The Ca 2+-dependent biosynthesis of lecithin, phosphatidylethanolamine and phosphatidylserine in rat liver subcellular particles

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1. 1. The Ca 2+-dependent incorporation of choline, ethanolamine and l-serine into rat liver phospholipids has been studied in vitro. The incorporation was located exclusively in the microsomal fraction. 2. 2. No incorporation took place in the presence of EDTA. The optimal Ca 2+ concentration was found to increase with decreasing pH, and vice versa. At pH 8.8 and 0.16 mM Ca 2+ the K m values of choline, ethanolamine and l-serine were 2.3 · 10 -5, 9.5 · 10 -6 and 6.2 · 10 -5 M, respectively. At pH 7.5 the K m and V for both choline and ethanolamine increased with increasing Ca 2+ concentration. At pH 8.8 high Ca 2+ concentrations were inhibitory. The K m and V of l-serine showed no significant changes. 3. 3. Choline incorporation was competitively inhibited by ethanolamine and l-serine; incorporation of l-serine was competitively inhibited by choline and ethanolamine. In contrast, ethanolamine incorporation was inhibited noncompetitively by l-serine and uncompetitively by choline. This indicates that the Ca 2+-stimulated incorporation of choline, ethanolamine and l-serine does not occur via a single common phosphatidyl-enzyme complex. 4. 4. Ethanolamine is incorporated preferentially into polyunsaturated phosphatidylethanolamine species in rat liver microsomes. The rate of incorporation into hexa-unsaturated species was twice that into mono- and di-unsaturated species, while choline was incorporated 10–15 times more rapidly into the hexa-unsaturated species. 5. 5. The results show that the incorporation rate of l-serine found in vitro can account for the rate of phosphatidylserine synthesis observed in vivo. The Ca 2+-dependent mechanism thus seems to be the major pathway of phosphatidylserine synthesis in the liver. In contrast, the synthesis of lecithin and phosphatidylethanolamine via this mechanism must be quantitatively negligible. 6. 6. A metabolic scheme is discussed which implies interconversion of the respective phospholipids.