Nadph

NADP+

Ca2+

figure 6-8 Proposed mechanisms for production of H202.

(A) Superoxide anion, 02~, is first produced from 02 by the NADPH oxidase enzyme and then converted by dismutation, mediated by the enzyme superoxide dismutase (SOD), on the inside of the apical membrane (see Fig. 6-7); the H202 is then released to the outside.

(B) H202 is generated directly from 02 by the NADPH oxidase outside the cell. As indicated, changes in intracellular [Ca2+] have been shown to stimulate the production of H202. [Modified with permission from Gentile, F., DiLauro, R., and Salvatore, G. (1995). Biosynthesis and secretion of thyroid hormones. In "Endocrinology" (L. J. DeGroot et al, eds.), Vol. 1, Chapt. 34, pp. 517-542. W. B. Saunders, Philadelphia, PA.]

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