A reexamination of the proposed inactivation of thyroid peroxidase in the rat thyroid by propylthiouracil

The antithyroid drug 6-propylthiouracil (PTU) was previously shown in our laboratory to have an unexpectedly prolonged inhibitory effect on iodination in the thyroid glands of rats. Eighteen hours after injection of a relatively small dose, iodination in the thyroid remained inhibited by more than 90%. We previously suggested that the prolonged inhibitory effect might be due to inactivation of thyroid peroxidase (TPO), a reaction previously shown to occur under certain conditions in an in vitro iodinating system containing highly purified TPO. However, the analytical procedure used in our earlier study did not exclude the possibility that sufficient PTU remained in the thyroid even after 18 h to inhibit TPO-catalyzed iodination by a reversible mechanism. Development of an improved analytical procedure, based on HPLC, led us to reexamine the mechanism of the prolonged inhibitory effect of PTU on iodination in rat thyroid glands. Rats were injected with [35S]PTU (1 mumol/100 g BW), and ultrafiltrates prepared from their homogenized thyroid glands were analyzed by HPLC. The major 35S-labeled metabolites were identified as sulfate/sulfite, PTU sulfinate, and PTU sulfonate. However, even after 18 h, a significant amount of unchanged [35S]PTU was also present. The calculated mean concentration of residual PTU was 20 microM, a sufficiently high level to explain the observed inhibition of iodination on the basis of a reversible mechanism. Experiments were also performed to examine the intrathyroidal distribution of 35S at intervals after the injection of [35S]PTU. All of the oxidation products of PTU showed marked increases between 2 and 16 h after injection. Based on our view that TPO is the major mediator of intrathyroidal metabolism of PTU, this observation is inconsistent with our previous proposal that TPO is inactivated after PTU injection. The results of the present study, therefore, lead us to withdraw our previous suggestion that TPO is inactivated after injection of PTU into rats. It is more likely that inhibition of iodination by PTU in the rat thyroid involves competition between PTU and tyrosyl residues of thyroglobulin for oxidized iodine, comparable to the reversible mechanism of inhibition observed in the TPO model system.