Effect of dietary fats on the delta 6- and delta 5-desaturation of fatty acids in rat liver microsomes.

Rats were given diets containing (% dietary energy): 46 arachis oil (AO), 36 partially-hydrogenated arachis oil (HAO) + 10 AO, 36 partially-hydrogenated marine oil (HMO) + 10 AO, or 46 of a combination of rape-seed oils high and low in erucic acid (RSO + LERSO). In the liver microsomes the content of arachidonic acid (20:4 omega 6) was reduced in the groups given HAO + AO and HMO + AO. The rates of delta 6-desaturation of linoleic acid into gamma-linolenic acid (18:3 omega 6) and of delta 5-desaturation of dihomo-gamma-linolenic acid into arachidonic acid were studied in vitro at two substrate levels: a high substrate level reflecting maximal microsomal desaturase activity in rat liver and a low substrate level reflecting desaturase activity under physiological conditions. Dietary HAO, rich in 18:1 isomers, suppressed the delta 6-desaturase activity but not the delta 5-desaturase activity. Dietary HMO, rich in 18:1, 20:1 and 22:1 isomers, reduced both delta 6- and delta 5-desaturase activities.