Inhibition of Cytochrome Oxidase (Paraphenylendiamine Oxidase) of the Thyroid Gland by Thiouracil and Other Compounds

Summary and Conclusions 1. Thiouracil and sulfonamides inhibit the cytochrome oxidase activity (paraphenylendiamine oxidase) of thyroid tissue (rat) in vitro. 2. The cytochrome oxidase activity of thyroid glands of rats treated with thiouracil is subnormal. 3. Inhibition of cytochrome oxidase activity may possibly be the factor responsible for the antithyroid activity of these drugs. 4. Glutathione, cysteine hydrochloride, and ascorbic acid inhibit cytochrome oxidase activity (paraphenylendiamine oxidase) of thyroid tissue in vitro. 5. Paraminobenzoic acid and paraxanthin do not influence the cytochrome oxidase activity of the thyroid gland.