Study on sex steroid-binding proteins (with emphasize on 17β-estradiol) in plasma of female and juvenile kutum (Rutilus frisii kutum)

A sex steroid-binding protein (SBP) that binds to 17β-estradiol with high affinity and moderate capacity was identified in the plasma of female and juvenile of kutum (Rutilus frisii kutum) sampled during the early stage of gonadal maturation in October and prior to spawning in March. Mean maximum specific binding (Bmax) and equilibrium dissociation constant (Kd) of the fish were as follows: In early stage of gonadal development (October), Bmax= 5800±970 fmol E2/mg protein, Kd 4.1±0.6nM, and prior to spawning (March) Bmax= 4000±895 fmol E2/mg protein, Kd 2.9±0.3nM. In juvenile sampled in October Bmax= 1600±130 fmol E2/mg protein, Kd 2.1±0.2nM and in March samples Bmax= 3500±780 fmol E2/mg protein, Kd 2.2±0.2nM. Plasma estradiol binding characteristics of the adult samples in October and March differed from the juvenile samples in having both Bmax and Kd significantly higher than juveniles. Plasma SBP levels displayed a moderate correlation with GSI (r2= 0.52) and CF (r2= 0.51) and a weak correlation with HSI (r2 = 0.28). Affinity was moderately correlated with CF (r2 = 0.68) and HIS (r2 = 0.50). A strong correlation was obtained between Bmax and Kd, high Bmax values coincided with high Kd values and vice versa