Effects of ethylenic bond position upon acyltransferase activity with isomeric cis-octadecenoyl coenzyme a thiol esters

Abstract

The specificity of the acyl-CoA:phospholipid acyltransferases has been studied using the 16 positional isomers of cis-octadecenoic acid. The results showed that the acyl-transferases acting at both the 1- and 2-positions of acyl-glycero-3-phosphorylcholine (acyl-GPC) discriminated between the acyl-CoA isomers in quite different ways. 1. 1. Acyl-CoA:1-acyl-GPC acyltransferase activity showed a distinct preference for the 9-, and 12-isomers. Of these three, the 9-octadecenoate (oleate) was the preferred substrate having a rate of 98 nmoles/min per mg.2. 2. Acyl-CoA:2-acyl-GPC acyltransferase reacted more rapidly with the 8-,10-,12-,13- and 15-isomers, and of these the 12-octadecenoate had the fastest rate (121 nmoles/min per mg).3. 3. As the enzymes were allowed to age at 4[deg], the activity was lost at slightly different rates for each isomer. The enzyme(s) esterifying the 1-position seemed to loose activity fairly uniformly with all isomers so that a similar pattern of reactivities was observed over a period of several days. The enzyme(s) esterifying the 2-position, however, differed in that after 2 days, the rate for the 9-isomer had dropped below that for the 12-isomer. This result suggests that different enzymes may exist for different acyl-CoA isomers.4. 4. High concentrations of sucrose (0.8 M) tended to stabilize the activities with the 9- and 12-isomers, but did not change the fact that the activity for the 9-isomer was lost more rapidly. Albumin, contrary to our expectations, increased the rate of loss of activity.5. 5. The enzyme activities were purified 10- to 15-fold above that of the crude tissue homogenate by treating the microsomal particles with sodium deoxycholate and albumin.6. 6. Acyltransferase rates for the esterification of the naturally occurring 7-,9-,11-, and 13-octadecenoates to position 1 and 2 of diacyl-GPC indicated a preferred position for each acid which is in accord with that reported for the distribution of these monoenoic acids in phospholipids isolated from rat liver.