Studies in the biosynthesis of hepatic and biliary lecithins

Male rats with biliary cannulae were injected with linoleate-1-(14)C, stearate-1-(14)C, palmitate-9-10-(3)H, phosphate-(32)P, l-methionine-methyl-(14)C, and choline-methyl-(3)H in various combinations and the incorporation of these isotopes into the phospholipids of liver, bile, and plasma was determined for 1-4 hr. The results summarized below favor the view (a) that exchange of saturated fatty acids plays a role in the formation of lecithins; (b) that the unsaturated fatty acids do not undergo significant exchange and determine the pathway of biosynthesis of lecithins; and (c) that there is either more than one pool of CDP-choline in liver or a pathway of biosynthesis of lecithin from choline not involving CDP-choline as an intermediate. Linoleoyl lecithin of liver attained higher specific activity with respect to phosphate-(32)P and choline-methyl-(3)H than did arachidonoyl lecithin. Lecithin in bile attained higher specific activities with respect to phosphate-(32)P, choline-methyl-(3)H, and linoleate-1-(14)C than the corresponding hepatic lecithins. Stearate-1-(14)C and palmitate-9-10-(3)H attained highest specific activities in the hepatic lecithin fraction rich in arachidonic acid. The specific activity of hepatic phosphatidyl ethanolamine was lower with respect to saturated fatty acids, but much higher with respect to (32)P than any lecithin. The ratio of specific activity of (3)H in methyl groups from choline to (14)C in methyl groups from methionine in hepatic sphingomyelin was lower than in hepatic linoleoyl lecithin.