Testicular androgen-binding protein (ABP): comparison of abp in rabbit testis and epididymis with a similar androgen-binding protein (TeBG) in rabbit serum

Abstract

Testicular androgen-binding proteins (ABP) in rabbit testis, caput epididymis and efferent duct fluid (EDF) were compared to a similar androgen-binding protein TeBG) in rabbit serum. The affinity of these proteins for 5α-dihydrotestosterone (DHT) at 0 °C ( K a ABP = 1.6 × 10 9 M −1 and K a TeBG = 1.9 × 10 9 M −1) and their steroid specificities were similar (DHT > androstanediol > progesterone and androstenedione). ABP and TeBG had also almost identical Stokes radii (42.8 ± 1.2 and 43.9 ± 0.8 Å, respectively), sedimentation coefficients (4.7 ± 0.2 S and 4.4 ± 0.2 S, respectively) and electrophoretic mobility ( R f = 0.4 in 6 1 2 % polyacrylamide gels). Calculation of molecular weights from Stokes radii and sedimentation rates indicated a molecular weight of 74,000 (69,000–78,000) for TeBG and 76,000 (71,000–82,000) for ABP. The corresponding frictional ratios were 1.61 for TeBG and 1.55 for ABP assuming a partial specific volume (– v) of 0.70 cm 3/g. Polyacrylamide gel electrophoresis (PAGE) at different gel concentrations gave a mean molecular radius of 2.74 nm, also indicating a molecular weight of about 75,000 (– v = 0.70 cm 3/g). ABP and TeBG could not be separated by PAGE; however, partial separation of ABP and TeBG was achieved by isoelectric focusing and ion-exchange chromatography on DEAE-cellulose. TeBG focused at pH 5.4, whereas ABP formed a distinct peak of bound radioactivity at pH 4.7. Also by ion-exchange chromatography, ABP in both testis and epididymal supernatants was shown to have an apparently higher surface charge than TeBG in rabbit serum. The concentration of ABP in efferent duct fluid (2 × 10 −7 M = 60 pmol/mg protein) was much higher than TeBG in male rabbit serum (5.2 × 10 −8 M = 0.7 pmol/mg protein). These findings ruled against the possibility that ABP in the testis and epididymis could have been derived directly from serum. It is concluded that ABP and TeBG are very similar if not identical proteins both serving as transport and carrier proteins in their respective compartments.