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We compared the binding characteristics of [3H]progesterone and 5 alpha-[3H]pregnanedione in cytosol and nuclear preparations of ovariectomized, estrogen-treated guinea pig uterus. There were similarities as well as differences in binding behavior. [3H]Progesterone and 5 alpha-[3H]pregnanedione were bound in cytosol with approximately the same apparent association constants and concentrations of binding sites. When centrifuged on sucrose gradients in 5 mM phosphate buffer, binding peaks with sedimentation coefficients of 7S were found with both. 5 alpha-[3H]Pregnanedione was bound by uterine nuclei and, like [3H]progesterone, required temperature-activated cytosol of estrogen-stimulated uterus. A series of unlabeled steroids had similar relative abilities to displace both [3H]progesterone and 5 alpha-[3H]pregnanedione from receptor complexes in cytosol or nuclei. When cytosol was incubated with increasing concentrations of [3H]progesterone or 5 alpha-[3H]pregnanedione the amount of 5 alpha-[3H]pregnanedione bound exceeded the amount of [3H]progesterone bound at steroid concentrations above 5 x 10(-9) M. This suggested that 5 alpha-pregnanedione was bound by additional components of the cytosol and the suggestion was strengthened by sucrose gradient analysis. At greater than saturating-steroid concentrations, the partition between 7S and 4 S binding proteins was different. [3H]Progesterone bound to 7S binding proteins in preference to 4S binders, whereas 5 alpha-pregnanedione showed relatively more 4S binding.

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