Compartmentalization of phospholipids for lipoprotein assembly on the basis of molecular species and biosynthetic origin.

Specific pools of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) are compartmentalized on the basis of their biosynthetic origin for assembly into lipoproteins by rat hepatocytes in culture (Vance, J.E. and Vance, D.E. (1986) J. Biol. Chem. 261, 4486-4491). The present experiments confirm that PC and PE derived from serine, rather than from ethanolamine, are preferred for assembly into lipoproteins. One possible explanation for this compartmentalization is that defined molecular species of phospholipids are selected for secretion. Thus, the molecular species distribution of PC and PE of cultured rat hepatocytes incubated with one of [3H]choline, [3H]ethanolamine or [3H]serine was compared in the cells and in the lipoproteins secreted into the culture medium using high-performance liquid chromatography. The percent distribution of molecular species of PC and PE labeled from ethanolamine was the same in the cells and the medium. On the contrary, the percent distribution of molecular species of PC derived from [3H]choline, and PC and PE derived from [3H]serine, was different in the cells and secreted lipoproteins; the species 1-stearoyl-2-arachidonyl-PC and -PE were selectively not secreted. However, selection of defined molecular species of cellular PC labeled from serine would only account for the specific radioactivity of secreted PC labeled from serine being 46% higher than in the hepatocytes at 4 h, whereas the specific radioactivity of medium PC was actually 300% higher than in the cells. A comparison of the labeling pattern of molecular species of PC and PE using [3H]serine and [3H]ethanolamine as precursors suggests that PE methyltransferase may prefer certain species of PE as substrate if the PE was derived from either serine or ethanolamine.