Purification and characterisation of prostatic binding protein and its subunits

The prostatic binding protein, previously described in rat ventral prostate, was isolated. The purified protein binds pregnenolone with an affinity of 1.2 X 10(6) M-1 and contains an average of 0.84 binding site per molecular. Its carbohydrate content is 3.2%. Its Mr, estimated by gel filtration, is 51 000 but in the presence of 6 M guanidine hydrochloride or 0.1% dodecylsulfate it dissociates into two subunits (S and F), which can be separated by polyacrylamide gel electrophoresis or by chromatography on hydroxyapatite. The Mr of these subunits is about 17 000, when estimated by gel filtration in 6 M guanidine hydrochloride, or 19 000 for subunit F and 20 000 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.