Regulation of the Fatty Acid Pattern of Phospholipids in Rabbit Sarcoplasmic Reticulum

The activities for the acylation of glycerol 3-phosphate, 1-acyl-sn-glycerol 3-phosphate and 2-acyl-sn-glycero-3-phosphorylcholine with several fatty acyl-CoA's were estimated with rabbit sarcoplasmic reticulum. The highest incorporation rate was observed at the transfer of linoleate to glycerol 3-phosphate, and linolenate was incorporated most slowly. In the acylation of 1-acyl-sn-glycerol 3-phosphate, the incorporation rate of linolenate was highest, with that of linoleate coming next, and other fatty acids were incorporated at practically the same lower rate. However, strict substrate specificity was not observed in the synthesis of phosphatidic acid in rabbit sarcoplasmic reticulum. The highest rate was observed in the incorporation of palmitate at the 1-position of 2-acyl-sn-glycero-3-phosphorylcholine, suggesting the participation of this enzyme in the regulation of fatty acid composition at the 1-position of phosphatidylcholine of rabbit sarcoplasmic reticulum. Since the specific activity of 1-acyl-sn-glycero-3-phosphorylcholine acyltransferase is about 10 times higher than that of 2-acyl-sn-glycero-3-phosphorylcholine acyltransferase, the fatty acid pattern of phosphatidyicholine of rabbit sarcoplasmic reticulum may be controlled by 1-acyl-sn-glycero-3-phosphorylcholine acyltransferase to a larger extent than by 2-acyl-sn- glycero-3-phosphorylcholine acyltransferase. Thus, the specificities of 1-acyl-sn-glycerol 3-phosphate and 2-acyl-sn-glycero-3-phosphoryl- choline acyltransferases are largely consistent with the general fatty acid patterns at the 1- and 2-positions of phospholipids. However, the fatty acid composition of phosphatidylcholine does not completely reflect the specificities of these enzymes.