Binding Characteristics of a Major Protein in Rat Ventral Prostate Cytosol That Interacts with Estramustine, a Nitrogen Mustard Derivative of 17β-Estradiol

The tissue distribution of [³H]estramustine, the dephosphorylated metabolite of estramustine phosphate (Estracyt), in the male rat was compared to that of [³H]estradiol 30 min and 2 hr following i.p. administration. In contrast to estradiol, estramustine was found to be efficiently concentrated in the ventral prostate gland by a soluble protein. The binding characteristics of this protein were studied in vitro using cytosol preparations of the gland. With a dextran-coated charcoal technique, the protein was found to bind estramustine with a broad pH optimum between pH 7 and pH 8.5, with an apparent K_d of 10 to 30 nm, and with a binding capacity of about 5 nmol/mg cytosol protein. The estramustine/protein complex was not retained by DNA-cellulose. None of the natural steroids tested inhibited the binding of 10 nm [³H]estramustine by more than 35% (progesterone), even when added in 4500-fold excess. The presence of a nitrogen mustard moiety at position 3 of the steroid was necessary for high-affinity binding to the protein. The protein was calculated to constitute about 20% of the total cytosol protein content.