Kinetics of the Iodination and the Coupling Reaction in Thyroglobulin Catalyzed by Lactoperoxidase and Chloramine T

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Abstract
The kinetics of the iodination reaction and the coupling reaction in thyroglobulin catalyzed by lactoperoxidase and chloramine T have been compared in order to elucidate whether or not the enzyme is necessary for these reactions once the active iodine has been generated by the peroxidase system.In the presence of added iodide, lactoperoxidase and chloramine T catalyze both reactions. Lactoperoxidase does so in a gradual fashion, while chloramine T acts almost instantaneously. Chloramine T is more rapid but less efficient than lactoperoxidase in both iodotyrosine and iodothyronine formation. At short time intervals the difference is very small, but it becomes larger at longer reaction time intervals.We were able to study the coupling reaction independently of the iodination reaction by using chemically prelabeled thyroglobulin in the absence of added iodide. Lactoperoxidase catalyzes the reaction much more efficiently than chloramine T. Chemically prelabeled thyroglobulin releases iodide during incubation in the presence of the peroxidase system, but not in the presence of chloramine T. Thus iodide might play a role in the coupling reaction as already suggested by Dème et al. [Eur. J. Biochem. 70, 435–440 (1976)] for thyroid peroxidase‐catalyzed iodination of thyroglobulin.From the present results it appears that peroxidase is a more potent catalytic agent than chloramine T for both the iodination and the coupling reactions. This suggests that the enzyme is involved in both reactions beyond simply providing the active form of iodine, though further work is needed to clarify the mechanisms involved.