ACTIVE SYNTHESIS OF C-24 5,C-N-3 FATTY-ACID IN RETINA/

The formation of C-14-labelled long-chain and very-long-chain (n-3) pe ntaenoic and hexaenoic fatty acids was studied in bovine retina by fol lowing the metabolism of [C-14]-docosapentaenoate [C-22:5,C- n-8 fatty acid (22:5 n-3)], [C-14]-docosahexaenoate (22:6 n-3), and [C-14]aceta te. With similar amounts of 22.5 n-3 and 22:6 n-3 as substrates. the f ormer was actively transformed into 24:5 n-3, whereas the latter was v irtually unmodified. Labelled 24:5, 26:5, 24:6 and 22:6 were formed fr om [1-C-14]22:5 n-3, showing that pentaenoic fatty acids including 24: 5 n-3 can be elongated and desaturated within the retina. When retinal microsomes were incubated with [1-C-14]22:5 n-3, 24:5 n-3 was the onl y fatty acid formed. In retinas incubated with [C-14]acetate, 24:5 n-3 was the most highly labelled fatty acid among the polyenes synthesize d, 24:6 n-3 being a minor product. Such selectivity in the elongation of two fatty acids identical in length, 22:5 n-3 and 22:6 n-3, despite the fact that 22:5 is a minor and 22:6 a major fatty acid constituent of retina, suggests that the active formation of 24:5 n-3 plays a key role in n-3 polyunsaturated fatty acid (PUFA) metabolism. This compou nd might give rise to even longer pentaenes via elongation, and to the major PUFAs of retina, 22:6 n-3, by 6-desaturation and chain shorteni ng. Of all retinal lipids, a minor component, triacylglycerol (TG), in corporated the largest amounts of [C-14]22:5 and 22:6. TG also concent rated most of the [C-14]24:5 formed in retina, whether from [C-14]22:5 n-3 or from [C-14]acetate, suggesting an important role for this lipi d in supporting PUFA metabolism and the synthesis of 22:6 n-3.