An Androgen Binding Protein in the Testicular Cytosol of Human Testis: COMPARISON WITH HUMAN PLASMA TESTOSTERONE-ESTROGEN BINDING GLOBULIN

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After removal of plasma contamination, an androgen binding protein (hABP) was detected in a 105,000-g supernate of human testicular homogenate. The physicochemical properties of hABP have been compared with a similar androgen binding protein in human plasma, testosterone-estrogen binding globulin (TeBG). hABP had high affinity (Kd = 7.8 nM) and low capacity (0.27 pmol/mg protein) for 5α-dihydrotestosterone (DHT). Binding affinity of human TeBG for DHT was greater (Kd = 0.66 nM, binding capacity 0.68 pmol/mg protein). On the basis of sedimentation rates and Einstein Stokes radii of hABP and TeBG, the mol wt of the two proteins were similar in the range of 87,000-92,000. The ligand specificities of hABP and TeBG were the same. The binding of [3H]DHT to hABP and TeBG were reversible processes at 0°C. The half-lives for the dissociation of [3H]DHT from hABP and TeBG were 100-120 min and 67-70 min, respectively. Heat sensitivity of hABP and TeBG were similar. hABP had a sharp pH binding curve with an optimum at 8, whereas TeBG had a stable pH optimum between 6.5 and 9. hABP and TeBG were eluted from an ion exchange column at 100 mM and 80 mM sodium chloride concentrations, respectively. Concanavalin A and ricin Sepharose affinity chromatography showed that TeBG is bound to the columns nearly quantitatively, whereas hABP is bound to the columns only partially.

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