Comparison of type I 5'-deiodination of thyroxine and of reverse-triiodothyronine in rat and chicken liver homogenates.

The characteristics of 5'-deiodination in the chicken liver have been compared to those in the rat. Using 6-n-propyl-2-thiouracil (PTU) at 1 mM in vitro, it was shown that, in accordance with former in vivo studies, only type I 5'-deiodinase exists in the liver of the chicken. When PTU was used in a concentration (10 microM) close to its Ki species differences could be demonstrated as for PTU sensitivity. 5'-Deiodinase in the chicken liver was more susceptible against the inhibitor. The early thiol-independent phase of deiodination was longer in the chicken than in the rat. Reverse-triiodothyronine (r-T3) seemed to be a more suitable substrate for this enzyme in the chicken when compared to either T4 degradation or r-T3 degradation in the rat. It is concluded that major characteristics of 5'-deiodination in the chicken are similar to those in the rat; however, the slight variances observed might explain some species differences found earlier.