Plasma steroid-binding proteins

Two steroid-binding proteins circulate in plasma, corticosteroid-binding globulin and sex hormone-binding globulin. They both have several different but connected, physiologic functions. Each is the major determinant of the concentration of the physiologically important hormones that they bind. CBG regulates the concentration of free cortisol and progesterone, and SHBG regulates the concentration of free testosterone, dihydrotestosterone, and, to a lesser extent, estradiol. It is this small free fraction of the appropriate hormone that is the active principal in affecting hormone action. In the past few years, it has been shown that both of these proteins have high affinity, specific receptors on the plasma membranes of a variety of cells. It has also been shown that when SHBG's binding sites are occupied it cannot bind to its receptor; only unliganded SHBG can. There are, as yet, no published reports on the control of CBG binding by steroids. For both SHBG and CBG, if an appropriate steroid is present when the binding protein is itself bound to its receptor, rapid induction of adenylate cyclase activity and the accumulation of intracellular cAMP occur. Finally, CBG has been shown to be a member of the superfamily of serine proteinase inhibitors. When it is exposed to a serine protease, it is cleaved and release all, or most, of its bound cortisol.