Comparison of lactoperoxidase- and thyroid peroxidase-catalyzed iodination and coupling

Abstract

Thyroid peroxidase (TPO) and lactoperoxidase (LPO) were compared with respect to their ability to catalyze protein iodination and T4 formation. Goiter thyroglobulin was used as the protein acceptor in most cases, since thyroglobulin is most likely the physiological substrate for peroxidase in the thyroid gland. Experiments were also performed with BSA, fibrinogen and casein. With goiter thyroglobulin as the protein acceptor LPO and TPO were about equally active in catalyzing T4 formation at pH 7.0. At pH 7.0 LPO was also as effective as TPO in catalyzing the conversion of DIT to T4 (coupling reaction). Iodinations were performed with concentrations of iodide varying from 10 μM to 10 mM. With both TPO and LPO, inhibition of iodination was observed when the concentration of I− exceeded 1 mM. Attempts to calculate Km (I−) for LPO were complicated by marked deviation of Lineweaver-Burk plots from linearity. However, the results indicated that at pH 7.0 TPO was more effective than LPO at concentrations of iodide approaching the physiological range (10-40 μM). In this respect TPO appears to be better adapted for thyroid function than LPO. However, at concentrations of I− near 1 mM, LPO became nearly as effective or more effective than TPO. With goiter thyroglobulin as acceptor the maximally observed moles of I bound per mole of enzyme per min were 8.2 × 103 for LPO and 6.3 × 103 for TPO. The pH optimum for TPO-catalyzed iodination (6.6-7.0) was closer to the physiological range than that for LPO-catalyzed iodination (> 6.0). From the results of this study we conclude that TPO possesses no marked specificity in its ability to catalyze iodination and T4 formation. © 1974 by The Endocrine Society.