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

Department of Obstetrics and Gynaecology (S.M.-Q., C.U., G.L.H.), University of British Columbia, and Child and Family Research Institute, Vancouver, British Columbia, Canada V5Z 4H4; and Fisheries and Oceans Canada (R.H.D.), West Vancouver, British Columbia, Canada V7V 1N6

Address all correspondence and requests for reprints to: Geoffrey L. Hammond, Ph.D., Child and Family Research Institute, 950 West 28th Avenue, Vancouver, British Columbia, Canada V5Z 4H4. E-mail: ghammond{at}cw.bc.ca.

When the biochemical characteristics of coho salmon SHBG (csSHBG) plasma were examined, two different steroid-binding profiles were obtained corresponding to recombinant csSHBG- 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- also binds androstenedione (K_d = 2.8 nM) and ethinylestradiol with high affinity, whereas csSHBG-β binds estradiol (K_d = 0.8 nM) preferentially. When analyzed by gel filtration, csSHBG- displays the properties of a 153-kDa homodimer, whereas csSHBG-β elutes as a 68-kDa monomer. The unique steroid-binding properties of csSHBG- and csSHBG-β allowed us to develop an assay for their measurements in immature (pre-smolt) and mature coho salmon blood. Plasma csSHBG- 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- expression in pre-smolts and mature fish is the liver, but low levels of csSHBG- mRNA are present in stomach/intestine of mature fish. In pre-smolts, high levels of csSHBG-β mRNA are present in gills and ovary, whereas csSHBG-β mRNA is most abundant in muscle and stomach/intestine of mature fish. Based on the differences in csSHBG- and csSHBG-β plasma levels and their tissue expression profiles, we conclude that gills and/or muscle contribute mainly to plasma SHBG-β in coho salmon. The assays we have developed will enable studies of how SHBG-/SHBG-β biosynthesis is regulated throughout the salmonid life cycle and how they influence steroid hormone action in these fish.