Cytochrome b5-dependent delta 9 desaturation of fatty acids in gastric microsomes.

Stearyl-CoA was shown to stimulate the reoxidation rate of cytochrome b5 of gastric microsomes and to decrease the reduction rate of trypsin-purified hog liver cytochrome b5 by the NADH-cytochrome b5 reductase of these microsomes. This latter effect was (1) proportional to microsome concentration and to stearyl-CoA concentration with an apparent Km of 3.3 . 10(-6) M and a Vmax of 71 nmol per min and per mg microsomal protein, (2) insensitive to ATP and inhibited by 1.4 mM KCN, (3) mimicked by palmityl-CoA but not by stearic nor palmitic acid. Direct assays carried out using [14C]stearyl- and [14C]palmityl-CoA as substrates showed a production of 0.12 nmol of oleic and palmitoleic acid, respectively, per min per mg of microsomal protein. In the presence of Tb5 antibodies the reaction was inhibited by 40%. These results support the occurrence of cytochrome b5-dependent fatty acid delta 9 desaturation in gastric microsomes.