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The metabolism of phosphatidylserine (PS), phosphatidylethanolamine (PE) and phosphatidylcholine (PC), derived from [3H]serine, has been studied in rat hepatocytes. After an initial pulse with radioactivity for 10 min and a chase for up to 240 min, cells were harvested and PS, PE and PC isolated. At the end of the pulse, greater than 90% of [3H]serine derived phospholipid radioactivity was associated with PS. In the subsequent chase, newly-made PS was degraded rapidly with less than 25% of the label lost from PS appearing in the PE and PC pools. In contrast, [3H]serine-labeled PE turnover was not detectable. Very little newly-made PS was converted to PC. PE and PC were further fractionated into molecular species by high-performance liquid chromatography. We report that [3H]serine-labeled PE is deacylated/reacylated with the major product of remodeling being 18:0-20:4 PE. In contrast, [3H]serine-labeled PC is not significantly remodeled.

The remodeling of the fatty acyl moieties of phosphatidylcholine (PC) has been studied in choline-deficient and choline-supplemented hepatocytes prepared from a choline-deficient rat. Choline-deficient hepatocytes were prelabeled with [Me-3H]choline for 30 min and subsequently incubated for up to 12 h in the presence or absence of choline. Analysis of the molecular species of PC from choline-deficient cells showed that, at the end of the pulse, approx. 75% of the label was incorporated into palmitate-containing species and only approx. 16% of the labeled species contained stearate. During the chase period there was a redistribution of label and after 12 h approx. 56% of the total radioactivity was associated with palmitate containing species and 37% was recovered in stearate-containing species. A similar distribution of radioactivity was observed in choline-supplemented cells. Measurement of the specific radioactivity of the major molecular species of PC was consistent with a precursor-product relationship between palmitate-containing species and stearate-containing species with arachidonate or linoleate on the sn-2 position. A model is presented which takes into account remodeling of both the sn-1 and sn-2 positions of PC.