An Androgen Binding Protein in the Testicular Cytosol of Human Testis

Abstract

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 (K_d = 7.8 nM) and low capacity (0.27 pmol/mg protein) for 5α-dihydrotestosterone (DHT). Binding affinity of human TeBG for DHT was greater (K_d = 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. Differences between hABP and TeBG, plus the reduction of plasma contamination as marked by albumin, suggest that the human mature testis contains an androgen binding protein separate from that circulating in plasma.