects of safflower oil supplementation. Am] Clin Nucr 1981; 34: 1-7. series of essential farty acids. Delta-6-desaturase, the enzyme 26. Horrobin DF. The roles of essential fany acids and prostaglandins in the which inserts a double bond between w12 and w13 carbon premenstrual syndrome.] Reprod Med 1985 (in press). 27. Bang HO, Dyerberg J. Lipid metabolism and ischemic heart disease in atoms, is the rate-limiting stage in the conversion of cis Greenland Eskimos. Adv Nucr Res 1980; 3: 1-22.
Pages to are hidden for
"Fatty acid nomenclature"Please download to view full document