Interaction of (+/-)-7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene with relaxed circular pBR322 DNA

Correlation of frameshift mutagenicity with DNA intercalation by CGS 20928A using an in vitro DNA unwinding assay

A compound's mutagenicity in different Salmonella tester strains can suggest its mechanism of reaction with DNA. Clear confirmation of such a mechanism, however, requires a direct test of the compound's reaction with DNA, often relying on specific in vitro studies. We report the use of a rapid in vitro test designed to measure DNA unwinding, a characteristic of DNA intercalators and many frameshift mutagens. CGS 20928A, an adenosine antagonist, produced a significant (> 2-fold) increase in revertants only for Salmonella tester strain TA1537, and only without metabolic activation. These data indicated that the compound was a direct acting frameshift mutagen and possibly intercalated into DNA. Our DNA unwinding assay indicated that at concentrations of > 0.1 mM CGS 20928A behaved like known intercalating compounds in that it unwound DNA. These concentrations of compound are comparable to those found mutagenic to TA1537. By comparison, the frameshift mutagen and known intercalating compound 9-aminoacridine unwound DNA in this assay in a concentration dependent fashion between 6-12 microM. ICR-191, another acridine frameshift mutagen, also unwound DNA. A compound structurally related to CGS 20928A, which was not mutagenic in Salmonella tester strains, did not produce any DNA unwinding even at 10 mM. Because the assay uses microgram quantities of material, it should be ideal for screening small amounts of congeneric series suspected of frameshift mutagenicity.

Differences in unwinding of supercoiled DNA induced by the two enantiomers of anti-benzo[a]pyrene diol epoxide

The unwinding of supercoiled phi X174 RFI DNA induced by the tumorigenic (+) and non-tumorigenic (-) enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) has been investigated by agarose slab-gel and ethidium titration tube gel electrophoresis. The differences in adduct conformations were verified by flow linear dichroism techniques. Both enantiomers cause a reversible unwinding by the formation of noncovalent intercalative complexes. The effects of covalently bound BPDE residues on the electrophoretic mobilities of the RF I DNA form in agarose gels were investigated in detail in the range of binding ratios rb approximately 0.0-0.06 (covalently bound BPDE residues/nucleotide). In this range of rb values, there is a striking difference in the mobilities of (+)-BPDE- and (-)-BPDE-adducted phi X174 DNA in agarose slab-gels, the covalently bound (+)-BPDE residues causing a significantly greater retardation than (-)-BPDE residues. Increasing the level of covalent adducts beyond rb approximately 0.06 in the case of the (+)-BPDE enantiomer, leads to further unwinding and a minimum in the mobilities (corresponding to comigration of the nicked form and the covalently closed relaxed modified form) at rb 0.10 +/- 0.01; at still higher rb values, rewinding of the modified DNA in the opposite sense is observed. From the minimum in the mobility, a mean unwinding angle (per BPDE residue) of theta = 12 +/- 1.5 degrees is determined, which is in good agreement the value of theta = 11 +/- 1.8 degrees obtained by the tube gel titration method. Using this latter method, values of theta = 6.8 +/- 1.7 degrees for (-)-BPDE-phi X174 adducts are observed. It is concluded that agarose slab gel techniques are not suitable for determining unwinding angles for (-)-BPDE-modified phi X174 DNA because the alterations in the tertiary structures for rb < 0.06 are too small to cause sufficiently large changes in the electrophoretic mobilities. The major trans (+)-BPDE-N2-guanosine covalent adduct is situated at external binding sites and the mechanisms of unwinding are therefore different from those relevant to noncovalent intercalative BPDE-DNA complexes or to classical intercalating drug molecules; a flexible hinge joint and a widening of the minor groove at the site of the lesion may account for the observed unwinding effects. The more heterogeneous (-)-BPDE-nucleoside adducts (involving cis and trans N2-guanosine, and adenosine adducts) are less effective in causing unwinding of supercoiled DNA for reasons which remain to be elucidated.

