Differential binding of testosterone and estradiol to isoforms of sex hormone-binding globulin: selective alteration of estradiol binding in cirrhosis.

The MCR of testosterone is decreased but the MCR of estradiol is unchanged in men with cirrhosis and elevated serum sex hormone-binding globulin (SHBG) concentrations. Previous studies indicated that SHBG from cirrhotic men selectively delivers estradiol, but not testosterone, to peripheral tissues of rats in vivo. These results suggest that estradiol and testosterone may bind to different SHBG isoforms in serum and that the estradiol-binding isoforms may be selectively altered in cirrhosis. This hypothesis was tested by polyacrylamide gel isoelectric focusing and fast protein liquid chromatography chromatofocusing. After Concanavalin-A affinity purification of serum glycoproteins from pregnant women, normal men, normal women, and cirrhotic men, the glycoprotein fraction was reconstituted, labeled with [3H]testosterone or [3H]estradiol, and applied to isoelectric focusing gels. Testosterone was bound selectively by the most acidic isoforms of SHBG, pI 4.5-5.4, and there was a significant anodal shift of the estradiol-binding isoforms in serum from cirrhotic men compared to that in serum from normal men. The selective binding of testosterone to the most acidic isoforms of SHBG was confirmed by fast protein liquid chromatography chromatofocusing, wherein the binding reactions were measured at neutral pH after separation of the isoforms. These biochemical studies and previous physiological experiments question the conventional view that testosterone and estradiol bind to a single competitive binding site on SHBG. Rather, testosterone is selectively bound by the most acidic SHBG isoforms. The estradiol-binding isoforms undergo a significant anodal shift in cirrhosis; this abnormality may result in the lack of decrease in estradiol MCR in cirrhosis.