Biosynthesis of docosahexaenoate-containing glycerolipid molecular species in the retina

Vertebrate retinas are highly enriched in long-chain polyunsaturated fatty acids (PUFA), especially docosahexaenoic acid (22:6n-3, DHA). In the present study, we investigated the role of de novo synthesis in the enrichment of 22:6n-3 in characteristic molecular species of retinal glycerolipids. Following the incubation of fresh dark-adapted retinas with [2-(3)H]-glycerol, individual glycerolipids were isolated and converted into either diacylglycerol acetates (DGAC) or diacylglycerol benzoates (DGBZ), followed by high-performance liquid chromatography (HPLC) and flow-through radioactivity detection. Total lipids from rat retinas incubated with [3H]-glycerol were analyzed. Unlike what was observed with frog retinas, relative larger of amounts of di-22:6 molecular species were synthesized de novo. In both rat and frog retinas, there was synthesis of glycerolipid molecular species containing two PUFA (one of which was 22:6) in larger amounts than predicted by their steady-state mass levels. These results demonstrate that the unique molecular species of retinal glycerolipids are derived only in part through de novo synthesis, but that molecular rearrangement (remodeling) and differential turnover must also play a role in maintaining the high levels of 22:6 found in rod phohtoreceptor outer segments (ROS) membranes.