A pregnenolone-binding protein in soluble fraction of guinea pig adrenal cortex.

A pregnenolone-binding component has been detected in the soluble fraction of the guinea pig adrenal cortex. Enzymatic degradation studies revealed that the binding component was a protein. The binding was destroyed at 60 degrees but was not inhibited by sulfhydryl reactants. Pregnenolone was bound optimally at pH 7 to 7.5 The equilibrium association constant at 0 degrees was 10(7) M-1. The pregnenolone-binding protein had an apparent molecular weight of 58,000, as determined by gel filtration. With the exception of pregnenolone sulfate, structurally similar steroids did not interfere with pregnenolone binding. No such binding activity was detected in the guinea pig liver and kidney. Serum contained pregnenolone-binding activity which was distinguishable from the adrenal cytosol factor by a variet of physicochemical means. The physiological importance of this finding remains to be determined.