Double-stranded DNA stereoselectively binds benzo(a)pyrene diol epoxides

Abstract

BEFORE it can exert its mutagenic and carcinogenic activities^1,2 benzo(a)pyrene (BP) requires metabolic activation and it is widely considered^3–5 that the initiating event in these processes is covalent binding to DNA. The predominant form of the activated hydrocarbon is a diol epoxide^6–11, of which four possible isomers are (+) and (−) enantiomers of 7β,8α-dihydroxy-9α-10α-epoxy-7,8,9,10-tetrahydro-BP (anti-BP diol epoxide) and (+) and (−) enantiomers of 7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydro-BP (syn-BP diol epoxide). Enzymatic formation of the (+) anti and (+) syn configurations have been reported elsewhere^12,13. We have shown that the racemic (±) anti-BP diol epoxide forms two stable covalent adducts each with the exocyclic amino groups of deoxyguanosine, deoxyadenosine and probably deoxycytidine¹⁴. We report here that we have now resolved the enantiomers of (±) anti-BP diol epoxide and reacted the optically pure hydrocarbons with DNA. Each enantiomer reacts with the exocyclic amino group of deoxyguanosine to form a pair of diasteriomers in a ratio which depends on the secondary structure of the nucleic acid. The (+) enantiomer reacts preferentially or asymmetrically with deoxyguanosine in double stranded DNA but the two enantiomers react equally with the same position in single-stranded DNA. The basis for the stereoselective binding is unknown but may involve formation of sterically orientated physical complexes before covalent attachment of the hydrocarbon to DNA. The mode of binding in the deoxyadenosine sites, on the other hand, must be different, as the (+) and (−) enantiomers react equally in either single- or double-stranded DNA.