Of carcinogenic adducts

If a-hydroxylation of NPYR is its mechanism of activation, one would expect the formation of carcinogen-DNA adducts containing a 4-oxobutyl- or related residue. Adducts have been isolated from the liver RNA of NPYR treated rats, but their structures have not been determined (25). Carcinogen DNA adducts have also been isolated from cultured human esophagus, colon, and bronchus (26, 27, 28). 

The mechanism of carcinogenesis by PAHs is believed to involve alkylation of an informational macromolecule in a critical, but at present unknown, manner. Such an interaction with a protein has been modelled by alkylation of a peptide this showed a conformational change occurred on alkylation. It has not yet been possible to study the structure of DNA alkylated by an activated carcinogen this is because DNA is a fiber and the structural order in it is not sufficient for a crystal structure determination. However the crystal structures of some alkylated portions of nucleic acids are described, particularly some nucleosides alkylated by chloromethyl derivatives of DMBA. In crystals of these alkylation products the PAH portion of the adduct shows a tendency to lie between the bases of other nucleoside... 

Identification of the chemical nature of these adducts is, however, possible only by chromatographic comparison with standards prepared by reacting the ultimate carcinogens with DNA. Proof of identity by co-chromatography should be undertaken using several chromatographic systems which depend for their separation upon different properties of the adduct molecules derivatization prior to reanalysis may be included. 

Studies on the comparative abilities (13) of B[a]P metabolites to bind to DNA in microsomal systems showed that the 7,8-dihydrodiol was the most efficient. This led to the proposal (69) that dihydrodiol epoxides were the ultimate carcinogenic metabolites. Chemical synthesis of all possible isomers (70.71) has allowed complete structural identification of the adducts (72-74). 

One of the major problems has been to determine the site of attachment of the PAH to the base. Some information may be obtained directly from the nmr spectra eliminating certain points of attachment. As mentioned above, if the C-8 proton of guanine or adenine can be identified, then this cannot be the point of attachment of the carcinogen. Estimation of the pKa s of the adducts either by titration (108) or partition (110) has, however, provided additional valuable information. Mass spectral fragmentation patterns can be of help in determining the site of substitution as well as in determining which bases are involved in binding (108.111-113). Substantial advances have been made in recent years on the mass spectral analysis of involatile compounds and derivatization is not always essential (114-118). X-ray analysis of DNA adducts has, to date, only been applied to model systems (119-121). 

Areas in which additional information is still needed relates to the role and relative importance of different adducts and the mechanisms by which they initiate cells. General principles are developing which will allow better predictions to be made at each of the stages of chemical carcinogenesis outlined in Table I. The ultimate goal therefore, would be, by a combined analysis of all these steps, to predict accurately the carcinogenicity of newly discovered or untested PAH derivatives. 

Grubor, N.M., Shinar, R., Jankowiak, R., Porter, M.D., and Small, G.J. (2004) Novel biosensor chip for simultaneous detection of DNA-carcinogen adducts with low-temperature fluorescence. Biosens. 

Nature of the critical cellular adduct. It is to be expected that the properties and behavior of the adduct formed with DNA will be of key importance. For example, the carcinogenicity of vinyl chloride has been attributed to its 7-N-(2-oxoethyl) derivative of guanine [60, 61]. 

Apart from gene silencing, other effects of DNA methylation include spontaneous deamination, enhanced DNA binding of carcinogens and increased UV absorption by DNA, all of which increase the rate of mutations, DNA adduct formation and subsequent gene inactivation... 

Nesnow S. 1994. Mechanistic linkage between DNA adducts, mutations in oncogenes, and tumorigenicity of carcinogenic aromatic hydrocarbons in strain A/J mice. In Chemical mixtures and quantitative risk assessment. Abstract of the second annual HERL symposium, Nov. 7-10., Raleigh, North Carolina Health Effects Research Laboratory, U.S. Environmental Protection Agency. 

CC 1065 108 is a highly toxic antibiotic isolated from Streptomyces zelensis containing a reactive spirocyclopropane ring. It cleaves DNA through a mechanism similar to the cleavage which occurs upon treatment of DNA with the carcinogenic ptaquiloside 25 (vide supra. Sect. 2.6), namely by depurination of alkyladenine adducts, Eq. (43) [142]. 

Chronic exposure to hepatotoxic doses of DMN has also been found to suppress humoral and cellular immunity in mice. DMN is geno-toxic in a wide variety of assays inducing DNA synthesis, chromosomal aberrations, sister chromatid exchange, and bacterial mutations. " The formation of DNA adducts by metabolites of DMN may play a critical role in the carcinogenic process."... 

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