Interaction of environmental chemicals with the plasma sex-steroid binding protein in the spotted seatrout

We have recently purified and characterized a high-affinity sex-steroid binding protein (SBP) with a mol weight of 135 kDa from the plasma of the spotted seatrout (Cynoscion nebulosus). The SBP shows high affinity for 17-estradiol (Kd 3 nM), and testosterone (Kd 6 nM), and competition studies demonstrate that other endogenous steroids including 11-ketotestosterone (the teleostean androgen), progesterone, pregnenolone, cortisol and the maturation-inducing steroid (17α, 20β, 21-trihydroxy-4-pregnen-3-one) bind with much lower affinities, and are essentially unbound at their physiological concentrations. The binding of a variety of estrogenic chlorinated pesticides, polychlorinated biphenyl (PCB) congeners and alkyl phenols to the seatrout SBP were examined in competition assays. The most potent compounds tested were the hydroxylated polychlorinated biphenyl (PCB) congeners (IC50 1 μM) followed by methoxychlor, nonylphenol and several DDT isomers, with IC 50s of approximately 4–8 μM. Interestingly, several of the naturally occurring phytoestrogens were inactive in this assay system. Scatchard analysis confirmed that the active compounds inhibit ligand binding through competitive mechanisms and that exposure to combinations of chemicals results in additive, and not synergistic, binding activities.