Purification and Characterisation of Prostatic Binding Protein and Its Subunits

Abstract

The prostatic binding protein, previously described in rat ventral prostate, was isolated. The purified protein binds pregnenolone with an affinity of 1.2 × 10⁶ M⁻¹ and contains an average of 0.84 binding site per molecule. Its carbohydrate content is 3.2%. Its M_r, estimated by gel filtration, is 51000 but in the presence of 6 M guanidine hydrochloride or 0.1% dodecylsufate it dissociates into two subunits (S and F), which can be separated by polyacrylamide gel electrophoresis or by chromatography on hydroxyapatite. The M_r of these subunits is about 17000, when estimated by gel filtration in 6 M guanidine hydrochloride, or 19000 for subunit F and 20000 for subunit S, when measured by dodecylsulfate/polyacrylamide gel electrophoresis. Their isoelectric points, estimated by isoelectric focusing in 8 M urea, are 4.6 for subunit F and 4.9 for subunit S. Prostatic binding protein and both subunits have a very similar amino acid composition. Upon reduction of disulfide bridges each subunit dissociates further into two components: one of these components is the same in both subunits.