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Malate dehydrogenase (MDH, EC 1.1.1.37) catalyzes the reversible reduction of oxaloacetate to malate in the presence of NADH. In eukaryotic cells the enzyme is generally found to be present as two distinct isoenzymes; one form is present in the cellular cytosol and the other is present exclusively in the mitochondria. These 2 isoenzymes form part of a shuttle system (the malate-aspartate shuttle) that functions as the major mechanism for the transportation of reducing equivalents between the cytosol and the mitochondria. As part of our ongoing studies on the mechansim of action and metabolic function of the malate dehydrogenases (Bernstein et al. 1978; Bernstein & Everse, 1978; Bernstein & Grisham 1978), we recently investigated the kinetic properties of the 2 isoenzymes present in rat Novikoff hepatoma tissues. These studies were initiated to evaluate whether or not the enzymes in the malate-asparate shuttle of tumour tissues are structurally and functionally identical to those of normal tissues. Fresh tumour or liver was homogenized with a glass tissue homogenizer in 0.1 M potassium phosphate buffer, pH 7.5, containing 0.25M sucrose. The homogenate was centrifuged for 10 min at 10,000 g to remove tissue debris. The supernatant was then centrifuged for 30 min at 20,000 g to obtain a high-speed supernatant that contained the cytoplasmic enzymes. The supernatantant did not contain any isocitrate dehydrogenase activity or transhydrogenase activity and was therefore judged to be free of mitochondrial enzymes. This high-speed supernatant was used without further fractionation for the determination of the cytoplasmic MDH

The cytoplasmic malate dehydrogenase in neoplastic tissues, presence of a novel isoenzyme?