REGULATION OF UNSATURATED FATTY ACIDS BIOSYNTHESIS .I. EFFECT OF UNSATURATED FATTY ACID OF 18 CARBONS ON MICROSOMAL DESATURATION OF LINOLEIC ACID INTO GAMMA-LINOLENIC ACID

1. (1) [1-14C]Linoleic acid is desaturated by the microsomes of rat liver to γ-[1-14C]linolenic acid, and this reaction is inhibited by CN-. 2. (2) The addition of α-linolenic acid or of several monoenoic acids, such as oleic, petroselinic, vaccenic and elaidic acids, provoked an inhibition. 3. (3) Unlabeled cis-linoleic and γ-linolenic acids increase the desaturation, whereas all-trans-linoleic acid decreased the conversion of [1-14C]linoleic acid into yγ-linolenic acid. 4. (4) The activating effect evoked by cis-linoleic and γ-linolenic acids disappeared on the addition of heated microsomes or lysolecithin, so this activating effect may be the result of a competition between the transacylating and desaturating enzymes present in the microsomes when a small amount of labeled acid is incubated. 5. (5) When the concentration ratio of [1-14C]linoleic acid to microsomal protein was increased so that the desaturating enzyme was the limiting factor, cis-linoleic, trans-linoleic and γ-linolenic acids all inhibited linoleic acid desaturation. 6. (6) The desaturation of oleic, linoleic and α-linolenic acids may be regulated by competitive reactions among the three acids, other acids, their reaction products or by feed-back inhibition. The transacylating reaction may also modify the desaturation. Insulin and dietary conditions may induce an increase of desaturating enzyme activity. © 1969.