Partial Characterization of a Sex Steroid-Binding Protein in Plasma from Arctic Charr (Salvelinus alpinus L.)

Abstract

A sex steroid-binding protein (SBP) that binds 17β-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 (Bmax) and equilibrium dissociation constant (Kd) of males (Bmax = 2122 fmol E2/mg protein, Kd = 1.9 nM), females (Bmax = 4115 fmol E2/mg protein, Kd = 3.0 nM), and juveniles (Bmax = 4355 fmol E2/mg protein, Kd = 1.8 nM) resembled binding characteristics of SBP from related species. The early-maturing females displayed both Bmax and Kd 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 SBP levels and affinity. The association rate for 17β-estradiol was rapid (t1/2 ≈ 1–2 min) compared with the dissociation rate (t1/2 ≈ 3 h). Several native hormones (estrogens, androgens, and progesterone) were able to compete with tritiated 17β-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.