In this study we have investigated the potential of nuclear activation system to convert diethylstilbestrol (DES) to reactive metabolites, which bind to low mobility nonhistone nuclear proteins. Reaction of DES with nuclei in the presence of cumene hydroperoxide or NADPH revealed binding of DES in low mobility group (LMG) nonhistone nuclear proteins analyzed by both organic solvent extraction and gel electrophoresis methods. Gel electrophoresis experiments revealed that five LMG proteins of MW 130, 108, 72, 51, and 45 KDa were irreversibly bound to 3H-DES. The kinetic constants, Km and Vmax, of this binding reaction in the presence of cumene hydroperoxide were 39 uM and 1225 pmol/mg protein/30 min, respectively. This binding was significantly inhibited by cytochromes P450 inhibitors. Low molecular weight thiols, i.e., glutathione and cysteine, or thiol modifiers such as n-ethylmaleimide, dithionitrobenzoic acid, and hydroxymercuric benzoate, drastically inhibited binding. The binding of DES metabolites to both transcriptionally active and inactive chromatins LMG proteins was observed. In summary, DES is metabolized to transcriptionally active chromatin LMG protein binding metabolites presumably by nuclear cytochromes P450. These data suggest that an analogous in vivo modification in the transcriptionally active chromatin LMG nonhistone proteins by DES metabolites may influence gene transcription.