The pathway from arachidonic to docosapentaenoic acid (20 : 4n-6 to 22 : 5n-6) and from eicosapentaenoic to docosahexaenoic acid (20 : 5n-3 to 22 : 6n-3) studied in testicular cells from immature rats

The concentration-dependent metabolism of 1-C-14-labelled precursors of 22: 5n-6 and 22:6n-3 was compared in rat testis cells. The amounts of [C-14]22- and 24-carbon metabolites were measured by HPLC. The conversion of [1-C-14 ]20:5n-3 to [3-C-14]22:6n-3 was more efficient than that of [1-C-14]20:4n-6 to [3-C-14]22:5n-6. At low substrate concentration (4 mu M) it was 3.4 tim es more efficient, reduced to 2.3 times at high substrate concentration (40 mu M). The conversion of [1-C-14]22:5n-3 to [1-C-14]22:6n-3 was 1.7 times more efficient than that of [1-C-14]22:4n-6 to [1-C-14]22:5n-6 using a low, but almost equally efficient using a high substrate concentration. When un labelled 20:5n-3 was added to a cell suspension incubated with [1-C-14]20:4 n-6 or unlabelled 22:5n-3 to a cell suspension incubated with [1-C-14]22:4n -6, the unlabelled n-3 fatty acids strongly inhibited the conversion of [1- C-14]20:4n-6 or [1-C-14]22:4n-6 to [C-14]22:5n-6. In the reciprocal experim ent, unlabelled 20:4n-6 and 22:4n-6 only weakly inhibited the conversion of [1-C-14]20:5n-3 and [1-C-14]22:5n-3 to [C-14]22:6n-3. The results indicate that if both n-6 and n-3 fatty acids are present, the n-3 fatty acids are preferred over the n-6 fatty acids in the elongation from 20- to 22- and fr om 22- to 24-carbon atom fatty acids. In vivo the demand for 22-carbon fatt y acids for spermatogenesis in the rat may exceed the supply of n-3 precurs ors and thus facilitate the formation of 22:5n-6 from the more abundant n-6 precursors. (C) 2000 Elsevier Science B.V. All rights reserved.