Enzymatic Regulation of 3-sn-Phosphatidylcholine and Triacylglycerol Synthesis in States of Altered Lipid Metabolism

Specific activities of phosphorylcholine-glyceride transferase and diglyceride acyltransferase, which catalyze the biosynthesis of 3-sn-phosphatidylcholine and triacylglycerol, respectively, from the common substrate 1,2-diacyl-sn-glycerol, were measured in liver microsomal preparations from alloxan-diabetic rats and from rats treated with triiodothyronine and propylthiouracil. Diglyceride acyltransferase was induced in both the ketotic diabetic and the triiodothyronine-treated animals but was repressed in the propylthiouracil-treated group. Phosphorylcholine-glyceride transferase specific activity was unchanged by propylthiouracil treatment. In the ketotic diabetic and triiodothyronine-treated animals the magnitude of changes in phosphorylcholine-glyceride transferase activity differed from that of diglyceride acyltransferase. The direction and magnitude of changes in the specific activity of these enzymes are compatible with their acting as regulators of 3-sn-phosphatidylcholine and triacylglycerol synthesis at the 1,2-diacyl-snglycerol branch point in the complex lipid biosynthetic pathway. The data are compatible with a possible association between the regulation of synthesis de novo of 3-sn-phosphatidylcholine and the assembly and release of lipoproteins from the liver. Methods for the chemical synthesis of phosphorylcholine and palmityl coenzyme A are described which are suitable for preparation of radioactively labeled substrates.