ACYL-GPC AND ALKENYL ALKYL-GPC-ACYL-COA ACYLTRANSFERASES/

In mammalian tissues, phosphatidylcholine, or 1,2-diacyl-glycerophosph ocholine (GPC), is the most abundant form of choline-containing phosph olipids. In some electrically active tissues, a significant portion of the choline-containing phospholipids is 1-alkenyl-2-acyl-GPC (plasmen ylcholine). The 1-alkyl-2-acyl-GPC is found in significant amounts in circulating cells such as neutrophils and macrophages but in low amoun ts in other tissues. Structural studies of phosphatidylcholine indicat e that there is an asymmetric distribution of acyl groups on the molec ule. Saturated fatty acids are usually esterified at the sn-1 position of the glycerol backbone, whereas unsaturated fatty acids are esterif ied at the sn-2 position. Similarly, unsaturated acyl groups are usual ly found in the sn-2 position of plasmenylcholine. The remodelling of the sn-2 acyl group in phosphatidylcholine by the deacylation-reacylat ion process has been demonstrated in a number of tissues. Phospholipas e A(2) is responsible for the hydrolysis of the acyl group at the sn-2 position, whereas 1-acyl-GPC acyl-CoA acyltransferase is responsible for the reacylation reaction. The acyltransferase is located in the mi crosomal fraction and displays specificity towards the polyunsaturated acyl groups. The enzyme can be solubilized by detergent, but the enzy me activity in soluble form is difficult to maintain. The acyltransfer ase for the reacylation of 1-alkenyl-GPC is also located in the micros omal fraction and is somewhat specific towards polyunsaturated acyl gr oups. In guinea pig heart mitochondria, however, a new form of 1-alken yl-GPC acyltransferase was identified which appeared to be different f rom the microsomal form. The acyltransferase for the acylation of 1-al kyl-GPC into platelet-activating factor has been studied in several ti ssues including human neutrophils. At present, the contribution of the acyltransferase in attaining the observed molecular composition of th e choline-containing phospholipids in the tissue has not been defined. We postulate that the intrinsic acyl-CoA specificity of the acyltrans ferase, the flux of 1-acyl-GPC, 1-alkenyl-GPC and 1-alkyl-GPC, as well as the pool size of acyl-CoA are major factors in producing the final composition of the molecular species of the choline-containing phosph olipids. (C) 1997 Elsevier Science B.V.