Biosynthesis of Phosphatidylcholines in Rat Liver

The incorporation of [1,2‐ 14 C 2 ]choline and methyl groups from L‐[ Me ‐ 14 C]methionine into subfractions of phosphatidylcholines of liver and blood plasma in the rat has been studied. Fractionation by thin‐layer chromatography on AgNO 3 ‐impregnated plates followed by reversedphase partition thin‐layer chromatography permitted the isolation of 8 different molecular types of phosphatidylcholines. In the liver, incorporation of [1,2‐ 14 C 2 ]choline occurred most rapidly in palmitoyl‐oleoyl and palmitoyl‐linoleoyl phosphatidylcholine but was very slow in the stearoyl‐arachidonoyl species. Initially, after injection of [1,2‐ 14 C 2 ]choline, the distribution of isotope in the subfractions of different degree of unsaturation of the phosphatidylcholines in blood plasma was very similar to that of the phosphatidylcholines in the liver. Approximately 80% of the incorporated isotope was found in the mono‐ and dienoic fractions. After administration of L‐[ Me ‐ 14 C]methionine the tetra and hexaenoic liver phosphatidylcholines contained 75 to 85% of the total [ 14 C]phosphatidylcholine. The proportion of isotope in these fractions of blood plasma was lower. Among the polyunsaturated phosphatidylcholines in liver those containing palmitic and docosahexaenoic acid were found to incorporate isotope from L‐[ Me ‐ 14 C]methionine to a greater extent than would be expected from a stoichiometric methylation of all types of phosphatidylethanolamines. The stearoyl‐arachidonoyl subfraction exhibited the lowest metabolic reactivity. In both male and female rats the amounts per whole liver of the mono‐ and dienoic fractions were greatly reduced during fasting.