Regulation of pathways of ornithine metabolism. Effects of thyroid hormone and diabetes on the activity of enzymes at the "ornithine crossroads' in rat liver.

Abstract

There is a significant increase in four of the urea cycle enzymes in liver from hypothyroid rats, arginase alone showed an opposite trend; these changes are reversed by physiological doses of thyroxine; hyperthyroidism results in a significant decrease in ornithine transcarbamoylase activity. The pattern of change in thyroidectomized and alloxan-diabetic rats showed marked similarities in respect to urea cycle and ornithine-metabolizing enzymes which are discussed in the light of the common feature of hypoinsulinism of diabetes and depressed response to insulin in hypothyroidism. The profile of ornithine-metabolizing enzymes is consonant with the decreased protein synthesis and turnover in hypothyroidism.