Long Chain Fatty Acid Activation in Subcellular Preparations from Rat Liver

Abstract

Abstract A study of the activation of several long chain saturated and unsaturated fatty acids by rat liver preparations was undertaken. Investigation of reaction requirements and appropriate modifications revealed that rat liver is extremely active in long chain acyl-coenzyme A synthetase or synthetases (acid: CoA ligase (AMP), EC 6.2.1.3) activity. With palmitate as substrate, the cell membrane fraction was found richest in activating enzyme, with the microsomal fraction next. In the microsomal activation of saturated fatty acids, maximal activity was reached with palmitate. The coenzyme A requirements found were very similar with palmitate, stearate, and erucate (Km 3.3 to 3.9 x 10-5 m), but these were markedly different from the coenzyme A requirements with oleate, linoleate, linolenate, and arachidonate (Km 1.56 to 3.3 x 10-4 m); this suggests the possible occurrence of at least two different long chain acyl-CoA synthetases in rat liver microsomes. In experiments with oleate, linoleate, linolenate, and arachidonate, the activating enzyme was found to be markedly susceptible to substrate inhibition, which complicated exact assay. The stimulation of activation of these unsaturated fatty acids by a heated microsomal fraction was traced mainly to their fatty acid-binding ability. Liver phospholipids greatly stimulated oleate activation, and this was thought to be due to the association of oleate with phospholipid. For the palmitate-activating enzyme, it appeared that bound phospholipid was essential for activity.