Interaction of an ultimate carcinogen, benzo[a]pyrene diol epoxide, with nucleosomal core particles: Apparent lack of protection of DNA by histone proteins

Abstract

The binding of chemical carcinogens to nuclear macromolecules, especially to DNA, is thought to be central to the initiation of carcinogenesis. Previous studies of the interactions of one such ultimate carcinogen, benzo[a]pyrene diol epoxide (BPDE-I) with nuclei, chromatin and purified DNA, demonstrated that although some BPDE-I-DNA interactions were altered in chromatin, covalent binding to chromatin DNA at saturating chromatin concentrations was quantitatively the same as binding to purified DNA. We have now extended these studies to include the basic subunit of chromatin, the nucleosomal core particle. Association constants for BPDE-I and a nonreactive analogue were determined by absorbance and fluorescence spectroscopy using either core particles or purified DNA and were found to be lower, by a factor of 30, for core particles. One of the major pathways of interaction of BPDE-I with DNA is the catalysis of BPDE-I hydrolysis by the exocyclic amino group of deoxyguanosine in native DNA. This detoxification reaction is inhibited about 30-fold in core particles compared with DNA, consistent with the hypothesis that intercalation is important in this catalytic reaction. In contrast to these findings, at DNA concentrations that allow maximal binding, similar amounts of BPDE-I are bound covalently to either free DNA or the DNA contained in core particles. This finding suggests that the interaction of DNA with histones to form the subunit structure of chromatin does not significantly protect DNA from damage by this ultimate carcinogen. The pattern of DNA adducts formed with core particle DNA shows a subtle shift toward the pattern seen with denatured DNA.

Identification of a Novel, N7-Deoxyguanosine Adduct as the Major DNA Adduct Formed by a Non-Bay-Region Diol Epoxide of Benzo[a]pyrene with Low Mutagenic Potential

“…Furthermore, there was no chromatographic evidence for heterogeneity of this adduct. The chromatographic system used routinely allows separation of diastereomeric, N2-dGuo adducts derived from BPDE-I (MacLeod et al, 1988;MacLeod et al, 1989). These data strongly suggest that the reaction of BPDE-III with N2-dGuo in double-stranded DNA is enantiomerically selective, producing predominantly a single diastereomer.…”

Efficient protection against oxidative DNA damage in chromatin

“…Although the constituents of chromatin and the formation of higher order structures sterically hinder such DNA-reactive compounds to some extent , the DNA-bound proteins are not as efficient in scavenging these nonradical compounds as they are in scavenging hydroxyl radicals. Indeed, in the presence of low concentrations of the DNA-damaging compounds benzo[alpyrene diol epoxide [35] and dimethylmethane sulfonate [36], histones do not confer much protection at all against the induction of DNA damage. As long as there are some regions of the DNA accessible to the drugs, the drug molecules will eventually find DNA and form lesions in it.…”

Ellipticine increase the superhelical density of intracellular SV40 DNA by intercalation

“…Chen et al show that actinomycin D preferentially intercalates into the nucleosomes of the transcriptionally active chromatin (36). The carcinogen, benzp[a]pyrene diol epoxide, has also been shown to intercalate within nucleosomes (37). Altered organization of nucleosome structure either in the active nucleolar gene of Dictyostelium or in chromatin at high pH has been shown to allow psoralen to bind and crosslink DNA within nucleosomes (38).…”