Kinetic Studies on the Chain Length Specificity of Long Chain Acyl Coenzyme A Synthetase from Rat Liver Microsomes

The kinetics of activation of saturated fatty acids by long chain acyl-CoA synthetase from rat liver microsomes has been studied with a method of selective extraction of free fatty acids based on the insolubility of acyl-CoA in diethyl ether. Saturated fatty acids with a chain length ranging from C18 to C6 were assayed at concentrations varying from 0.5 to 10 µm. Under these conditions, Vmax is maximum for palmitic, myristic, and lauric acids, and decreased drastically with octanoic and hexanoic acids. As judged by the values of Km, the affinity of long chain acyl-CoA synthetase is greatest with palmitic acid and decreases very progressively as the chain length of the fatty acids decreases. Finally, palmitic acid is a competitive inhibitor of the activation of octanoic acid. From these results, it has been concluded that there is evidence for only one long chain acyl-CoA synthetase in rat liver microsomes, and that this enzyme has a rather broad chain length specificity.