The influence of thyroidal status on rat hepatic mitochondrial NADH duroquinone reductase

Hepatic mitochondrial NADH duroquinone reductase and alpha-glycerophosphate dehydrogenase activities were measured in rats with altered thyroidal status. Whereas alpha-glycerophosphate dehydrogenase activity was decreased in hypothyroid rats, NADH duroquinone reductase was increased approximately 3-fold in both thiouracil-fed and thyroidectomized rats. In hyperthyroid animals, NADH duroquinone reductase activity was decreased, whereas there was the expected elevation in mitochondrial alpha-glycerophosphate dehydrogenase. Maximum velocity measurements of NADH duroquinone reductase demonstrated that the increase in enzyme activity associated with hypothyroidism occurred without an alteration in Michaelis constants for the reaction. Rats bearing mammary carcinomas induced by N-nitrosomethylurea also showed an increase in hepatic NADH duroquinone reductase when rendered hypothyroid, but the enzyme was unaffected by thyroxine administration.