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 Kd of approximately 2 nM and a binding capacity of approximately 1 nmol/mg cytosolic protein, corresponding to approximately 5% of the total protein content. However, gel permeation chromatography analysis as well as velocity sedimentation analysis on sucrose gradients of [3H]TCDD-labeled rat prostatic cytosol indicated binding of [3H]TCDD to two discrete species. These analyses indicated a sedimentation coefficient of 3.6-3.8S, a Stokes radius of 25-28 A, 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 A, a relative molecular weight of approximately 100,000 and an estimated concentration of 5-20 fmol/mg cytosolic protein. Binding of [3H]TCDD 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]TCDD were found to be similar to the characteristics of a 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]TCDD, [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.