To characterize the molecular species composition of ethanolamine glycerophospholipids (EGP) in the primate retina and to examine the effects of different dietary fats, the authors fed rhesus monkeys diets containing widely ranging amounts of n-3 fatty acids.
From birth, infant monkeys were fed either a control soybean oil diet, containing 8% of total fatty acids as 18:3 (n-3), or a safflower oil-based n-3 fatty acid deficient diet containing < 0.4% 18:3 (n-3). A subset of the n-3 deficient group was later repleted with 1.6% ethyl docosahexaenoate, 22:6 (n-3), starting at 10 months of age. Tissues were taken from all monkeys upon termination at 21 to 51 months of age. The diacyl, alkenylacyl, and alkylacyl EGPs were quantitated by high-pressure liquid chromatography (HPLC).
Twenty-eight molecular species were identified in the retina of control monkeys. Ether phospholipids comprised 36% of the retinal ethanolamine glycerophospholipids. Species containing polyunsaturated fatty acids in both the sn-1 and sn-2 positions (dipolyenes) were present only in the diacyl subclass and comprised 16% of the total species. Species having n-3 fatty acids in the sn-2 position contributed 59%, 36%, and 70% of total species in the diacyl, alkenylacyl, and alkylacyl subclasses, respectively. In the molecular species of the n-3 fatty acid deficient monkeys, the major change was the loss of most of the 18:0-22:6(n-3) species and its partial replacement with 18:0-22:5(n-6). In contrast, the species 18:1-22:6(n-3) decreased only slightly, from 6.2% to 4.8% of total diacyl species. Although the total concentration of dipolyenes (15% to 20% of the total species) was not affected by diet, their fatty acid compositions were changed drastically. The dipolyene species 22:6(n-3)-22:6(n-3) nearly disappeared in the n-3 deficient monkeys. Concomitantly, two new species, 22:5(n-6)-22:6(n-3) and 22:5(n-6)-22:5(n-6), appeared at 2.6% and 2.0%, respectively. Deficient monkeys given the ethyl ester of 22:6(n-3) in the diet recovered to a near-normal molecular species composition, except in the ether lipids, in which 16:0-20:4 remained low.
Diets of differing n-3 fatty acid content had profound qualitative and quantitative effects on the molecular species of retinal phospholipids, and the replacement of 22:6(n-3) by 22:5(n-6) in the retinas of n-3 deficient monkeys was asymmetric and functionally incomplete.