Isolation and characterization of sex-steroid-binding protein from rat and rabbit plasma

The sex-steroid-binding plasma protein (SBP), which had been reported to be absent in the rat by several workers, was detected in 6-week-old male rats, but not in female rats aged 5-43 days. The rat SBP was purified to apparent homogeneity by affinity chromatography on testosterone-17-alpha-ethynylcarboxyaminoethyl Sepharose 4B followed by hydroxyapatite column chromatography, and its properties were compared with those of rabbit SBP prepared in exactly the same fashion. The concentration of SBP in adult male rabbit was 25-fold greater than that in 6-week-old male rat, but the binding characteristics of both SBPs were very similar. They specifically bind testosterone, 5 alpha-dihydrotestosterone, and 5 alpha-androstane-3 alpha, 17 beta-diol, but have virtually no affinity for estradiol, progesterone, or cortisol. Equilibrium dissociation constants for dihydrotestosterone were estimated to be 11.7 nM for rat SBP and 14.9 nM for rabbit SBP.