Nicotinic acid metabolism: stereochemical course of the (2R,3S)-2,3-dimethylmalate lyase reaction.

1. The enantiomers of chiral propionate were prepared by cleavage of (2R,3S)-2,3-dimethylmalate and of (2R,3S)-[3-3H1]-2,3-dimethylmalate with the corresponding lyase in tritiated and in ordinary water, respectively. 2. Using commercially available citrate lyase, an enzymic method was elaborated for the formation of (3S)-citryl-CoA from acetyl-CoA and citrate in a preparative scale (mumol). The product was used in the presence of propionate (or acetate) and partially inactivated citrate lyase to prepare propionyl-CoA (or acetyl-CoA). The method was applied to the chiral propionates as well as symmetrically labelled [2-3H2]propionate and is suggested to be superior to others for the preparation of propionyl-CoA (or acetyl-CoA) from mumol of radioactively labelled acid. 3. On configurational determination with propionyl-CoA carboxylase the [2-3H12]propionyl-CoA derived from unlabelled dimethylmalate retained nearly all of the tritium label, whereas that derived from the labelled substrate lost the tritium label completely; [2-3H2]propionyl-CoA lost 50% of its label. 4. It was concluded from these results that the cleavage reaction on (2R,3S)-2,3-dimethylmalate lyase proceeds with inversion of configuration at C-3 of the substrate which becomes C-2 of propionate.