Chain Elongation of α- and γ-Linolenic Acids and the Effect of Other Fatty Acids on Their Conversion in Vitro

The chain elongations of α- and γ-linolenic acids with malonyl coenzyme A with rat liver microsomes were studied. γ-Linolenic acid was converted to eicosatrienoic acid in about 70% yield, whereas α-linolenic acid was only converted to about 30% under the same conditions. Unsaturated acids of the linoleic acid family present in the incubation mixture increased the conversion of both linolenic acids. The conversion of γ-linolenic acid was inhibited most effectively by trienoic acids: 9,12,15-octadecatrienoic, 5,8,11- octadecatrienoic, and 5,8,11-heptadecatrienoic acids. α- Linolenic acid was mainly inhibited by its own metabolic products, eicosapentaenoic and docosahexaenoic acids, when these were added. Saturated fatty acids inhibited the conversion of both acids, but in quite different fashions. The inhibition of α-linolenic acid was independent of the chain length of the saturated acids, whereas γ-linolenic acid was most effectively inhibited by tetradecanoic and pentadecanoic acids. The strong inhibition by the saturated fatty acids of the elongation of unsaturated fatty acids may serve as a regulation of the composition of tissue lipids.