Xenoestrogen interaction with human sex hormone-binding globulin (hSHBG)

This study reports on some environmental chemicals with estrogenic activity (xenoestrogens) and their binding interaction for human plasma sex-hormone binding globulin (hSHBG). The binding affinity constant of these xenoestrogens was measured in equilibrium conditions by solid phase binding assay, and their ability to displace endogenous testosterone and estradiol from hSHBG binding sites was determined with an ammonium sulfate precipitation assay in native plasma from normal men and women. The data showed that some of these xenoestrogens bind hSHBG, with a reversible and competitive binding activity for both [3H]testosterone and [3H]17beta-estradiol and with no apparent decrease in the number of hSHBG binding sites. Their respective binding affinity constants were low, ranging from 0.02 to 7.8 10(5) 1 x mol(-1). However, in native plasma from normal men and women, they were able to dose-dependently increase concentrations of hSHBG-unbound testosterone and/or estradiol. In this study, 4-nonylphenol and 4-tertoctylphenol, two alkylphenols used as surfactants in many commercial products, and bisphenol A and O-hydroxybiphenyl, widely used in the plastics industry, were identified as potent hSHBG-ligands. Additionally, the flavonoid phytoestrogens genistein and naringenin were also identified as hSHBG ligands, whereas their glucoside derivatives, genistin and naringin, had no binding activity for hSHBG. From these data, it is suggested that hSHBG binding may transport some contaminant xenoestrogens into the plasma and modulate their bioavailability to cell tissues. On the other hand, xenoestrogens may also displace endogenous sex steroid hormones from hSHBG binding sites and disrupt the androgen-to-estrogen balance. Whether xenoestrogen SHBG ligands could reach high enough concentrations in the blood to expose humans to any such effect merits further investigation.