The nature of the deoxyribonucleosides involved in the binding of carcinogenic hydrocarbons to the DNA of mouse embryo cells

The reaction of (+/-)-7alpha, 8beta-dihydroxy-9beta, 10beta-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene with DNA

The reaction of (+/-)-7alpha,8beta-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo(a)pyrene (BP diol epoxide) with DNA, followed by enzymatic hydrolysis and LH 20 Sephadex column chromatography, yielded products containing a purine deoxyribonucleoside linked to a 7,8,9,10-tetrahydrobenzo(a)pyrene derivative. DNA containing tritiated adenine or pyrimidines gave little or no tritiated products, showing that reaction occurred chiefly with the guanine residues. Results with 8-(3H)-guanine-labelled DNA, and studies on the protonation and ionization of the products in acid or alkaline media, led to the formulation of the main product as an N2-substituted guanine derivative. The reaction of BP diol epoxide with deoxyadenosine and deoxyguanosine gave further nucleoside derivatives which were probably minor products in the reaction with DNA. Treatment with strong acid unexpectedly converted each of the isolated products to free guanine or adenine.

The 7-methylbenz(a)anthracene deoxyribonucleoside products isolated from DNA after metabolism of the carcinogen by rat liver microsomes in the presence of DNA

Metabolism of 7-methylbenz[a]anthracene (7MeBA) by 3-methylcholanthrene-induced rat liver microsomes in the presence of added native or denatured DNA resulted in covalent binding of the hydrocarbon to the nucleic acid. Enzymatic degradation and column chromatographic fractionation showed that the hydrocarbon-deoxyribonucleoside products were separable from the products similarly obtained from DNA having 7MeBA bound following treatment of mouse embryo cells in culture with this hydrocarbon. Comparison of the microsome catalysed hydrocarbon-deoxyribonucleoside products with those obtained by reaction with DNA of 7MeBA-5,6-oxide suggested that this K-region epoxide made a significant contribution to the liver microsome-induced DNA binding.

Retention of tritium during the binding of tritiated benz(a)pyrene to DNA

Chemical conversion of generally tritiated benzo(a)pyrene to 6 and 1,6-substituted derivatives resulted in 30% and 48% loss of tritium respectively. Metabolism of [3H], [14C]-benzo(a)pyrene by rat liver microsomes yielded 3-hydroxybenzo(a)pyrene with 30% loss of tritium, a mixture of quinones with 50% loss of tritium and three dihydrodiol metabolites which had retained all the tritium of the parent hydrocarbon. DNA isolated from mouse embryo cells which had been exposed to [3H], [14C]-benzo(a)pyrene, and DNA with this hydrocarbon bound following in vitro rat liver microsomal incubation were degraded enzymically and the hydrocarbon-deoxyribonucleoside products isolated. The tritium contents of the products obtained from both DNA samples were very close to those of the original double labelled benzo(a)pyrene. These results are inconsistent with a phenol or quinone intermediate being responsible for the reaction with DNA, but fully consistent with a diol epoxide intermediate as proposed by Sims et al. (1974).