The composition and metabolism of molecular species of glycerolipids, including phosphatidic acid (PA), phosphatidylinositol (PI) and diacylglycerol (DG), were studied in four frog retinal fractions prepared by discontinuous sucrose gradient centrifugation. Six glycerolipid classes were isolated from the lipid extracts of each fraction and converted to their corresponding 1,2-diacylglycerol acetates by acetolysis for quantitation of their molecular species by HPLC. Rod outer segments (ROS) showed a distinctive molecular species composition in all glycerolipid classes except phosphatidylcholine (PC). The relative amounts of dipolyunsaturated species in ROS were higher in phosphatidylethanolamine (PE), phosphatidylserine (PS), and PA, compared to the other retinal fractions. PI and DG of ROS had a similar molecular species composition and contained only small amounts of dipolyunsaturated species. A unique feature of the molecular species of ROS PI and DG was that they had high amounts of species containing docosahexaenoic acid (22:6ω3), while PI and DG from the other retinal membranes consisted mostly of species containing arachidonic acid (20:4ω6). Following in vitro incubation of frog retinas with [2-3H] glycerol, the mass and radioactivity distributions among molecular species were determined following HPLC fractionation. The unique species composition of PS in ROS is determined mainly by selective translocation from the inner segments to ROS, since the dpm %, representative of newly synthesized species, was similar to the steady state mole %. In contrast, the dpm % of PE and PA in ROS did not resemble their steady state composition (mole %). Instead, these were similar to newly synthesized species composition of the same glycerolipid classes in the other membrane fractions. This suggests that the distinctive species composition of PE and PA in ROS is determined not by selective translocation from the inner segments, but by remodeling processes taking place in the ROS. However, both the selective translocation and the remodeling processes seem to be responsible for the molecular species compositions of PC, PI, and DG in ROS. The dpm %s of these in ROS were not similar to those of the same glycerolipid classes in the other membrane fractions indicating the selective translocation of the newly synthesized species to the ROS. In addition, the remodeling processes on these glycerolipids were suggested because the dpm %s were not similar to the steady state species compositions of PC, PI, and DG in ROS. © 1990.
