DNA strand scission by benzo[a]pyrene diol epoxides

Effects of benzo(a)pyrene adducts of DNA synthesis in vitro

Two diol epoxides of benzo(a)pyrene (BP), and benzo(a)pyrene 4,5-oxide, have been used to make adducts in the homopolymers polyribocytidylic acid, (rC); polyriboadenylic acid (rA), polydeoxycytidylic acid (dC) and polydeoxyadenylic acid (dA). With appropriate oligomers as primers these modified and unmodified polynucleotides were used as templates for DNA synthesis with avian myeloblastosis virus DNA polymerase (AMV) or E. coli Pol I DNA polymerase. We have found that: (1) the size of the DNA product is not markedly decreased by the presence of these these polycyclic aromatic hydrocarbon adducts in the templates; (2) the presence of adducts does not lead to increased incorporation of erroneous bases. These results, supported by kinetic data, suggest that these polymerases can bypass a site containing an adduct on the template without leaving a gap or causing misincorporation of a base and they imply that mutagenesis by BP may not be attributable to either of these mechanisms.

In vitro breakage of plasmid DNA by mutagens and pesticides

Covalently closed circular molecules of the colicinogenic plasmid E1 can serve as sensitive indicators for detecting in vitro breakage of DNA. After these molecules are radioactively labeled and purified by cesium chloride density-gradient centrifugation, they are incubated with the compounds to be tested. Samples are analyzed on alkaline sucrose gradients to determine the fraction of unbroken molecules and a breakage rate is calculated. Positive results were obtained for all three mutagenic alkylating agents (MMS, EMS, and MNNG) and of the 11 pesticides tested, dexon, dichlorvos, malathion, and methyl parathion induced breaks in molecules at a rate significantly greater than the controls.

Base specificity in the binding of benzo[a]pyrene diol epoxide to simian virus 40 DNA

[14C]Simian virus 40 (SV40) DNA was reacted with [3H]7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene to give 0.60 adducts per genome. The modified DNA was digested to completion with Hind III restriction endonuclease and the 6 fragments isolated by polyacrylamide gel electrophoresis. Hydrocarbon binding to the fragments was proportional to their guanine--cytosine (G--C) content, reflecting selective reaction of the hydrocarbon with deoxyguanosine residues. No sites unusually susceptible to alkylation were detected.

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.