Mercury (II) Stimulation of Malate Dehydrogenase Activityti’

The reduction of NAD catalyzed by pig heart mitochondrial malate dehydrogenase can be stimulated by low concentrations, approximately 0.05 mM, of Hg-‘+. This activation is dependent on the presence of the substrate malate at the time the enzyme is exposed to Hg-‘-+. The stimulation is both pHand temperature-dependent, with maximum stimulation occurring near pH 8.8 and 30”. The activation is not the result of an alteration in the K, values of the enzyme for either NAD or malate. The reverse reaction, the oxidation of NADH, is markedly inhibited by Hg+ in potassium phosphate buffer, pH 7.4, but is unaffected in Tris-HCl buffer at the same pH. These observations suggest that the effects of Hg++ on enzymatic activity are the results of various structural changes produced following mercaptide formation with different sulfhydryl groups of the enzyme. Hg++ stimulation of NAD reduction could also be demonstrated with malate dehydrogenase from other animal sources, as well as with the enzymes from Escherichia coli, Aerobacler aerogenes, and Salmonella typhimurium. In contrast, the enzymes from Bacillus subtilis and Bacillus polymyxa could not be stimulated, while those from Bacillus licheniformis and plant sources were inhibited by Hg++.