Abstract
Docosahexaenoic acid (22:6ω3) is uniquely enriched in photoreceptor outer segment phospholipids, comprising up to one-half of the fatty acids of phosphatidylethanolamine and phosphatidylserine. The current study was designed to investigate the incorporation of 22:6ω3 into outer segment phospholipids over 12 days and to determine whether the retina contained the enzymes necessary for elongation and desaturation of the major dietary precursor of 22:6ω3, the essential fatty acid linolenic acid (18:3ω3). Sprague-Dawley rats were injected intravitreally with [ 14C]22:6ω3 or [ 14C]18:3ω3 and kept in cyclic light ( 12 hr light 12 hr dark ) for 2 hr to 12 days. Phospholipids from rod outer segments and the remaining retinal debris were separated by two-dimensional thin-layer chromatography. [ 14C]22:6ω3 radioactivity was initially highest in phosphatidylcholine and rapidly decreased from 45% of total phospholipid labeling at 2 hr to 26% by 1 and 3 days in ROS, while phosphatidylethanolamine labeling increased from 49 to 68% by 3 days and phosphatidylserine labeling increased from 3 to 14% over 12 days. Phenacyl derivatives of total fatty acids were separated by HPLC. A substantial conversion of [ 14C]18:3 to [ 14C]20:5, [ 14C]22:5 and [ 14C]22:6 was noted after 1 day, with increasing conversion to [ 14C]22:6 over the 12-day period. When only one eye was injected with [ 14C]18:3ω3, negligible radioactive fatty acids were detected in the contralateral eye from 1 to 12 days post-injection demonstrating that conversion of 18:3 to 22:6 occurred primarily within the injected eye. All enzymes for elongation and desaturation of 18:3 to 22:6 appear to be present in the eye. However, the conversion of 22:5 to 22:6 by delta-4 desaturase is evidently rate-limiting and may affect phopholipid replacement during photoreceptor outer segment renewal if this pathway proves to be essential for the supply of 22:6 during disk membrane formation.
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