Hepatic phosphatidylcholines: evidence for synthesis in the rat by extensive reutilization of endogenous acylglycerides.

Studies were performed to compare the extent of fatty acid incorporation into liver phosphatidylcholines (PCs) by acyl remodeling and by de novo synthesis. To this end, isolated rat livers were first perfused with palmitoleic acid (16:1) and [2-3H]glycerol and then with 17:1 fatty acid and nonradiolabeled glycerol that resulted in the formation of new molecular species of radiolabeled PCs containing 16:1 and 17:1 acyl groups. The specific activities of newly formed molecular species of the de novo precursors of acylglyceride synthesis, phosphatidic acids (PAs) and diglycerides (DGs), and the products of synthesis, PCs and triglycerides (TGs), were measured at periods during both the labeling period of perfusion with 16:1 (first 15 min) and the more prolonged chase period with 17:1 (up to 120 min). At the end of the labeling period, the specific activity of all the 16:1-containing PAs, DGs, and 16:1-16:1-16:1 and 16:1-16:1-18:2 TGs were the same and were much higher than any molecular species that did not contain 16:1. The specific activities of these molecular species are indicative of the specific activity of molecular species synthesized exclusively by de novo synthesis (i.e., by acylation of glycerol 3-phosphate) during the labeling period. In contrast, the specific activity of 16:1-16:1 PC was only 2/3 that of the other 16:1-16:1 glycerides, and the specific activities of the other 16:1-containing PCs were only about 1/3 that of the corresponding 16:1-containing PAs, DGs, and 16:1-16:1 TGs. After the labeling period and during the chase period with perfusion of 17:1 and nonradiolabeled glycerol, the specific activities of major 16:1 PCs exceeded the specific activities of their corresponding PAs and DGs and remained considerably higher than these precursors of de novo synthesis for the duration of perfusions. However, during this period, the specific activities of major 16:1 PCs were less than their corresponding molecular species of TGs. During the chase period, new 17:1 molecular species of PCs were formed that were also radiolabeled. The specific activity of 16:1-17:1 PC, the 17:1 PC with the highest specific activity, always exceeded its corresponding PA and DG precursors. During the chase period, non-16:1 and non-17:1 molecular species of PCs that comprised the bulk of hepatic PCs were also radiolabeled and the specific activities of these molecular species progressively increased during this period.(ABSTRACT TRUNCATED AT 400 WORDS)