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 fa tty acids (PUFA) in isolated rat seminiferous tubules was investigated by following the metabolism of three 1-C-14-labeled n-6 tetraenoic fa tty acids (20:4, 24:4, and 32:4) and [U-C-14]acetate. In contrast to [ C-14]32:4, which was poorly incorporated and altered, [C-14]20:4 and [ C-14]24:4 were efficiently taken up by the tubules, esterified into li pids, elongated to VLCPUFA, and desaturated to pentaenoic fatty acids; the rate of [C-14]24:4 desaturation to [C-14]24:5 was notably high. T he main products with [C-14]acetate as precursor were labeled saturate s and VLCPUFA, most of the label in tetraenoic and pentaenoic acids ap pearing in 24:4 and 24:5, respectively. These two C24 polyenes, connec ted by a DELTA6 desaturation, may play a central role in n-6 PUFA meta bolism, in their capacity as potential precursors of longer polyenes v ia elongation and of shorter ones, such as 22:5n-6, via retro-conversi on. Triacylglycerols, rich in C22 and C24 polyenes, incorporated the g reatest amounts of both [C-14] acetate-derived and exogenous C-14-PUFA , suggesting that this lipid class is involved in the traffic and meta bolism of testicular PUFA. The detection of a series of unusual odd-ch ain tetraenoic and pentaenoic acids, also labeled with [U-C-14]acetate , suggests that a PUFA chain shortening mechanism occurs in testis inv olving alpha- in addition to beta-oxidation. We speculate that alpha-o xidation plays a role in the retroconversion of PUFA.