Some properties of microsomal fatty acid activating enzyme of rat liver

Abstract

Some properties of microsomal long-chain fatty acid activating enzyme(s) have been studied by hydroxamate formation method or by following formation of [1- 14C]-acyl-CoA from [1- 14C]fatty acids. Unlike with palmitate, hydroxamate formation with laurate requires the presence of lecithin. Lecithin appears to bind fatty acids reversibly and the detergent-type inhibition of the enzyme by excess of fatty acids is thereby prevented. In lecithin-supplemented medium saturated fatty acids from decanoate to stearate were activated at appreciable rates by microsomes of rat liver as well as by homogenates of heart. Separate activity peaks were seen with laurate and palmitate which suggests that more than one enzyme may be involved in the activation of longchain saturated fatty acids. Rates of activation of hexadecanedioate, 12-hydroxystearate, 12-hydroxyoleate, elaidate and oleate are also given. The acyl-CoA synthetase activity, with palmitate and oleate as substrates, is optimal in the pH range 7.2–8.8. At 37 °C, the enzyme(s) is (are) unstable but this effect is independent of pH in the range 7.0–8.8. Presence of high concentration of potassium phosphate or of 5 mM ATP stabilize the enzyme activity. High concentrations of salt increase the apparent K m values for ATP and CoA, as well as the V of the acyl-CoA synthetase reaction. The enzyme shows high affinity for fatty acids but exact determination of K m value is not possible because of marked unspecific binding of free fatty acids to microsomes.