Metabolism of Propionic Acid in Animal Tissues

The conversion of propionyl coenzyme A to succinyl-CoA involves carboxylation to methylmalonyl-Cob (a), isomerization of methyhnalonyECoA (a) to its enantiomorph methyhnalonylCoA (b), and isomerization of the latter to succinyl-CoA (3). Since methylmalonyl-CoA (a) has recently been shown to have the D configuration (4, 5), the reversible reaction catalyzed by methylmalonyl-CoA racemase will henceforth be written as n-methylmalonyl-CoA e L-methylmalonyl-CoA and that catalyzed by methylmalonyl-CoA mutasel as L-methylmalonylCoA G succinyl-CoA. Methylmalonyl-CoA mutase is present in animal tissues and in propionic acid bacteria, and its activity is dependent on the presence of cobamide coenzyme which is firmly attached to the mutase of animal origin. A 5000-fold purification of the sheep liver holoenzyme was reported previously (3). However, this preparation was only about 70% pure, as judged by sedimentation in the ultracentrifuge, and the amounts then obtained were insufficient for study of the properties of the enzyme. An ultracentrifugally homogeneous preparation of the sheep liver holoenzyme has now been obtained, and the apoenzyme has been prepared by resolution with acid in the presence of ammonium sulfate (6). A report of the properties of the enzyme, including molecular weight, coenzyme content and kinetics, the equilibrium constant of the reaction, and the effect of inhibitors, is the subject of this paper. The apoenzyme has been found to be highly sensitive to SH-binding reagents.