A study of the effect of alpha-linolenic acid on the desaturation of dihomo-gamma-linolenic acid using rat liver homogenates.

The effects of alpha-linolenic acid on delta 5 desaturation of 20: 3 n-6 dihomo-gamma-linolenic acid) to 20: 4 n-6 (arachidonic acid) and on delta 6 desaturation of 18: 2 n-6 (linoleic acid) to 18:3 n-6 (gamma-linolenic acid) were investigated in two different studies using rat liver homogenates. In the first study, rats were fed a semi-synthetic fat-free diet. When unlabelled 18:3 n-3 was added to the incubation of 14C 18:2 or 14C 20:3, the desaturation rates decreased. delta 6 Desaturation was more strongly inhibited than delta 5 desaturation. The position isomer, gamma-linolenic acid, had no significant effect on delta 6 and delta 5 desaturation. In the second study, the rats were fed a diet including alpha-linolenic acid in the form of rapeseed oil containing no erucic acid. The control animals were fed a peanut oil diet. When alpha-linolenic-fed rats were compared to control animals, we found that delta 6 desaturase activity was lower in the alpha-linolenic-fed animals. In contrast, there was no significant effect on the desaturation rate when 14C 20:3 n-6 was used as a substrate. Our results lead to the conclusion that alpha-linolenic acid, when given to rats as dietary rapeseed oil, partly inhibited delta 6 desaturation in vitro, but did not affect significantly subsequent delta 5 desaturation in the biosynthesis of arachidonic acid. The inhibition of the delta 5 desaturation step, when alpha-linolenic acid was added to the incubation, might be due to an unknown competitive mechanism. These observations have been discussed in relation with the current literature.