Conformation of DNA modified with a dihydrodiol epoxide derivative of benzo[a]pyrene

Unwinding of supercoiled Col E1-DNA after covalent binding of the ultimate carcinogen N-acetoxy-N-2-acetylaminofluorene and its 7-iodo derivative

The unwinding of superhelical Col E1-DNA was studied by means of gel electrophoresis and electron microscopy after covalent binding of N-acetoxy-N-2-[14C]acetylaminofluorene (N-Aco-[14C]AAF) and its 7-iodo derivative (N-Aco-[14C]AAIF). Studies with both compounds indicated that complete unwinding of the supercoiled DNA required the binding of hydrocarbon residue to about 3% of the bases. Thus the unwinding angle per residue of N-2-acetylaminofluorene (AAF) and its 7-iodo derivative was of 22 degrees +/- 3 and 18 degrees +/- 3 respectively. Our results are in good agreement with those obtained by Drinkwater et al. [9]. Precedent studies from this laboratory have shown that N-Aco-AAF and its 7-iodo derivative induce different local conformation change in native DNA (insertion-denaturation model and outside binding model respectively). The unexpected ability of the 7-iodo derivative to unwind supercoiled DNA is discussed.

Modification of deoxyribonucleic acid by a diol epoxide of benzo[a]pyrene. Relation to deoxyribonucleic acid structure and conformation and effects on transfectional activity

The effects of secondary structure on DNA modification by (+/-)-7 beta, 9 alpha-dihydroxy-9 alpha, 10 alpha-epoxy-7,8,9,10-tetrahydrobenzol[a]pyrene [(+/-)BPDE I] were investigated. No differences in the total extent of (+/-) BPDE I binding to double- and single-stranded calf thymus DNA were found. High-performance liquid chromatography (LC) of the nucleoside adducts obtained from hydrolysates of native and denatured calf thymus, as well as from superhelical and linear plasmid DNA, indicated that in all cases the major adduct (60--80% of total adducts) was formed by reaction of the (+) enantiomer of BPDE I with the N-2 position of dG residues in the DNA. A minor adduct formed from the reaction of the (-) enantiomer with dG residues was also detected and was present in greater amounts in denautred DNA than in native DNA. Small amounts of BPDE I--dA and BPDE I--dC adducts were also detected in both the single- and double-stranded DNAs. Restriction enzyme analysis of BPDE I modified SV40 and phage lambda DNA provided evidence that the modification of DNA by this carcinogen is fairly random with respect to nucleotide sequence. Partial hydrolysis of modified plasmid DNA by the single-strand-specific S1 nuclease and LC analysis of the nucleoside adducts in the digested and undigested fractions of the DNA revealed no preferential excision by the S1 nuclease of the different BPDE I--deoxynucleoside adducts. Functional changes in BPDE I modified DNA were demonstrated. With increasing extents of modification, there was a decrease in the ability of plasmid DNA to transfect a receptive Escherichia coli strain to antibiotic resistance.

Evidence for translational control of the binding of 7, 12-dimethylbenz[a]anthracene to DNA of murine epidermal cell in culture

The effects of various inhibitors of aryl hydrocarbon hydroxylase (AHH), antioxidants, inhibitors of DNA, RNA, and protein synthesis, and protease inhibitors on the binding of [7,12-3H]dimethylbenz[a]anthracene ([3H] DMBA) to DNA of murine epidermal cells in culture have been investigated. 7,8-Benzoflavone, 5,6-benzoflavone and methyrapone (inhibitors of AAH) and antioxidants, butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), efficiently reduced the binding of [3H] DMBA to cellular DNA. Inhibitors of DNA and RNA synthesis did not affect this process whereas inhibitors of protein synthesis suppressed the binding of [3H] DMBA to cellular DNA. Protease inhibitors p-tosylamide-2-phenylchloromethyl ketone (TPCK) and p-tosyl-L-lysine chloromethyl ketone (TLCK) also reduced the interaction between DMBA and DNA. Thus, it appears that binding of DMBA to cellular DNA is regulated at the level of translation or/and post translation.

