STEROID-PROTEIN INTERACTIONS

Abstract

Abstract : The species specificity which had been observed in rat and human transcortin for the species specific corticosteroid has not been found to be a general phenomenon. The corticosteroidbinding proteins in the sera of various mammalian species belong to the alpha gloublin as was demonstrated by the method of equilibrium paper electrophoresis. Electrophoretic studies showed that the transcortin containing fraction of ran serum migrates faster than albumin at pH values below 7, whereas at higher pH it behaves as an alpha globulin. The alpha- acid glycoprotein (orsomucoid) has been prepared by a combination of precipitation and chromatographic procedures. The interaction of progesterone with this glycoprotein has been found to be highly dependent on temperature; the association constant decreases with increasing temperature. A strong dependency on pH has also been observed; the association constant is highest at pH 8 and decreases to about 1/20 of this value at pH 2.3. The number of binding sites for progesterone has been determined to be n = 1 at pH 7.4, 4 degrees C; the free energy of binding has the relatively high value of -7.5 kilocalories per mole. Partial removal of sialic acid from the orosomucoid preparations reduced the binding affinity for progesterone. In biological assay studies on the ligated uterus horn of the mouse it was observed that progesterone is inactivated by binding to the alphaacid blycoprotein.