THYROXINE-SUPPORTED OXIDATION IN THE MYELOPEROXIDASE SYSTEM

Thyroxine and other iodothyronines (concentrations in the nanomolar ra nge) stimulated the oxidation of NADH in the myeloperoxidase-H2O2-Cl s ystem. In the absence of chloride, thyroxine had only a marginal effec t. This suggests that thyroxine increased the generation of chlorinati ng oxidants. A peroxidase-catalysed oxidation product of thyroxine, 3, 5-diiodotyrosine, was inactive. Preincubation of thyroxine in the myel operoxidase system showed that thyroxine was oxidized to a product cap able of stimulating NADH oxidation. Reduction and alkylation of myelop eroxidase under nondenaturing conditions also increased the oxidative activity of the enzyme. It is postulated that both iodoacetamide and a thyroxine-derived oxidation product (presumably a quinone) alkylate s ulphydryl groups near the active centre of myeloperoxidase making it m ore accessible for its substrate.