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 charr (Salvelinus alpinus L.) sampled during the early stage of gonadal maturation in June and prior to spawning in October. Maximum specific binding (B-max) and equilibrium dissociation constant (K-d) of males (B-max = 2122 fmol 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 nM) 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 characteristics between fully matured males or females and immature juveniles were observed in the October samples. Interestingly, despite large individual variations there was a strong correlation between SEP levels and affinity. The association rate for 17 beta -estradiol was rapid (t(1/2) approximate 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. Gel filtration chromatography demonstrated a peak of estradiol binding at approximately 60 kDa, when eluted on a Sephadex S-200 HR column. (C) 2001 Academic Press.