A novel class of inactive steroid-binding sites in female rat liver nuclei

Nuclear salt extracts from intact female rat liver showed insignificant levels of progesterone-, oestradiol-, testosterone- or dexamethasone-specific binding. However, brief exposure of nuclear extracts to dextran-coated charcoal (DCC) induced binding for all the above classes of steroids. This 'DCC-effect', which was reproduced neither by gel filtration nor by extensive dialysis of the nuclear extract, could not be ascribed to removal of endogenous free or loosely bound steroids. We show that rat liver nuclei contain a class of secondary binding sites (BSII), which exhibit moderate or low affinity for steroid ligands, positive co-operativity, and cross-reaction between classes of steroids. The capacity of BSII sites to bind steroids depends strictly on prior neutralization by DCC of endogenous heat-stable non-dialysable inhibitor(s). The putative roles of these BSII binding sites are discussed in relation to component(s) probably responsible for inhibitory activity.