Accepton concentration effect in the selectivity of acyl coenzyme A: U aclglycerylphosphorylcholine acyltransferase system in rat liver.

In the acylation of 1-acylglycerylphosphorylcholine (1-acyl-GPC) in rat liver, neither the specificity observed with Triton X-100 treated microsomes were consistent with the selectivity observed in vivo. This apparent discrepancy was solved as follows. 1. Crude microsomes or deoxycholate-treated microsomes had approximately the same levels of activities for oleoyl-, linoleoyl- and arachidonoyl-CoAs. Triton X-100 treated microsomes had much higher activity for arachidonate but very little activity for oleate, which was found to be the consequence of selective inactivation by Triton X-100 of oleoyl-CoA:1-acyl-GPC acyl transfer reaction. These observations favor a concept that different enzymes or different sites on a single enzyme exist in rat liver microsomes for the transfers of different acyl-CoAs. 2. Although the maximum velocities for oleoyl-CoA and arachidonoyl-CoA were approximately the same, more arachidonate was incorporated than oleate at very low concentrations of the acceptor even when both acyl-CoAs were present at saturating concentrations. Thus, higher selectivity for arachidonate (and linoleate to a lesser extent) observed with rat liver in vivo could be correlated with relatively higher affinity in this reaction for 1-acyl-GPC observed in vitro, which exerts its effect at low concentrations of the acceptor.