Human fibroblast chromatin states as effectors of the dna binding characteristics of benzo[a]pyrene anti-7,8-dihydrodiol 9,10-epoxide and two nonalkylating dna-binding molecules

Pure populations of mitotic or nonmitotic diploid human fibroblasts (>98% pure) were exposed to [³H)benzo[a)pyrene (CAS: 50-32-8) anti-7,8-dihydrodiol 9,10-epoxide: r-7, t-8 dihydroxy-t-9,10-oxy-7, 8,9,10-tetrahyd robenzo[ a) pyrene (or antidiol- epoxide). In addition, metaphase chromosomes, interphase chromatin, or naked DNA was isolated from the pure cell populations and then titrated to saturation with anti-diol-epoxide, chromomycin A₃, or 3,8-diamino-5-ethyl-6-phenylphenanthridinium bromide (ethidium bromide). At saturation, anti-diol-epoxide had covalently modified 1.5% of the total deoxyguanosine residues in naked DNA, and this was reduced to 29 and 15% of this level in saturating the available anti-diol-epoxide-binding sites in chromosomes or chromatin, respectively. A similar hierarchy of accessible binding sites (naked DNA> chromosomes> chromatin) was also observed for the noncovalent interaction of chromomycin A₃ or ethidium bromide with the human cell DNA. Deproteinization of the chromosome or chromatin preparations returned the level of drug binding to that seen with naked DNA. The results clarify the association between proteins and DNA in human chromatin and suggest how cell-cycle-dependent changes in DNA-associated proteins or higher-order changes in protein-DNA conformation can act to alter the access of molecules to DNA-binding sites.