A Comparison of sex steroid binding protein (SBP) in four species of teleost fish

A sex steroid binding protein (SBP) binding E2 with high affinity has been detected in a pleuronectid (greenback flounder Rhombosolea tapirina), two sparids (black bream Acanthopagrus butceri and snapper Pagrus auratus), and its presence has been confirmed in a salmonid (rainbow trout Oncorhynchus mykiss). SBP binding characteristics were measured using a hot saturation assay for trout, bream and snapper, and a cold saturation assay for flounder. Bound and unbound steroid were separated by incubation with dextran-coated charcoal (DCC). Affinity for E2 was highest in trout (kD = 0.44 nM), followed by bream (kD = 3.39 nM) and snapper (kD = 10.7 nM). The lowest affinity was found in flounder (kD = 84.7 nM). Binding capacity, however, was greatest in flounder (Bmax = 164 nM), followed by trout (Bmax = 92 nM), and then bream and snapper (Bmax = 50 and 39 nM, respectively). Binding of E2 to SBP had a very rapid rate of association, and most dissociation occurred within 5 min. To confirm that the plasma protein measured here was SBP, the relative binding affinities of SBP for a range of steroids were measured. In trout, bream and snapper, SBP bound E2 with the highest affinity, followed by T. In contrast, the relative affinity of T for flounder SBP was more than twice that of E2. The rank orders of affinity of binding indicate the importance of an unhindered 17β-hydroxyl group, and a 3-hydroxyl or 3-ketone group for high affinity binding to SBP. These requirements for high affinity binding are present in most animals possessing SBP and indicate conservation of the SBP molecule through evolution.