Carcinogen-binding Proteins in the Rat Ventral Prostate: Specific and Nonspecific High-Affinity Binding Sites for Benzo(a)pyrene, 3-Methylcholanthrene, and 2,3,7,8-Tetrachlorodibenzo-p-dioxin

The polychlorinated dibenzodioxin [³H]-2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and the carcinogens [³H]benzo(a)pyrene and [³H]-3-methylcholanthrene bound to saturable binding sites in cytosol from the rat ventral prostate. Analysis of equilibrium binding parameters in diluted cytosol preparations indicated an apparent K(d) of ~2 nM and a binding capacity of approximately 1 nmol/mg cytosolic protein, corresponding to ~5% of the total protein content. However, gel permeation chromatography analysis as well as velocity sedimentation analysis on sucrose gradients of [³H]TCCD-labeled rat prostatic cytosol indicated binding of [³H]TCCD to two discrete species. These analyses indicated a sedimentation coefficient of 3.6-3.8S, a Stokes radius of 25-28 Å, and a calculated relative molecular weight of 42,000-45,000 for the most abundant binding species. The other binding species sedimented at 4-5S under high ionic strength conditions and at 8-10S under low ionic strength conditions and had a Stokes radius of approximately 60 Å, a relative molecular weight of ~100,000, and an estimated concentration of 5-20 fmol/mg cytosolic protein. Binding of [³H]TCCD to this species was displaceable by a 200-fold M excess of 2,3,7,8-tetrachlorodibenzofuran. Therefore, this species was tentatively identified as the TCDD receptor. The properties of the high-capacity binder of [³H]TCCD were found to be similar to the characteristics of protein previously purified from the rat ventral prostate, prostatic secretory protein, which binds androgens as well as estramustine, a nitrogen mustard derivative of estradiol. The binding of estramustine to diluted prostatic cytosol was shown to be competitively inhibited by 2,3,7,8-tetrachlorodibenzofuran. Moreover, purified prostatic secretory protein bound [³H]TCCD, [³H]benzo(a)pyrene, as well as [³H]-3-methylcholanthrene. It is suggested that binding to this protein is responsible for the high-binding capacity of carcinogens in cytosol from the rat ventral prostate.