Demonstration of a thyroid hormone dependent protein in liver cytosol of male rats

Examination of SDS polyacrylamide electrophoretic patterns of liver cytosol from male Sprague Dawley rats revealed a protein (P(T)) with an approximate molecular weight of 24,000 in the cytosol of euthyroid rats which was virtually absent in cytosol from hypothyroid rats. As determined by densitometry, P(T) constituted approximately 5% of the total cytosol protein in euthyroid rats but only 0.5% of cytosol protein in hypothyroid rats. Decreased P(T) appeared related to hypothyroidism and not decreased parathormone or calcitonin since protein patterns in cytosol from rats rendered hypothyroid either by surgical thyroidectomy or treatment with propylthiouracil were identical. Only a faint band corresponding to P(T) was noted in kidney cytosol and the protein was not observed in cytosol from brain, lung, heart, spleen or testis. Injection of graded doses of T₃, 25-2,000 μg/100 g body weight (b.w.), into hypothyroid rats restored P(T) to 95% of the euthyroid level by the third day in all animals injected with 500 μg T₃ or greater. The absence of P(T) in liver cytosol from euthyroid female rats prompted further studies. Treatment of ovariectomized female Sprague Dawley rats with testosterone propionate, 1 mg/100 g b.w., for 10 days restored P(T) to the level seen in male rats and injection of male rats with estradiol valerate, 1 mg/100 g b.w., for 10 days resulted in disappearance of P(T). Since it is responsive to androgens as well as thyroid hormone, P(T) may be the same protein as the urinary protein reported by Roy. The studies demonstrate an easily identifiable liver cytosol protein which could serve as a valuable model for study of the mechanism of thyroid hormone and androgen.