Hydrogen peroxide (H2O2) is an essential electron acceptor for thyroid peroxidase-catalyzed iodination and coupling reactions. In the presence of iodide, its production is a limiting step in thyroid hormone biosynthesis. Several studies have demonstrated that the thyroid particulate fraction contains a Ca2+- and NADPH-dependent H2O2 generator (NADPH-O2:oxidoreductase), the so-called thyroid NADPH-oxidase. It has recently been demonstrated that cellular H2O2 release is under the tonic control of TSH in primary cultures of dog thyrocytes. The present study evaluates the effect of TSH on the thyroid NADPH-oxidase and cytochrome c reductase activities, two enzymes believed to be involved on H2O2 generation in the thyroid gland. There was almost no detectable NADPH-dependent H2O2 generator in the membranes of cells grown for 18 h without TSH. But cells grown in the presence of TSH (0.1 mU/ml) had a Ca2+- and NADPH-dependent H2O2-generating activity that increased up to the third day in culture, as did the cell iodide organification capacity. This increase was also partially blocked by 12-O-tetradecanoylphorbol 13-acetate and cycloheximide. Forskolin and 8-bromo-cAMP both reproduced the action of TSH on the Ca2+- and NADPH-dependent H2O2 generator. In contrast, the thyroid NADPH-cytochrome c reductase activity in particles from control cells was similar to that of TSH-treated cells and was unaffected by forskolin or 12-O-tetrade-canoylphorbol 13-acetate. These results suggest that NADPH-cytochrome c reductase activity is not regulated by TSH and, thus, reinforce the idea that this enzyme is not involved in thyroid H2O2 generation. On the other hand, the Ca2+- and NADPH-dependent H2O2 generator, so-called thyroid NADPH-oxidase, is induced by TSH through the cAMP cascade. Thus, it seems to be another marker of thyroid differentiation, in addition to thyroperoxidase and thyroglobulin, and could play a key role in thyroid hormone production.
ASJC Scopus subject areas