Evidence for nonintercalative complexes formed from the reversible binding of benzo[a]pyrene metabolites to closed-circular, single-stranded M13mp19 DNA

The fluorescence excitation spectrum of complexes formed from the reversible binding of the proximate carcinogen, trans-7,8-dihydroxy-7,8-dihydro-benzo[a]pyrene (BP78D) to closed-circular, single-stranded, viral M13mp19 DNA (SS M13 DNA) exhibits a red-shift of 5 nm compared to the spectrum of BP78D measured without DNA or with native, calf thymus DNA. In SS M13 DNA which is 0.10 mM in PO4-, the fluorescence intensity of BP78D is 2.3 times smaller than the intensity measured without DNA; however, the fluorescence lifetime (42.7 nsec) of BP78D with SS M13 DNA is 1.7-1.8 times larger than the lifetimes of BP78D measured without DNA or with calf thymus DNA. These results are consistent with the conclusion that, in addition to binding sites which cause fluorescence quenching, SS M13 DNA contains sites which permit formation of BP78D inclusion complexes that have weaker interactions with nucleotide bases than those occurring in intercalated complexes. The association constant (1.45 +/- 0.01 x 10(5) M-1) for the binding of BP78D to SS M13 DNA is more than 9.0 times larger than that for binding to calf thymus DNA. It is 7.1 times larger than that for the binding of the less genotoxic metabolite, trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene (BP45D) to SS M13 DNA. UV Photoelectron data and results from ab initio molecular orbital calculations suggest that a difference in polarizability contributes to the greater SS M13 DNA binding of BP78D compared to that of BP45D.

Sequence selectivity, a test of the nature of the covalent adduct formed between benzo[a]pyrene and DNA

A theoretical study is presented of the energetic and structural properties of covalent adducts of benzo[a]pyrene and a DNA fragment. Energy optimisation is performed with the use of minimiser with constraints and an advanced semiempirical energy formula. Three types of adducts are studied: an external complex with the benzopyrene located in the DNA minor groove and two types of intercalative complexes with the carcinogen situated on the 3' side and 5' side of the covalently bound guanine. For each of the adducts the effects of DNA base sequence are examined. It is shown that the results for the intercalative complex with the carcinogen situated on the 5' side of the modified guanine correlate with the experimentally determined sequence preference.

Kinetic flow dichroism study of conformational changes in supercoiled DNA induced by ethidium bromide and noncovalent and covalent binding of benz[a]pyrene diol epoxide

The dynamic conformational changes due to the noncovalent intercalative binding of ethidium bromide and racemic trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE), and the covalent binding of BPDE to supercoiled phi X174 DNA, have been studied by gel electrophoresis and a novel application of a kinetic flow linear dichroism technique. The magnitude of the linear dichroism (delta A) of the DNA oriented in the flow gradient is sensitive to the hydrodynamic shape of the DNA molecule which is affected by the binding of the drug or the carcinogen BPDE. While the linear dichroism of ethidium bromide supercoiled DNA is time independent, the delta A spectra of BPDE-DNA reaction mixtures vary on time scales of minutes, which correspond to the reaction rate constant of BPDE to form 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene hydrolysis products and covalent DNA adducts. The rapid noncovalent intercalation of BPDE causes an initial large increase in delta A (up to 250%, corresponding to the dichroism observed with relaxed circular DNA), followed by a slower decrease in the linear dichroism signal. This decrease in delta A is attributed to the removal of intercalated diol epoxide molecules and the resulting reversible increase in the number of superhelical turns. The kinetic flow dichroism spectra indicate that the noncovalent BPDE-DNA complexes are intercalative in nature, while the covalent adducts are characterized by a very different conformation in which the long axes of the pyrenyl residues are oriented at a large angle with respect to the average orientation of the planes of the DNA bases.(ABSTRACT TRUNCATED AT 250 WORDS)

Origins of stereospecificity in DNA damage by anti-benzo[a]pyrene diol-epoxides. A molecular modelling study

A general computational procedure for the modelling of intercalated DNA-ligand complexes has been developed, and is used here to model intercalated complexes of the (+)-anti and (-)-anti enantiomers of benzo[a]pyrene diol-epoxide (BPDE) with cytosine-3',5'-guanosine double-stranded DNA sequences (dCpG). Results are presented indicating differences between the behaviours of the two enantiomers which have implications for the understanding of the stereospecificity of DNA strand breakage by benzo[a]pyrene diol-epoxides.

