Differential incorporation of docosahexaenoic and arachidonic acids in frog retinal pigment epithelium.
Vertebrate retinas contain a high level of docosahexaenoic acid (DHA, 22:6n-3) and a relatively low level of arachidonic acid (AA, 20:4n-6). Although it is not known how DHA is selectively concentrated in the retina, the retinal pigment epithelium (RPE) is intimately involved in this process as these cells phagocytize shed rod outer segments (containing DHA) and take up DHA from the circulation. To explore the role of the RPE in the DHA enrichment in the retina, native frog RPE cells (RPE-eyecups) were incubated with [3H]DHA or [3H]AA for short times, or pulsed for up to 1 h with the two labeled fatty acids plus labeled glycerol simultaneously, followed by a chase for up to 7 h. The incorporation of label into various lipids was monitored by thin-layer or high performance liquid chromatography. In short-time incubations, a greater amount of DHA was incorporated into neutral lipids (primarily triglycerides, TG) compared with AA, while similar amounts of both fatty acids were found in phospholipids. On a relative basis, DHA was esterified mainly into TG, with AA into phospholipids. The uptake of each fatty acid correlated linearly with concentration (30-120 nM) and the incorporation pattern was distinct. Interestingly, the uptake and incorporation of labeled DHA or AA were not affected by the presence of unlabeled AA or DHA, respectively. In a pulse-chase experiment, RPE cells were active in the de novo synthesis of glycerolipids, particularly TG, which achieved a specific activity as high as diglycerides and ten times higher than the major phospholipids. The incorporation of DHA into most glycerolipids occurred to a large extent during de novo glycerolipid synthesis, whereas AA was incorporated mainly by deacylation-reacylation reactions. Our results suggest different metabolic pathways for handling DHA and AA in frog RPE. The preferential incorporation of DHA into TG suggests that TG might play an important role in the selective enrichment of DHA in the retina and in the recycling of photoreceptor DHA between the RPE and the retina.