Effects of Thyroxine and Cortisol on Liver Threonine Dehydratase and Tryptophan Pyrrolase in Rats Fed a High Protein Diet

The activities of threonine dehydratase (l-threonine hydro-lyase EC4.2.1. 16) and tryptophan pyrrolase (l-tryptophan: Oxygen oxido-reductase EC 1.13.1.12) were measured in tissues from rats subjected to hormonal and dietary treatments. Both treatment with cortisol and a high protein intake resulted in elevations in the activities of these enzymes in intact, hypophysectomized, and thyroidectomized rats. The combined effect of cortisol treatment and a high protein intake on the activities of threonine dehydratase and tryptophan pyrrolase was as great or greater than the sum of the two treatments individually. Thyroidectomy resulted in a two- to threefold increase in the activity of threonine dehydratase, but did not affect the activity of tryptophan pyrrolase. Thyroxine injections depressed threonine dehydratase activity in intact, hypophysectomized, and thyroidectomized rats. Thyroxine also depressed the activity of threonine dehydratase in rats fed a high protein diet or injected with cortisol. Tryptophan pyrrolase activity was not altered in intact rats treated with thyroxine but was depressed in hypophysectomized rats fed control or high protein diets.