A comparative study of estramustine and pregnenolone binding to prostatic binding protein: Evidence for subunit cooperativity

Abstract

The binding of estramustine, a nitrogen mustard derivative of oestradiol to purified rat prostatic binding protein was studied as a test for a possible identity between this protein and the very similar estramustine-binding protein, described by Forsgren et al. [5]. In accordance with this hypothesis estramustine binds to purified prostatic binding protein with a high affinity (2.5 × 10 7M −1). This affinity markedly exceeds the affinity of pregnenolone for this protein (0.9 × 10 6M −1) or for a complex of prostatic binding protein, with prostatic proline-rich polypeptide, (4.7 × 10 6M −1). In competition experiments estramustine completely suppresses the binding of [ 3H]pregnenolone, whereas the binding of [ 3H]estramustine is only partially suppressed by pregnenolone, even at high concentrations. Prostatic binding protein was separated in its F- and S-subunit by DEAE-Sepharose chromatography performed in the presence of 8 M urea. Only the S-subunit, most probably in its dimer form. displays marked estramustine and pregnenolone binding, with affinities of respectively 3.7 and 1.2 × 10 6M −1. Recombination of both subunits results in a strong increase of estramustine binding, but not of pregnenolone binding.