Long and very long chain polyunsaturated fatty acids of the n-6 series in rat seminiferous tubules. Active desaturation of 24:4n-6 to 24:5n-6 and concomitant formation of odd and even chain tetraenoic and pentaenoic fatty acids up to C32.

The formation of long and very long chain (VLC) n-6 polyunsaturated fatty acids (PUFA) in isolated rat seminiferous tubules was investigated by following the metabolism of three 1-14C-labeled n-6 tetraenoic fatty acids (20:4, 24:4, and 32:4) and [U-14C]acetate. In contrast to [14C]32:4, which was poorly incorporated and altered, [14C]20:4 and [14C]24:4 were efficiently taken up by the tubules, esterified into lipids, elongated to VLCPUFA, and desaturated to pentaenoic fatty acids; the rate of [14C]24:4 desaturation to [14C]24:5 was notably high. The main products with [14C]acetate as precursor were labeled saturates and VLCPUFA, most of the label in tetraenoic and pentaenoic acids appearing in 24:4 and 24:5, respectively. These two C24 polyenes, connected by a delta 6 desaturation, may play a central role in n-6 PUFA metabolism, in their capacity as potential precursors of longer polyenes via elongation and of shorter ones, such as 22:5n-6, via retroconversion. Triacylglycerols, rich in C22 and C24 polyenes, incorporated the greatest amounts of both [14C] acetate-derived and exogenous 14C-PUFA, suggesting that this lipid class is involved in the traffic and metabolism of testicular PUFA. The detection of a series of unusual odd-chain tetraenoic and pentaenoic acids, also labeled with [U-14C]acetate, suggests that a PUFA chain shortening mechanism occurs in testis involving alpha- in addition to beta-oxidation. We speculate that alpha-oxidation plays a role in the retroconversion of PUFA.