KINETIC SELECTIVITY OF CHOLINEPHOSPHOTRANSFERASE IN MOUSE-LIVER - THE K(M) FOR CDP-CHOLINE DEPENDS ON DIACYLGLYCEROL STRUCTURE

The effects of different 1,2-diacyl-sn-glycerols on the kinetic properties of CDP-choline: 1,2-diacylglycerol cholinephosphotransferase (EC 2.7.8.2) from mouse liver microsomes have been studied. Initial-velocity experiments were carried out with various concentrations of several species of diacylglycerol at different fixed concentrations of CDP-choline. Kinetic analysis of these data showed a family of intersecting lines consistent with a sequential kinetic mechanism of catalysis. The K(m) and V(max.) values derived from rate data revealed a pronounced effect of diacylglycerol species utilization on the K(m) value for CDP-choline. There was a biphasic relationship between diacylglycerol chain length and the K(m) for CDP-choline. Substitution of an unsaturated fatty acid in the sn-2 position of distearin also dramatically increased the CDP-choline K(m) value as well as the V(max). 1,2-Dipalmitoyl-sn-glycerol was the preferred substrate over other disaturated species, but 1,2-dihexanoyl-sn-glycerol could not be utilized. These results demonstrate the kinetic mechanism of in vitro catalysis and suggest a regulatory role for CDP-choline concentration in the diacylglycerol species selectivity of cholinephosphotransferase resulting in the de novo biosynthesis of different molecular species of phosphatidylcholine.