Purification and characterization of the malate dehydrogenase from Streptomyces aureofaciens

The malate dehydrogenase (MDH) from Streptomyces aureofaciens was purified to homogeneity and its physical and biochemical properties were studied. Its amino-terminal sequence perfectly matched the amino-terminal sequence of the MDH from Streptomyces atratus whose biochemical characteristics have never been determined. The molecular mass of the native enzyme, estimated by size-exclusion chromatography, was 70 kDa. The protein was a homodimer, with a 38-kDa subunit molecular mass. It showed a strong specificity for NADH and was much more efficient for the reduction of oxaloacetate than for the oxidation of malate, with a pH optimum of 8. Unlike MDHs from other sources, it was not inhibited by excess oxaloacetate. This first complete functional characterization of an MDH from Streptomyces shows that the enzyme is very similar in many respects to other bacterial MDHs with the notable exception of a lack of inhibition by excess substrate.