Partial characterization of a sex steroid-binding protein in plasma from Arctic charr (Salvelinus alpinus L.)

A sex steroid-binding protein (SBP) that binds 17 beta -estradiol with high affinity and moderate capacity was identified in the plasma from Arctic ch arr (Salvelinus alpinus L.) sampled during the early stage of gonadal matur ation in June and prior to spawning in October. Maximum specific binding (B -max) and equilibrium dissociation constant (K-d) of males (B-max = 2122 fm ol E-2/mg protein, K-d = 1.9 nM), females (B-max = 4115 fmol E-2/mg protein , K-d = 3.0 nM), and juveniles (B-max = 4355 fmol E-2/mg protein, K-d 1.8 n M) resembled binding characteristics of SEP from related species. The early -maturing females displayed both B-max and K-d values significantly higher than those of males (June samples). No significant differences in binding c haracteristics between fully matured males or females and immature juvenile s were observed in the October samples. Interestingly, despite large indivi dual variations there was a strong correlation between SEP levels and affin ity. The association rate for 17 beta -estradiol was rapid (t(1/2) approxim ate to 1-2 min) compared with the dissociation rate (t(1/2) approximate to 3 h). Several native hormones (estrogens, androgens, and progesterone) were able to compete with tritiated 17 beta -estradiol for the binding site. Ge l filtration chromatography demonstrated a peak of estradiol binding at app roximately 60 kDa, when eluted on a Sephadex S-200 HR column. (C) 2001 Acad emic Press.