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

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.

Abstract

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.