THE COA-INDEPENDENT TRANSACYLASE IN PAF BIOSYNTHESIS - TISSUE DISTRIBUTION AND MOLECULAR-SPECIES SELECTIVITY

Microsomal membranes from six different rat tissues (spleen, lung, kid ney, brain, testis, and liver) were found to possess CoA-independent t ransacylase activity that could both acylate lyso-[H-3]PAF [H-3]hexade cyl-2-lyso-sn-glycero-3-phosphocholine) and then deacylate the -[H-3]h exadecyl-2-acyl-sn-glycero-3-phosphocholine product via the transacyla tion of added exogenous k-1'-enyl-2-lyso-sn-glycero-3-phosphoethanolam ine. Platelet-activating factor -3]hexadecyl-2-acetyl-sn-glycero-3-pho sphocholine) was produced when acetyl-CoA was added to the spleen micr osomes during generation of lyso-[H-3]PAF by the transacylases. More e xtensive studies with subcellular fractions from spleen revealed that, in addition to microsomes, the transacylase activities were also pres ent in the 15 000 X g membrane fraction but not in the cytosol. Analys is of molecular species of 1-[H-3]hexadecyl-2-acyl-sn-glycero-3-phosoc holine before and after addition of lk-1'-enyl-2-lyso-sn-glycero-3-pho sphoethanolamine 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 mi crosomal membranes. The transfer of acyl groups by the transacylase ap peared to be equally effective for either arachidonic or docosapentaen oic(n - 6) fatty acids, whereas linoleic and oleic fatty acids were no t transferred from -[H-3]hexadecyl-2-acyl-sn-glycero-3-phosphocholine following the addition of lk-1'-eny-2-lyso-sn-glycero-3-phosphoethanol amine. Similar experiments with the membrane fraction of undifferentia ted HL-60 cells showed that arachidonic acid supplementation of intact cells enhanced both the CoA-independent transacylation of lyso-[H-3]P AF and the subsequent deacylation of [H-3]hexadecyl-2-acyl-sn-glycero- 3-phosjphocholine caused by addition of -1'-enyl-2-acyl-sn-glycero-3-- phosphoethanolamine. Differentiation of the HL-60 cells into a neutrop hil-like form had no effect on the transacylase activity. Our results indicate the PAF-related transacylase is widely distributed among tiss ues and, although highly selective for polyunsaturated acyl groups, do es not discriminate selectively among the polyunsaturates.