Ca2+/Nicotinamide Adenine Dinucleotide Phosphate-Dependent H2O2 Generation Is Inhibited by Iodide in Human Thyroids

Abstract

A calcium and NAD(P)H-dependent H2O2-generating activity has been studied in paranodular thyroid tissues from four patients with cold thyroid nodules and from nine diffuse toxic goiters. H2O2 generation was detected both in the particulate (P 3,000 g) and in the microsomal (P 100,000 g) fractions of paranodular tissue surrounding cold thyroid nodules (PN), with the same biochemical properties described for NADPH oxidase found in porcine and human thyroids. In PN tissues, the particulate NADPH oxidase activity (224 ± 38 nmol H2O2·h−1·mg−1 protein) was similar to that described for the porcine thyroid enzyme. However, no NADPH oxidase activity was detectable in the particulate fractions from eight diffuse toxic goiter patients treated with iodine before surgery; all but one also received propylthiouracil or methimazole in the preoperative period. Thyroid cytochrome c reductase (diffuse toxic goiters = 438 ± 104 nmol NADP+·h−1·mg−1 protein; PN = 78 ± 10 nmol NADP+·h−1·mg−1 protein) and thyroperoxidase (diffuse toxic goiters = 621 ± 179 U·g−1 protein; PN = 232 ± 121 U·g−1 protein) activities were unaffected by iodide. Thus, the human NADPH oxidase seems to be inhibited by iodinated compounds in vivo and probably is an enzyme involved in the Wolff-Chaikoff effect. Our findings reinforce the hypothesis that thyroid NADPH oxidase is responsible for the production of H2O2 necessary for thyroid hormone biosynthesis.