| Abstract: | A study was made of the effect of a highly purified preparation of an unusual estrogen-binding protein (UEBP) of rat liver on 3H-estradiol (3H-E2) and 3H-testosterone (3H-T) metabolism by female rat liver homogenates. The UEBP decreased the rate of metabolic conversions of 3H-E2 and 3H-T in a dose-dependent and specific manner. The effectiveness of the inhibitory action of the UEBP declined with increase of metabolic activities of homogenate enzymes. The effects of the UEBP were reduced fully or partly by the ligands specifically binding to the UEBP, estrone (E1), and estriol (E3) respectively. The latter fact was used to demonstrate the effectiveness of the endogenous male rat liver homogenate UEBP in the control of 3H-E2 metabolism. The UEBP did not exhibit any oxidoreductase activity on the use of 3H-E1, 3H-E2, 3H-E2, 3H-T and 3H-androstenedione with NAD+, NADH, NADP+, and NADPH as cofactors. These data present the first direct experimental evidence of the regulatory function of the UEBP in the time course of changes in sex steroids. |
