Relative Antithyroid Effects of 2-Thiouracil, 2-Thiouridine and 2-Thio-UMP

SummaryThe presence of uridine phosphorylase and thymidine phosphorylase, enzymes capable of converting thiouracil to thiouridine, was demonstrated in thyroid tissue from several species. Uridine kinase which catalyzes further metabolism of thiouridine to thio-UMP was also present. The activity of UMP pyrophosphorylase which converts thiouracil to thio-UMP in a one-step reaction was insignificant.Comparisons of antithyroidal activity in intact rats and with partially purified porcine thyroid peroxidase demonstrate that thiouridine was approximately 10% as potent as thiouracil in vivo but less than 1% as active as thiouracil as an inhibitor of thyroid peroxidase. Thio-UMP was about 5% as active as thiouracil on thyroid peroxidase.These results indicate that it is highly unlikely that thiouracil conversion to a nucleoside or nucleotide is involved in its antithyroidal action.