Regulatory Effects of Iodide and Thiocyanate on Tyrosine Oxidation Catalyzed by Thyroid Peroxidase

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Abstract
The effects of I, thiocyanate and perchlorate, 3 anions with the same molecular size, on the oxidation of tyrosine to 3,3''-bityrosine by several peroxidases were evaluated at pH 8.8, i.e., in conditions in which I is not oxidized. The following results were obtained: I greatly stimulates the rate of bityrosine formation in the presence of thyroid peroxidase. No effect was seen with horseradish peroxidase or lactoperoxidase. Maximal I effects were obtained with about 0.5 mM iodide and Km for iodide was equal to about 0.028 mM. Thyroid peroxidase may contain a simple class of regulatory binding sites for I. SCN- mimics I effects; maximal stimulatory effects were seen with about 0.5 mM thiocyanate and Km for SCN- was equal to 0.1 mM. The effects of SCN- and those of I were not additive. SCN- may bind to the same regulatory site as I, but with a slightly lower affinity. No effect of SCN- was seen with horseradish peroxidase or lactoperoxidase. ClO4-, another anion with the same molecular size as I and SCN-, had neither an effect on the oxidation of tyrosine to bityrosine nor did it prevent the stimulatory effect of I on this reaction. Bromide was without effect on the same reaction.