A binding-protein for aldosterone in human plasma

Abstract

Equilibrium dialysis of fresh human plasma revealed that aldosterone was bound to plasma proteins other than albumin. The apparent association constant of aldosterone to nonalbumin plasma proteins was one order of magnitude larger than that reported for aldosterone to corticosteroid-binding globulin (CBG), while its binding capacity was smaller than that of CBG. Bound aldosterone was not completely dissociated when the binding capacity of CBG was saturated with a large amount of cortisol. The addition of aldosterone under these experimental conditions further displaced its binding to nonalbumin plasma proteins. Progesterone inhibited binding of aldosterone but not to the same extent as cortisol did. Column electrophoresis of dialyzed plasma demonstrated, in addition to the peak of aldosterone binding to CBG and albumin, a peak of aldosterone comigrating with α 1-globulin. When the fractions containing this peak were pooled and subjected to equilibrium gel nitration on Sephadex G-200, bound aldosterone was eluted between CBG and albumin. Desialylated plasma also showed a peak of aldosterone binding different from the peaks corresponding to CBG and albumin on gel filtration. This aldosterone-binding protein was a glycoprotein precipitable with 60% saturation of ammonium sulfate and completely inactivated by heating at 60°C for 30 min. These results suggest the existence of an aldosterone-binding protein in human plasma different from CBG, albumin and α 1-acid glycoprotein.