Alteration of the aflatoxin cyclopentenone ring to a .delta.-lactone reduces intercalation with DNA and decreases formation of guanine N7 adducts by aflatoxin epoxides

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Abstract
The regio- and stereospecificity exhibited by reaction of aflatoxin B1 8,9-epoxide with DNA as well as the efficiency of reaction are remarkable and suggests that a specific orientation of bound epoxide facilitates formation of the transition state leading to guanine N7 adducts. We have compared aflatoxins B1 and B2 with aflatoxins G1 and G2 as to their binding with calf thymus DNA, d(ATGCAT)2, d(GCATGC)2, and plasmid pBR322. Aflatoxins B1 and B2 contain a cyclopentenone ring fused to the lactone ring of the coumarin. They have similar DNA association constants and intercalate with B-DNA, as demonstrated by NMR analysis of association with d(ATGCAT)2 and d(GCATGC)2, alteration of pBR322 electrophoretic mobility, and flow dichroism using linearly oriented calf thymus DNA. The less planar .delta.-lactone ring of aflatoxins G1 and G2 reduces DNA binding affinity by approximately 1 order of magnitude. Nevertheless, binding studies with d(ATGCAT)2 and d(GCATGC)2 suggest that aflatoxins G1 and G2 also bind B-DNA by intercalation. To establish the existence of a relationship between the association of these aflatoxins with DNA and adduct formation induced by aflatoxin epoxides, we compared the yield of guanine N7 adduct from aflatoxin B1 8,9-epoxide and from aflatoxin G1 9,10-epoxide at three concentrations of calf thymus DNA. As DNA concentration is decreased, two observations are made: (1) the number of adducts formed by either aflatoxin B1 8,9-epoxide or aflatoxin G1 9,10-epoxide is reduced with a concomitant increase in formation of the respective dihydrodiols, and (2) the ratio of adducts formed by aflatoxin G1 9,10-epoxide to those formed by an equivalent concentration of aflatoxin B1 8,9-epoxide decreases. We conclude that, for these two aflatoxin epoxides, the transition state for adduct formation at guanine N7 involves an intercalated complex with DNA. The reduced genotoxicity of aflatoxin G1 as compared to aflatoxin B1 may in part be due to decreased DNA binding affinity, resulting in the formation of fewer adducts.

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