ON THE RELATION BETWEEN A STEAROYL-SPECIFIC TRANSACYLASE FROM BOVINE TESTIS MEMBRANES AND A COPURIFYING ACYLTRANSFERASE

Bovine testis membranes contain a coenzyme A-dependent transacylase th at carl catalyze the preferential transfer of stearoyl groups from pho sphoglycerides to sn-2-acyl molecular species of lysophosphatidic acid and lysophosphatidylinositol [Itabe et al., (1992) J. Biol. Chern. 26 7, 15319-15325], We have now purified this enzyme 1000-fold and shown that it copurifies with an acyltransferase, The purified transacylase can use phosphatidic acid, phospatidylinositol, or phosphatidylinosito l-4-phosphate as an acyl donor and catalyzes the transfer of stearoyl groups in preference to palmitoyl groups or oleoyl groups. In contrast , the purified acyltransferase uses acyl-coenzyme A as an acyl donor a nd shows no such preference for stearoyl group transfer. Furthermore, phosphatidylinositol-4,5-bisphosphate inhibits the two enzymes to diff erent extents and by different mechanisms. Nevertheless, the enzymes a re similar in several respects: they use the same acyl accepters and, when assayed together, compete for the acyl acceptor, sn-2-oleoyl lyso phosphatidic acid; they lose activity in parallel unless stabilized by the addition of an anionic phosphoglyceride or stearoyl-coenzyme A; a nd they show similar sensitivities to heat and pH. One way to explain these results is to postulate that the transacylase reaction occurs in two successive steps: a stearoyl-specific first step in which a stear oyl group is transferred from an sn-1-stearoyl-2-acyl phosphoglyceride to coenzyme A, and a relatively non-acyl-chain-specific second step i n which a stearoyl group is transferred from stearoyl-coenzyme A to an sn-2-acyl lysophosphoglyceride. According to this line of reasoning, the transacylase assay that we have used measures the net effect of bo th steps, whereas the acyltransferase assay measures only the effect o f the second step.