Comparative Effects of Methylmalonyl Coenzyme A on Fatty Acid Synthetase Derived From Rat and Man 1

Summary

The effect of methylmalonyl CoA on fatty acid synthetase activity was compared on enzyme derived from rats and man. Partially purified fatty acid synthetase from both sources was shown to be strikingly similar. Enzyme from both sources catalyzed propionyl CoA equally well (approximately 90% the rate of acetyl CoA at equimolar concentrations), giving rise to odd-chain fatty acids. Neither enzyme catalyzed methylmalonyl CoA in measurable activities over a wide range of concentrations. Michaelis constants for malonyl CoA were essentially the same (K_m = 4 × 10^-5 and 3 × 10^-5 M for rat and human liver, respectively). Methylmalonyl CoA was demon-strated to be a competitive inhibitor of malonyl CoA, with K_i values of 2.4 × 10^-5 M for the enzyme from both sources. Human liver fatty acid synthetase was immunoreactive with antiserum prepared against purified rat liver fatty acid synthetase.