Thermal stabilization of the DNA duplex by adducts of aflatoxin B1

The trans-8,9-dihydro-8-(N7-guanyl)-9-hydroxyaflatoxin B1 cationic guanine N7 adduct of aflatoxin B1 thermally stabilizes the DNA duplex, as reflected in increased Tm values upon adduction. The magnitude of the increased Tm value is characteristically 2–3°C. The major rotamer of the neutral guanine N7 adduct trans-8,9-dihydro-8-(2,6-diamino-4-oxo-3,4-dihydropyrimid-5-yl-formamido)-9-hydroxy aflatoxin B1 (the FAPY major adduct) exhibits a 15°C increase in Tm in 5′-d(CTATFAPYGATTCA)-3′-5′-d(TGAATCATAG)-3′. Site-specific mutagenesis experiments reveal the FAPY major adduct induces GT mutations in Escherichia coli at a frequency six times higher than that of the cationic adduct (Smela, M. E.; Hamm, M. L.; Henderson, P. T.; Harris, C. M.; Harris, T. M.; Essigmann, J. M. Proc Natl Acad Sci USA, 99, 6655–6660). Thus, the FAPY major lesion may account substantially for the genotoxicity of AFB1. Structural studies for cationic and FAPY adducts of aflatoxin B1 suggest both adducts intercalate above the 5′-face of the modified deoxyguanosine and that in each instance the aflatoxin moiety spans the DNA helix. Intercalation of the aflatoxin moiety, accompanied by favorable stacking with the neighboring base pairs, is thought to account for the increased thermal stability of the aflatoxin cationic guanine N7 and the FAPY major adducts. However, the structural basis for the large increase in thermal stability of the FAPY major adduct in comparison to the cationic guanine N7 adduct of aflatoxin B1 is not well understood. In light of the site-specific mutagenesis studies, it is of considerable interest. For both adducts, the intercalation structures are similar, although improved stacking with neighboring base pairs is observed for the FAPY major adduct. In addition, the presence of the formamido group in the aflatoxin B1 FAPY major adduct may enhance duplex stability, perhaps via intrastrand sequence-specific hydrogen bonding interactions within the duplex. © 2002 Wiley Periodicals, Inc. Biopolymers (Nucleic Acid Sci) 65: 190–201, 2002