Stimulation of the chlorinating activity of human myeloperoxidase by thyroid hormones and analogues

Thyroxine and triiodothyronine concentrations of 50 nM or lower can stimulate the chlorinating activity of the myeloperoxidase-H2O2-Cl- antimicrobial system in vitro. The initial rates of the chlorinating reaction with monochlorodimedone were similar for both thyroid hormones. Maximum stimulation occurred around pH 6 and a linear relationship exists between stimulation and thyroxine concentrations up to at least 1 microM. Of the various thyroxine analogues tested, stimulation was in the order: T4 (or T3) greater than triiodothyropropionic acid greater than 3,5-T2. Diiodotyrosine did not have any significant stimulatory effect. The oxidised product of the phenolic ring of T4, presumably a hydroxyquinone, may act as an additional electron carrier and thereby facilitates redox reactions.