THE H2O2-GENERATING SYSTEM MODULATES PROTEIN IODINATION AND THE ACTIVITY OF THE PENTOSE-PHOSPHATE PATHWAY IN DOG THYROID

Iodide oxidation and binding to proteins require a thyroperoxidase and an ill defined H2O2-generating system. The NADP+ supply and, thus, NADPH oxidation are the limiting steps of the pentose phosphate pathway. The purpose of this work was to test the hypothesis that H2O2 generation is a limiting step of iodination and NADPH oxidation and, therefore, of the pentose phosphate pathway. H2O2 produced by dog thyroid slices was measured with the homovanillic fluorescence assay. Our data show that H2O2 generation is stimulated by both the cAMP cascade [as activated by TSH, forskolin and (Bu)2cAMP] and the Ca2+-phosphatidylinositol cascade (as activated by carbamylcholine, ionomycin, and 12-O-tetradecanoylphorbol-13-acetate). We used several physiological and pharmacological agents that modulate iodide organification. In all cases there was a strict parallelism between effects on H2O2 generation, iodide binding to proteins, and pentose phosphate pathway activity. Moreover, in TSH- or carbamylcholine-stimulated slices, glucose or Ca2+ depletion, which greatly depressed H2O2 generation, also greatly decreased iodide organification and the activity of the pentose phosphate pathway. The glutathione peroxidase-catalyzed H2O2 reduction in the cytosol, which involves NADPH oxidation and, therefore, increases the NADP supply, also enhances the activity of the pentose phosphate pathway. All of these data strongly support the hypothesis that H2O2 generation in dog thyroid controls iodination of proteins; through the NADPH oxidation resulting from H2O2 production and reduction, hydrogen peroxide also regulates the activity of the pentose phosphate pathway.