Aflatoxin B₁Epoxide, the Ultimate Carcinogenic form of Aflatoxin B₁: Synthesis and Reaction with Dna

Abstract

The ultimate carcinogenic form of aflatoxin B₁, the exo-8,9-epoxide, has long evaded both isolation and synthesis. The epoxide has now been prepared in essentially quantitative yield by oxidation of aflatoxin B₁ with dimethyldioxirane. The epoxide is sufficiently stable that it can be isolated and characterized but it undergoes rapid solvolysis in hydroxylic media. Highly efficient reaction occurs with DNA to give N⁷ adducts on guanine. The oligodeoxynucleotide duplexes d(ATCGAT)₂ and d(ATGCAT)₂ react with the epoxide to give the AFB₁ adduct at guanine N⁷. Stoichiometry of adduct formation is 1 per duplex in the first case and 2 per duplex in the second. NMR studies, including measurement of nuclear Overhauser effects by one and two dimensional techniques, reveal that the aflatoxin moiety is intercalated in the duplex on the 5’ side of the point of attachment. The difference in stoichiometry observed with the two duplexes reflects the fact that with d(ATCGAT)₂ intercalation of aflatoxin between the C:G and G:C base-pairs precludes entry of a second molecule into that site, whereas intercalation occurs between T:A and G:C base-pairs in the second case so that two sites of intercalation are available. Several lines of evidence indicate that non-covalent association of aflatoxin with DNA involves intercalation and lead to the hypothesis that the transition state for reaction of aflatoxin B₁ epoxide with DNA is also intercalative.