The nature of the stimulatory role of the supernatant fraction on triglyceride synthesis by the alpha-Glycerophosphate pathway.

Evidence is presented as to the nature and mechanism of the stimulatory effect of the supernatant fraction on the biosynthesis of triglycerides via the alpha-glycerophosphate pathway in the intestinal mucosa. When microsomes are employed as the enzyme source, the major lipid formed from either labeled palmitic acid orL: -alpha-glycerophosphate is phosphatidic acid and only a limited amount of triglyceride is synthesized. The addition of the supernatant fraction to microsomes results in a stimulation of triglyceride biosynthesis at the expense of phosphatidic acid. Employing the same microsomal fraction, the reaction sequence was followed step by step and the intermediates were isolated. The results suggest that the stimulatory role of the supernatant fraction can be attributed to the presence ofL: -alpha-phosphatidate phosphohydrolase (EC 3.1.3.4). The hydrolysis of the biosynthesized microsomal phosphatidic acid by the supernatant enzyme occurs at a faster rate than the hydrolysis of added phosphatidic acid prepared from egg lecithin. The initial acylation steps in the biosynthesis of triglycerides or phosphatidic acid via the glycerophosphate pathway occur only in the presence of fatty acid and the cofactors necessary for its activation. Under these conditions, fatty acyl-CoA will not substitute for the fatty acid activation system.