Long-chain acyl-CoA synthetase of rat testis microsomes. Substrate specificity and hormonal regulation

We investigated long-chain fatty-acyl-CoA synthetase activity in rat testicular microsomes. The apparent Michaelis constants (Km's) for the substrate fatty acids increased while their corresponding maximal velocities decreased in the order 18:3(n-3), 20:3(n-6), and 18:0. The reaction with 20:3 as substrate was diminished in the presence of a constant amount of either 18:0, 18:2(n-6), or 18:3(n-3) in a manner consistent with their action as simple competitive inhibitors, with the Ki values for 18:0 and 18:3(n-3) being of the same order of magnitude as their respective Km's. Adrenocorticotrophin and/or dexamethasone administration to intact rats caused a significant decrease in the thioesterification of all three substrates without producing any alteration in the fatty-acid composition of the microsomal membranes. These results indicate the presence of a broad-specificity activating enzyme in testis whose function is subject to hormonal regulation.