Effects of docosahexaenoic (22:6n-3), tetracosapentaenoic (24:5n-3) and tetracosahexaenoic (24:6n-3) acids on the desaturation and elongation of n-3 polyunsaturated fatty acids in trout liver microsomes

The effects of long chain n-3 polyunsaturated fatty acids (PUFA) on the desaturation and elongation systems involved in the conversion of 18:3n-3 to 24:6n-3 were investigated. Microsomes were prepared from the livers of rainbow trout and incubated with ¹⁴C-labelled 18:3n-3 and cofactors required for elongation and/or desaturation in the presence of 22:6n-3, 24:5n-3 or 24:6n-3. The formation of 24:6n-3 was significantly inhibited in the presence of 50 μM 22:6n-3, 24:5n-3 or 24:6n-3, whereas the amount of radiolabelled 20:5n-3 formed was inhibited by only 24:5n-3 or 24:6n-3 at the same concentration. When malonyl-CoA was omitted from the incubation system to allow the measurement of desaturation in the absence of elongation, the Δ6 desaturation of ¹⁴C-18:3n-3 to ¹⁴C-18:4n-3 was inhibited by approximately 25% in the presence of 24:5n-3 or 24:6n-3 but was not affected by 22:6n-3. The Δ5 desaturation of ¹⁴C-20:4n-3 was not affected by the presence of any of the long chain PUFA and no significant effect of 18:3n-3, 22:6n-3 or 24:6n-3 on the Δ6 desaturation of 24:5n-3 to 24:6n-3 was observed. To permit the measurement of individual elongation reactions, KCN was included in the incubation medium to inhibit desaturation and ¹⁴C-labelled 18:3n-3, 18:4n-3, 20:4n-3, 20:5n-3 and 22:5n-3 were examined as substrates. 18:4n-3 and 22:5n-3 were more extensively used for elongation than 18:3n-3, 20:4n-3 and 20:5n-3. The presence of 22:6n-3, 24:5n-3 or 24:6n-3 in the incubation system had no effect on any of the specific elongations of any of the substrates examined. It is concluded that, in the conversion of 18:3n-3 to 24:6n-3 by trout liver microsomes, the Δ6 desaturation of 18:3n-3 may be subjected to direct feedback inhibition and that 24:5n-3 may be preferred over 18:3n-3 as a substrate for Δ6 desaturation.