Purification and Characterization of Testosterone-Binding Globulin of Canine Serum

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Abstract
Testosterone-binding globulin (TeBG) of canine serum was purified to apparent homogeneity by affinity chromatography on testosterone-17α-ethynylcarboxyaminoethyl-Sepharose 4B followed by hydroxyapatite column chromatography. Canine TeBG was a glycoprotein containing 5.5% carbohydrates. Equilibrium sedimentation analysis in the presence and absence of 6 M guanidine hydrochloride gave molecular weights of 40,000 and 76,000, respec tively, suggesting that native TeBG consists of two subunits. Equilibrium dissociation constants at 0°C for testosterone and dihydrotestosterone were estimated to be 5.58 × 108 M and 1.43 × 108 M, respectively, and the number of binding site per native molecule was approximately unity for both androgens. Canine TeBG had virtually no affinity for estradiol, proges terone, or cortisol. Canine TeBG did not cross–react with a rabbit antiserum raised against bovine TeBG.