Delta 6 and delta 5 desaturase activities in liver from obese Zucker rats at different ages.

delta 6 Desaturation of linoleic acid (18:2 n-6) and delta 5 desaturation of dihomo-gamma-linolenic acid (20:3 n-6) were measured in liver microsomes from genetically obese Zucker rats (fa/fa) and from their lean littermates (Fa/--). Both groups were fed a balanced commercial diet. The rats were 6, 9 and 12 weeks old, which corresponded to stages in their active growth period. The content of total fatty acids and n-6 polyunsaturated fatty acids in whole liver and liver microsomes was also determined in order to ascertain how the desaturase activities measured in vitro reflected regulation of essential fatty acid metabolism in vivo. Contrary to values obtained for delta 6 desaturation, delta 5 desaturation at nonsaturating substrate levels were lower in obese rats than in lean controls. In contrast, at saturating substrate level, the maximal delta 5 desaturase activities were the same in both phenotypes and they increased with age. Study of delta 5 desaturation kinetics (1/V vs 1/S) showed that Vm did not differ between 12-week-old obese and lean rats, whereas KM in obese rats was much lower than in controls, expressing the very low affinity of the enzyme for the substrate in obese animals. The fatty acid composition of liver lipids reflected the results of desaturase activities in vitro. In particular, the ratios 20:4 n-6/20:3 n-6 were lower in obese rats than in lean rats, which can be explained by the lower conversion of 20:3 n-6 into 20:4 n-6 by delta 5 desaturation.(ABSTRACT TRUNCATED AT 250 WORDS)