Biosynthesis of rat liver phosphatidylethanolamines from intraportally injected ethanolamine

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.

Solubilization and purification of rat liver microsomal 1,2-diacylglycerol: CDP-choline cholinephosphotransferase and 1,2-diacylglycerol: CDP-ethanolamine ethanolaminephosphotransferase

1. The procedure, which involved 2-step sonication of microsomes at pH 7.4 and then at pH 8.5 in the presence of sodium deoxycholate and subsequent dialysis, resulted in 4-5-fold purification of choline-phosphotransferase and ethanolaminephosphotransferase with the yield of 40-50%. 2. Ethanolaminephosphotransferase was further purified 8.5-fold over microsomes by sucrose density gradient centrifugation of the partially purified preparation, while cholinephosphotransferase activity was considerably lost during this procedure. No separation of the two transferases from each other was achieved at this step. 3. Cholinephosphotransferase required Mg2+ as cofactor, and microsomal phospholipids for its maximal activity. On the other hand, Mn2+ was more effective than Mg2+ as cofactor for ethanol aminephosphotransferase, and this enzyme was inhibited by microsomal phospholipids. 4. Both transferases were stimulated several-fold by sodium deoxycholate and also showed similar optimal pH ranging from pH 8.0 to 8.5. 5. Km values for 1,2-diacylglycerol emulsion were 81.0 muM for cholinephosphotransferase and 63.0 muM for ethanolaminephosphotransferase, respectively. CDP-choline and CDP-ethanolamine competitively inhibited, with the same Ki value (both 350 muM), ethanolaminephosphotransferase and cholinephosphotransferase, respectively. The Ki values obtained were much greater than the corresponding Km values for the cytidine substrates (36.4 muM for CDP-choline and 22.0 muM for CDP-ethanolamine). 6. The partially purified enzymes were further treated with Triton X-100. When enzyme activities were assayed with Mg2+, cholinephosphotransferase, although considerably inactivated, was partially separated from ethanolaminephosphotransferase by sucrose density gradient centrifugation of Triton-treated preparations. Furthermore, cholinephosphotransferase (but not ethanol-aminephosphotransferase) itself was partially separated into Mg2+ -requiring and Mn2+ -requiring components. In contrast, ethanolaminephosphotransferase assayed with either Mg2+ or Mn2+ formed a single peak together with Mn2+ -requiring cholinephosphotransferase.

Influence of chain length and unsaturation on the effects of fatty acids on phosphoglyceride biosynthesis in isolated rat and pig hepatocytes

Hepatocytes isolated from rat or pig by collagenase perfusion were incubated with [3H]glcyerol and different albumin-bount fatty acids. Among C22 fatty acids docosahexaenoic acid stimulated phosphatidylethanolamine synthesis in rat hepatocytes most effectively. Addition of docosahexaenoic acid plus either palmitic or stearic acid resulted almost in the same stimulation whereas combinations of this acid with lauric or myristic acid had no effect. Lauric acid and myristic acid alone inhibited phosphatidylethanolamine synthesis. The chain length specificity for monoenoic fatty acids was similar, the hexadecenoic and octadecenoic acids (both cis and trans) being most stimulatory. The addition of 0.2 mM ethanolamine markedly stimulated phosphatidylethanolamine synthesis, but most effects of fatty acids were similar in its presence or absence.