Benzo(a)pyrene 7,8-dihydrodiol-9,10-oxide modification of DNA: relation to chromatin structure and reconstitution

Purified duck reticulocyte DNA was incubated in vitro with a 7,8-dihydrodiol-9,10-oxide derivative of benzo(a)pyrene (BPDE). The carcinogen-modified DNA was somewhat more susceptible to partial digestion by the single strand specific endonuclease S1 than unmodified DNA, suggesting slight denaturation of the helix at sites of modification. Chromatin was reconstituted in vitro utilizing this carcinogen-modified DNA and unmodified-chromatin associated proteins. This reconstituted chromatin showed the same kinetics and extent of digestion by Staphylococcal nuclease, and similar nucleosome profiles on sucrose density gradient centrifugation, as those obtained with native chromatin or chromatin reconstituted with unmodified DNA. Moreover, polyacrylamide gel electrophoresis of DNA fragments obtained from nuclease digests gel electrophoresis of DNA fragments obtained from nuclease digests of the reconstituted chromatins suggested that the chromatin containing carcinogen-modified DNA had the same subnucleosome structure as that reconstituted with unmodified DNA. In a separate set of studies intact duck reticulocyte chromatin was reacted directly with BPDE. Nuclease digestion studies indicated that 65% of the carcinogen was bound to the 'open' regions of chromatin, and 35% to 'closed' regions. These results indicate that although convalent binding of a benzo(a)pyrene (BP) derivative to DNA produces local distortions in conformation of the helix, this modification does not appear to interfere with the ability of the DNA to associate with histones to form nucleosome structures. In addition, although DNA in the open regions of chromatin is more susceptible to reaction with the BP derivative, there is appreciable reaction with the DNA associated with histones.

Computer-generated graphic models of the N2-substituted deoxyguanosine adducts of 2-acetylaminofluorene and benzo[a]pyrene and the O6-substituted deoxyguanosine adduct of 1-naphthylamine in the DNA double helix

Computer models of three deoxyguanosine-carcinogen adducts in double-helical DNA are presented. The carcinogen moiety is rotated and the best fit within the double helix is evaluated. The 2-acetylaminofluorene (AAF) derivative, 3-(deoxyguanosin-N2-yl)-AAF, is found to be situated within the minor groove, has very little freedom of rotation and causes little helical distortion. The (+)-anti-benzo[a]-pyrene (BP)-diol epoxide-N2 adduct, 10beta-(deoxyguanosin-N2-yl)-7beta, 8alpha,9alpha-trihydroxy-7,8,9,10-tetrahydro-BP, has a similar fit with a greater degree of steric interaction, suggesting that this adduct could cause some local destabilization. The 1-naphthylamine (NA) derivative, N1-(deoxyguanosine-O6-yl)-1-NA, resides within the major groove, does not perturb the helix and has considerable freedom of movement.

Conformation of dinucleoside monophosphates modified with benzo[a]pyrene-7,8-dihydrodiol 9,10-oxide as measured by circular dichroism

The conformational properties of GpU modified with the reactive derivative of benzo[a]pyrene, (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene, has been investigated utilizing circular dichroism spectroscopy. Binding of this carcinogen to the N2 of G residues in GpU resulted in the formation of four compounds (I to IV) representing two pairs of diastereoisomers. The molar ellipticity values of the modified dimers were approximately twofold higher than those of the modified guanosine monomers. These values were decreased appreciably when the spectra of the dimers were obtained at 80 degrees C or in methanol rather than at 25 degrees C in water, suggesting that under the latter conditions there is a stacking interaction between the carcinogen and the neighboring uridine residue. Based on these results, a conformation is proposed for modified GpU. It includes insertion of the benzo[a]pyrene moiety, by rotation of the modified guanine residue about its glycoside bond, coplanar to the neighboring uridine and perpendicular to the phosphodiester backbone.