Chemical Characterization of D-Lactate Dehydrogenase from Escherichia coli B

The d-lactate dehydrogenase (d-lactate:diphosphopyridine nucleotide oxidoreductase, EC 1.1.1.28) from Escherichia coli B has been purified to homogeneity. This enzyme, mol wt 115,000, is very sensitive to autoxidation. The amino acid composition has been determined. All 12 of the cysteinyl residues of the enzyme are in the reduced state. Reaction of these cysteinyl residues with arsenite, thiol reagents, or alkylating agents leads to inhibition or inactivation of the enzyme. Pyruvate or diphosphopyridine nucleotide protects the enzyme against alkylation by iodoacetamide. However, the addition of reduced diphosphopyridine nucleotide causes a significant increase in the rate of alkylation by iodoacetamide, suggesting that the binding of the reduced coenzyme to this lactate dehydrogenase causes a conformational change in the protein.