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.