Effect of thyrotropin and thyroxine on serum malic and isocitric dehydrogenases

Abstract

The effect of altered levels of thyrotropin and thyroxine on NAD- and NADP-linked enzyme systems in the rat was studied. Serum malic and isocitric dehydrogenases were determined in normal, iodine-deficient, thyroidectomized, thyroxine-treated, thyroxine- and TSH-treated, and hypophysectomized animals. Serum malic dehydrogenase was found to be significantly increased in those metabolic states where an elevation of TSH could be predicted. This change in enzyme activity appeared to be independent of expected circulating thyroxine levels. Serum isocitric dehydrogenase was unaffected by the above treatments. Thyrotropin did not appear to regulate directly serum malic dehydrogenase activity. The physiologic significance of these findings is not known.