Metabolism of phosphatidylcholine in the frog retina

The biosynthesis and the turnover of phosphatidylcholine were studied in the frog retina following either (a) injection into the animal of 32PO4, 33PO4, [1,3-3H]glycerol, [2-3H]glycerol, or [methyl-3H]choline, or (b) incubation of isolated retinas in solutions containing [methyl-3H]choline. 1. Examination of the pools of lipid precursors in the retina demonstrated that the choline and phosphate pools are long-lived compared to the glycerol pool, which is metabolically very active and turns over rapidly. 2. The peak in specific activity of phosphatidylcholine synthesized from labeled glycerol occurred earlier, and was higher in the microsomal fraction than in the rod outer segments, which is consistent with synthesis of phosphatidylcholine on the microsomes of the inner segment and subsequent incorporation into the rod outer segments. 3. Autoradiography of retinas incubated in vitro with tritiated choline revealed a diffuse labeling pattern in the rod outer segments. Biochemical studies following injection of labeled glycerol showed an exponential decline in specific radioactivity of phosphatidylcholine in the rod outer segments, which is consistent with a diffuse labeling of these membranes. 4. The half-life of phosphatidylcholine in the rod outer segments synthesized from labeled glycerol was found to be 18-19 days. Based on these values, calculations were made which indicated that phosphatidylcholine in the outer segments is turning over faster than integral disc membrane proteins.