Binding of sex hormones by male rat liver microsomes

In vitro microsomes of adult male rats possess binding sites specific for 17 beta-hydroxy-5 alpha-androstan-3-one (5 alpha-DHT), estradiol-17 beta (E2-17 beta) and progesterone (Prog), respectively. The characteristics of these microsomal steroid binding were as follows: 1. The equilibrium dissociation constants were estimated to be 35.4, 9.94 and 25.2 nM for 5 alpha-DHT, E2-17 beta and Prog, respectively. The maximum numbers of binding sites were estimated to be 0.25, 0.12 and 6.43 pmol/mg of microsomal protein for 5 alpha-DHT, E2-17 beta and Prog, respectively. 2. The ligand specificities of microsomal binding for 5 alpha-DHT, E2-17 beta and Prog were similar to so-called cytosolic androgen-, estrogen-, and progestin-receptors, respectively, with the following two exceptions, namely, unlabeled Prog best inhibited the microsomal [3H]-5 alpha-DHT binding and unlabeled DES did not significantly inhibit microsomal [3H]-E2-17 beta binding. 3. These microsomal steroid binding capacities were not affected significantly by different incubation at 2 degrees and 36 degrees C. The steroid-binding components of microsomes in physiological saline (0.13 M-NaCl) were no longer soluble after 1 M-NaCl extraction. Our data suggest strongly that sex steroids may act on mammalian liver by specific binding to microsomes, which brings about changes of various cytoplasmic events, as well as by their specific binding to so-called cytosolic receptors, which ultimately brings about direct gene activations.