Characterization and Measurement of the Plasma α- and β-Sex Hormone-Binding Globulin Paralogs in Salmon

When the biochemical characteristics of coho salmon SHBG (csSHBG) plasma were examined, two different steroid-binding profiles were obtained corresponding to recombinant csSHBG-α and csSHBG-β. These SHBG paralogs share only 24% sequence identity, and this explains their unique steroid-binding properties. Both proteins bind testosterone, but csSHBG-α also binds androstenedione (Kd = 2.8 nm) and ethinylestradiol with high affinity, whereas csSHBG-β binds estradiol (Kd = 0.8 nm) preferentially. When analyzed by gel filtration, csSHBG-α displays the properties of a 153-kDa homodimer, whereas csSHBG-β elutes as a 68-kDa monomer. The unique steroid-binding properties of csSHBG-α and csSHBG-β allowed us to develop an assay for their measurements in immature (pre-smolt) and mature coho salmon blood. Plasma csSHBG-α levels were 3- to 4-fold higher than those of csSHBG-β irrespective of developmental stage or sex and correlate with each other. The major site of csSHBG-α expression in pre-smolts and mature fish is t...