The CoA-independent transacylase in PAF biosynthesis: tissue distribution and molecular species selectivity.

Microsomal membranes from six different rat tissues (spleen, lung, kidney, brain, testis, and liver) were found to possess CoA-independent transacylase activity that could both acylate lyso-[3H]PAF (1-[3H]hexadecyl-2-lyso-sn-glycero-3-phosphocholine) and then deacylate the 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine product via the transacylation of added exogenous 1-alk-1′-enyl-2-lyso-sn-glycero-3-phosphoethanolamine. Platelet-activating factor (1-[3H]hexadecyl-2-acetyl-sn-glycero-3-phosphocholine) was produced when acetyl-CoA was added to the spleen microsomes during generation of lyso-[3H]PAF by the transacylases. More extensive studies with subcellular fractions from spleen revealed that, in addition to microsomes, the transacylase activities were also present in the 15 000 × g membrane fraction but not in the cytosol. Analysis of molecular species of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine before and after addition of 1-alk-1′-enyl-2-lyso-sn-glycero-3-phosphoethanolamine as the acyl acceptor demonstrated a high selectivity for polyunsaturated fatty acids (> 3 double bonds/acyl group) in both the acylation and deacylation processes that occurred in testicular microsomal membranes. The transfer of acyl groups by the transacylase appeared to be equally effective for either arachidonic or docosapentaenoic(n − 6) fatty acids, whereas linoleic and oleic fatty acids were not transferred from 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine following the addition of 1-alk-1′-enyl-2-yso-sn-glycero-3-phosphoethanolamine. Similar experiments with the membrane fraction of undifferentiated HL-60 cells showed that arachidonic acid supplementation of intact cells enhanced both the CoA-independent transacylation of lyso-[3H]PAF and the subsequent deacylation of 1-[3H]hexadecyl-2-acyl-sn-glycero-3-phosphocholine caused by addition of 1-alk-1′-enyl-2-acyl-sn-glycero-3-phosphoethanolamine. Differentiation of the HL-60 cells into a neutrophil-like form had no effect on the transacylase activity. Our results indicate the PAF-related transacylase is widely distributed among tissues and, although highly selective for polyunsaturated acyl groups, does not discriminate selectively among the polyunsaturates.