Evidence that hepatic triglycerides provide acylglycerides for synthesis of bile phosphatidylcholines

Abstract

To determine the biochemical origin of bile phosphatidylcholines (PCs), rat liver perfusions with 16:1 fatty acid (FA) and [3H]glycerol were performed to generate novel radiolabeled bile and liver PCs and their hepatic glyceride precursors. Results showed total equilibration of bile and liver 16:1-16:1 PC when the specific activity of precursor glycerol-3-phosphate was kept constant. However, when the specific activity of glycerol-3-phosphate decreased during the labeling period and during a prolonged chase period with 17:1 FA and nonradiolabeled glycerol, the specific activity of bile 16:1-16:1 PC was appreciably higher than this same PC in the liver and during the chase period was even higher than its hepatic 16:1-16:1 acylglycerol precursors, phosphatidic acid and diglyceride. During the chase period with 17:1 FA, new radiolabeled 16:1-17:1 PC was formed, and again the specific activity of this PC in bile was greater than this PC and 16:1-17:1 phosphatidic acid and diglyceride in the liver. Only the specific activity of liver 16:1-16:1-(FA) triglyceride equaled or was high enough to support the formation of new bile 16:1-16:1 PC. These studies indicate that bile PCs do not directly derive from preexisting hepatic PCs or by de novo synthesis through phosphatidic acids and diglycerides, but likely originate by remodeling from a pool of hepatic triglycerides.