Thyroglobulin-mediated one- and two-electron oxidations of glutathione and ascorbate in thyroid peroxidase systems

The one- or two-electron oxidation of thyroglobulin by the thyroid peroxidase system was found to be regulated by the iodine content of thyroglobulin. The catalytic intermediate of thyroid peroxidase observed at steady state of the reaction was Compound I and II when the iodine content in thyroglobulin was 0.2 and 0.7%, respectively, apparent rate constants for the rate-limiting steps being estimated at 4.7 x 10(7) and 4.8 x 10(4) M-1 S-1. The thyroglobulin-mediated oxidation of GSH occurred by way of two-electron transfer at 0.2% iodine content and by way of one-electron transfer at 0.7% iodine content. The spin-trapping experiment with 5,5-dimethyl-1-pyrroline-N-oxide showed that glutathione radicals were formed in the latter reaction but not in the former. In the reactions of thyroid peroxidase, the one- and two-electron oxidations of ascorbate were also mediated by 0.2 and 0.7% iodine thyroglobulins, respectively. The reactions were analyzed and mimicked with the use of p-cresol and p-acetaminophenol as a mediator in the reactions of lactoperoxidase and thyroid peroxidase.