A comparison of the intercalative binding of non-reactive benzo[a]pyrene metabolites and metabolite model compounds to DNA

The reversible DNA physical binding of a series of non-reactive metabolites and metabolite model compounds derived from benzo[a]pyrene (BP) has been examined in UV absorption and in fluorescence emission and fluorescence lifetime studies. Members of this series have steric and pi electronic properties similar to the highly carcinogenic metabolite trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) and the less potent metabolite 4,5-epoxy-4,5-dihydrobenzo(a)pyrene (4,5-BPE). The molecules examined are trans-7,8-dihydroxy-7,8-dihydrobenzo[a]-pyrene (7,8-di(OH)H2BP), 7,8,9,10-tetrahydroxytetrahydrobenzo[a]pyrene (tetrol) 7,8,9,10-tetrahydrobenzo[a]pyrene (7,8,9,10-H4BP), pyrene, trans-4,5-dihydroxy-4,5-dihydrobenzo[a]pyrene (4,5-di(OH)H2BP) and 4,5-dihydrobenzo[a]pyrene (4,5-H2BP). In 15% methanol at 23 degrees C the intercalation binding constants of the molecules studied lie in the range 0.79-6.1 X 10(3) M-1. Of all the molecules examined the proximate carcinogen 7,8-di(OH)-H2BP is the best intercalating agent. The proximate carcinogen has a binding constant which in UV absorption studies is found to be 2.8-6.0 times greater than that of the other hydroxylated metabolites. Intercalation is the major mode of binding for 7,8-di(OH)H2BP and accounts for more than 95% of the total binding. Details concerning the specific role of physical bonding in BP carcinogenesis remain to be elucidated. However, the present studies demonstrate that the reversible binding constants for BP metabolites are of the same magnitude as reversible binding constants which arise from naturally occurring base-base hydrogen bonding and pi stacking interactions in DNA. Furthermore, previous autoradiographic studies indicate that in human skin fibroblasts incubated in BP, pooling of the unmetabolized hydrocarbons occurs at the nucleus. The high affinity of 7,8-di(OH)H2BP for DNA may play a role in similarly elevating in vivo nuclear concentrations of the non-reactive proximate carcinogen.

Interaction of O,O,S-trimethyl phosphorothioate and O,S,S-trimethyl phosphorodithioate, the impurities of malathion with supercoiled PM2 DNA

The interaction of O,O,S-trimethyl phosphorothioate and O,S,S-trimethyl phosphorodithioate, the impurities found in malathion, with DNA at pH 8.0, was investigated. Supercoiled PM2 DNA was incubated with these compounds at pH 8.0 at 37 degrees C and then the superhelicity of the modified DNA was determined by gel electrophoresis. Both compounds caused unwinding of supercoiled DNA in dose- and incubation time-dependent manner. O,S,S-trimethyl phosphorodithioate was a more potent agent than O,O,S-trimethyl phosphorothioate. At 37 degrees C following 2.0 hours incubation, 100 mM O,S,S-trimethyl phosphorodithioate produced fully unwound DNA, whereas at 200 mM O,O,S-trimethyl phosphorothioate produced 80% unwound DNA following 12 hours' incubation. At the same condition, 5 mM methyl methanesulfonate, a potent alkylating mutagen, produced fully unwound DNA following 1 hour incubation at 5 mM. These results indicated that there were chemical interactions between these agents and DNA. The possibility of the interaction of OOS-TMP being as a covalent intercalation as well as strand nicking was discussed.

Depurination of benzo[a]pyrene-diolepoxide treated DNA

Rat liver DNA was treated in vitro with benzo[a]pyrene-diolepoxide (BPDE), the ultimate carcinogenic metabolite derived from the polycyclic hydrocarbon benzo[a]pyrene. On incubation of the reacted DNA, apurinic sites developed which gave rise to strand breakage in alkaline solution. The reduction in molecular weight produced by these breaks was measured by analytical ultracentrifugation. In the case of anti-BPDE this depurination was shown to occur in two stages. The first was mainly due to attack at the 7-position of guanine, to yield an adduct which was lost from the DNA within a few hours. The second stage was due to much slower loss of the major N2-guanine adduct. The separated enantiomers, (+)- and (-)-anti-BPDE, and syn-BPDE all caused depurination to various extents. It is argued that although these processes are important in a study of the action of BPDE on DNA in vitro, their contribution to the biological activity of BPDE is probably negligible.

Binding of benzo[a]pyrene to different chromatin domains following activation at the nuclear membrane

When isolated liver nuclei from methylcholanthrene-treated rats are incubated with benzopyrene, covalent adducts are formed between DNA and the ultimate carcinogen, benzopyrene diol epoxide. Brief digestion with DNaseI, or micrococcal nuclease has been used to demonstrate that benzopyrene metabolites bind more readily to DNA in chromatin regions with a more open, active conformation than to inactive chromatin.