Utilization of L-serine in the in vivo biosynthesis of glycerophospholipids by rat liver.

The incorporation of L-serine-U-14C, L-serine-3-14C, and D,L-serine-1-14-C into the glycerophospholipids of rat liver in vivo was determined over a period of 3 min to 13 hr following intravenous injection. The radioactivity from these serines was transferred to variable extent into the glycerol, fatty acid, and nitrogenous base parts of all the glycerophospholipids and neutral lipids. The half-lives and turnover rates of phosphatidylserine calculated from the precursor-product specific activity curves obtained with L-serine-U-14C were 14 min and 0.28 mumol/min/liver, respectively. The half-lives and turnover rates of phosphatidylserine as measured from the decay data of lipid serine from all markers averaged, respectively, 8.2 hr and 0.0008 mumol/min/liver. The discrepancy between these turnover rates was attributed to an understimation of degradation of phosphatidylserine due to its continued biosynthesis and/or an extensive reutilization of L-serine. By monitoring the formation of radioactive lipid ethanolamine, it was found that phosphatidylserine was decarboxylated at one-half the rate of lipid serine biosynthesis. It is suggested that as much as one-half of total phosphatidylserine may be degraded by other mechanisms, such as base exchange with choline, ethanolamine, and serine, as already demonstrated in vitro by other workers. The time course and nature of labeling of phosphatidylcholine was consistent with an extensive conversion of radioactive L-serine to 1-carbon fragments and a rapid methylation of phosphatidylethanolamine to phosphatidylcholine.