Differential responses to fasting and subsequent feeding by microsomal systems of rat liver: 6- and 9-desaturation of fatty acids.

Hepatic microsomal preparations from nonfasted, fasted, and fasted-fed rats were employed, together with cofactors, in studies of 9-desaturation of stearate-1-(14)C and 6-desaturation of linolenate-1-(14)C. Prior fasting sharply reduced 9-desaturation but did not affect 6-desaturation; feeding restored 9-desaturation. Position of desaturation was determined by permanganate-periodate oxidation and separation of the dicarboxylic acids. Feeding after fasting stimulated both desaturase systems but either dl-ethionine or actinomycin D prevented this. Dietary carbohydrate or saturated fat increased 9-desaturation and dietary protein increased 6-desaturation. Insulin treatment of nonfasted rats increased 9-desaturation but not 6-desaturation. High dietary unsaturated fat (20% safflower oil) stimulated 6-desaturation but inhibited the 9-desaturation response to feeding. The results indicate that the two desaturases are distinct and are inducible in response to different substances.