Direct desaturation of eicosatrienoyl lecithin to arachidonoyl lecithin by rat liver microsomes.

A microsomal enzyme system from rat liver was shown to catalyze desaturation, in presence of reduced pyridine nucleotides and oxygen, of 1-acyl-2-[14C]eicosatrienoyl-sn-glycero-3-phosphorylcholine to 1-acyl-2-[14C]arachidonoyl-sn-glycerophosphorylcholine. This desaturation was linear with time and proportional to microsomal protein concentration, and proceeded with no significant breakdown of the lecithin substrate. The microsomal enzyme system will also desaturate 1,2-di-[14C]eicosatrienoyl-sn-glycero-3-phosphorylcholine and [14C]eicosatrienoyl-CoA, but not free [1-14C]eicosatrienoic acid in the absence of ATP, Mg2+, and CoA. Desaturation of 1-acyl-2-[14C]eicosatrienoyl-glycerophosphorylcholine as well as [14C]eicosatrienoyl-CoA was dependent on oxygen and either NADH or NADPH, and was inhibited by cyanide but not by carbon monoxide, indicating the involvement of cytochrome b5 and not P450. The activity of both eicosatrienoyl-glycerophosphorylcholine desaturase and the eicosatrienoyl-CoA desaturase was increased in rats that had been starved for 48 h and refed a fat-free diet. These data indicate the existence of a new route to synthesis of arachidonate, namely, by desaturation of eicosatrienoyl lecithin to arachidonoyl lecithin.