Effect of triiodothyronine in the regulation of haem biosynthetic pathway

Triiodothyronine administration to thyroidectomized animals, decreased cytochrome P-450 content by 50%. Haem oxygenase was not modified by triiodothyronine treatment, either alone or with a suboptimal dose of CoCl2. Under the same conditions hepatic delta-amino-laevulinic acid synthase activity was not affected. Triiodothyronine caused a two fold increase in tryptophan pyrrolase activity. Both the hole and total enzyme were increased to the same degree. Porphobilinogen deaminase-Uroporphyrinogen III Co-synthase activity was induced by 67% over thyroidectomized values, in triiodothyronine treated rats, and only 32% above the sham operated controls. Our results suggest that under triiodothyronine stimulation, the decrease in cytochrome P-450 content is not due to an enhanced rate of degradation of the haem moiety, but it rather dissociates to increase the cellular haem pool, and saturates in part the newly synthesized apotryptophan pyrrolase.