Effect of dietary fatty acid composition on the biosynthesis of unsaturated fatty acids in rat liver microsomes.

Abstract

AbstractA study was made of the influence of semisynthetic diets of low and high unsaturation on the fatty acid composition and desaturation‐chain elongation enzymatic activity of the liver microsomal fractions of male Sprague‐Dawley rats of different ages. Groups of rats were fed 5 or 20% coconut oil (CO), or a 5 or 20% mixture of corn and menhaden oils (3∶7) (CME) from weaning to 100 wk of age. Growth rate and food consumption were measured during this period in which animals were sacrificed at 36, 57, 77 and 100 wk of age. Both the level and composition of the dietary fat supplements produced marked effects on the fatty acid composition of the liver microsomal lipids. In general, the fatty acid composition of the microsomal fractions reflected that of the dietary fat and was more unsaturated with the higher level of fat fed. The rate of conversion of linoleic to arachidonic acid in assays performed in vitro with liver microsomal preparations from animals of the different groups also showed marked differences. The 6‐desaturase‐chain elongation activity was higher in the 5% than 20% group and corresponded to the essential fatty acid (EFA) status of the animals in these groups as represented by the triene‐tetraene ratio of the microsomal lipid. The relationship of the 6‐desaturase activity to fatty acid composition of the microsomal lipid indicated that if varied directly with the level of 20∶3ω9, 18∶1 and 16∶1 and was inhibited by arachidonic acid. The activity of the 6‐desaturase enzyme system was lowest in the liver microsomal fraction obtained from the animals fed the CME diets and appeared to be suppressed by the high levels of 20∶5 and 22∶6 that accumulated in the microsomal lipid. Accordingly, the levels of arachidonic acid were lower in the microsomal lipid of these groups than those of the corresponding CO groups in spite of a greater abundance of linoleic acid in the diet. The data suggest that the activity of the 6‐desaturase‐chain elongation system is regulated by the fatty acid composition of the microsomal lipid as influenced by the composition of the dietary fat.