Differences in binding characteristics of sex steroid binding protein in reproductive and nonreproductive female rainbow trout (Oncorhynchus mykiss), black bream (Acanthopagrus butcheri), and greenback flounder (Rhombosolea tapirina).

The binding characteristics of sex steroid binding protein (SBP) were investigated in vitellogenic and nonreproductive female rainbow trout (Oncorhynchus mykiss), black bream (Acanthopagrus butcheri), and greenback flounder (Rhombosolea tapirina). The binding capacity of rainbow trout and black bream SBP was significantly greater in vitellogenic than in nonreproductive-stage fish. A decrease in binding affinity was observed in male trout injected with estradiol (E(2)) compared to control fish. This difference was not observed after partial purification of the SBP by gel filtration and may have resulted from competitive inhibition of E(2) binding by vitellogenin. No differences in flounder SBP binding characteristics were observed.