Fatty acids, cyclopropanoid

Methyl Branched, Cyclopropanoid, and Cyclopropenoid Fatty Acids... 

Fig. 2.17. Cyclopropanoid and cyclopropenoid fatty acids from plants.

Christopher, R. K. and Duffield, A. M. (1980) Use of vinyl methyl ether as a chemical ionization reagent gas for gas chromatographic chemical ionization mass spectrometric discrimination between cyclopropanoid and monoenoic fatty acid methyl esters. Biomed. Mass Spectrom., 1 (10), 429-32. 

Cyclopropanoid and cyclopropenoid fatty acids are often accompanied by epoxy fatty acids (Bohannon and Kleiman, 1978). 

Elucidation of the structures of the various cyclopropanoid fatty acids (Lukina, 1%2 Christie, 1970) and cyclopropenoid fatty acids (Christie, 1970 Carter and Frampton, 1964), and typical reactions of these unusual fatty acids, have been described. Methods for their isolation, identification, and analysis have been discussed as well (Lukina, 1962 Christie, 1970 Carter and Frampton, 1964). Chromatographic methods used in the isolation and analysis of cyclic fatty acids have been described in detail (Lie Ken Jie, 1980). Reference is made to improvements in methods for their characterization and determination by chemical methods (Brown, 1969), spectrophotometry (Coleman and Firestone, 1972), nuclear magnetic resonance (nmr) spectroscopy (Boudreaux et al, 1972 Pawlowski et al., 1972b), mass spectrometry (Eisele et al., 1974), and thermal methods of analysis (White et al., 1976). 

Syntheses of cyclopropanoid fatty acids (Christie and Holman, 1966 Christie et al., 1%8) and cyclopropenoid fatty acids (Gensler et al., 1970 Pawlowski et al., 1972a) have been described. 

It is well known that lactobacillic acid, a cyclopropanoid fatty acid occurring in certain bacteria, is formed by the addition of a methylene group to C/5--11-octadecenoic acid, a positional isomer of oleic acid, the preferred precursor of the CHj bridge being 5-adenosylmethionine (Christie, 1970 O Leary, 1965 Hofmann, 1%3 Law, 1971). 

While the transformation of oleic acid to dihydrosterculic acid and desaturation of the latter to sterculic acid were clearly established, the route of biosynthesis of the homologous cyclopropanoid and cyclopropenoid fatty acids could not be deduced from the results of experiments with seedlings and seed slices (Johnson et al., 1967 Hooper and Law, 1965). It appeared likely that dihydromalvalic acid was the immediate precursor of malvalic acid (Johnson et al., 1967), but it was not apparent whether dihydromalvalic acid was formed from a heptadecenoic acid or by a-oxidation of dihydrosterculic acid. The latter possibility appeared more likely in view of the labeling pattern of sterculic and malvalic acids isolated from H. syriacus seedlings that had been incubated with [l- C]acetate (Smith and Bu Lock, 1964). 

It is as yet unknown whether the substrates of the enzymes involved in the synthesis of dihydrosterculic acid are specific phospholipids, acyl-CoA, acyl-acyl carrier protein (ACP) or even free fatty acids. The preferred substrates of bacterial cyclopropanoid fatty acid synthesis are phosphatidyl-ethanolamines (Christie, 1970 O Leary, 1965 Hofmann, 1%3 Law, 1971). The true substrate of the desaturating enzyme is also yet to be identified. It is well known that exogenous stearic acid, a straight-chain saturated fatty acid, is not desaturated to oleic acid unless it is supplied as the ACP derivative (Shine et al., 1976). In contrast, dihydrosterculic acid, a cyclic saturated fatty acid, is desaturated to stercuiic acid when supplied as such. 

From the results obtained with callus cultures it was concluded, though not unequivocally established, that a-oxidation of dihydrosterculic to dihydromalvalic acid, and of stercuiic to malvalic acid, occurred without activation of the cyclopropanoid and cyclopropenoid fatty acids (Yano et al., 1972a). It appears that a-oxidation is the major or even the only pathway for the oxidation of fatty acids in these callus cultures (Yano et al., 1972a). 

It appears that cyclopropanoid fatty acids occur mainly esterified in position 2 of the various glycerolipids (Christie, 1970 Hofmann, 1963), the position usually occupied by a polyunsaturated acyl moiety. Phospholipids, in which either a cyclopropanoid fatty acid or a polyunsaturated fatty acid is esterified in position 2, exhibit similar physical properties (Christie, 1970 Hofmann, 1963). Although a systematic study of the distribution of the cy-clopropenoid fatty acids in the various positions of glycerolipids is yet to be carried out, it seems that these unusual fatty acids in seed oils are also predominantly esterified in position 2 (Christie, 1970). 

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