Antithyrotoxic Activity of Hemoglobin in the Rat

Abstract

Dietary hemoglobin and the acid-insoluble precipitate obtained from an alkaline hydrolysate of hemoglobin had good antithyrotoxic activity, as measured by the α-glycerophosphate dehydrogenase (GPD) assay. Both interfered with the intestinal absorption of thyroxine, as measured by the level of radioactivity in blood after feeding ¹³¹I-labeled thyroxine to rats. Globin was inactive in both test systems, whereas crystalline hemin was very active in the absorption test, but only moderately active in the antithyrotoxic (GPD) assay. Hemin interfered with the catalytic role of thyroxine in the oxidation of DPNH by the horseradish peroxidase enzyme system; this indicated the formation of a complex between thyroxine and hemin. At least a part of the antithyrotoxic activity of hemoglobin can be attributed to an inhibition of the intestinal absorption of thyroxine by its iron porphyrin moiety.