Superoxide anion is the initial product in the hydrogen peroxide formation catalyzed by NADPH oxidase in porcine thyroid plasma membrane.

The plasma membrane fraction from porcine thyroid is known to exhibit an NADPH-dependent production of hydrogen peroxide (H2O2), which is utilized for the oxidative biosynthesis of thyroid hormones catalyzed by thyroid peroxidase. The H2O2 formation is cyanide-insensitive, ATP-activatable, and Ca2+-dependent (Nakamura, Y., Ogihara, S., and Ohtaki, S. (1987) J. Biochem. (Tokyo) 102, 1121-1132). It remains unknown, however, whether H2O2 is produced directly from molecular oxygen (O2) or formed via dismutation of superoxide anion (O2-). We therefore attempted to analyze the mechanism of H2O2 formation by utilizing a new method for the simultaneous measurement of O2- and H2O2, in which diacetyldeuteroheme-substituted horseradish peroxidase was employed as the trapping agent for both oxygen metabolites. When NADPH was incubated with the membrane fraction in the presence of the heme-substituted peroxidase, a massive O2 consumption was observed together with the formation of compound III, and O2- adduct of the peroxidase. The amounts of compound III formed and O2 consumed were stoichiometric with each other, while formation of compound II, an indicative of H2O2, was not observed during the reaction. On the other hand, when an excess amount of superoxide dismutase was included in the reaction mixture, compound II was produced with complete suppression of the compound III formation. NADH minimally supported both O2 consumption and formation of compound III or II. These results indicate that the NADPH oxidase in the plasma membrane of thyroid produces O2- as the primary metabolite of O2 and hence that H2O2 required for the thyroid hormone synthesis provided through the dismutation of O2-.