Marked differences in base selectivity between DNA and the free nucleotides upon adduct formation from Bay- and Fjord-region diol epoxides.

Distributions of adducts formed from each of the four optically active isomers of 3,4-dihydroxy-1,2-epoxy-1,2,3, 4-tetrahydrobenzo[c]phenanthrene and of 7,8-dihydroxy-9,10-epoxy-7,8, 9,10- tetrahydrobenzo[a]pyrene (BcPh and BaP diol epoxides) on reaction with an equimolar mixture of deoxyadenosine and deoxyguanosine 5'-monophosphates were compared with the known adduct distributions from these diol epoxides (DEs) upon reaction with calf thymus DNA in vitro. In the presence of an equimolar (100 mM total) mixture of dAMP and dGMP, the efficiency of formation of all types of adducts relative to tetraols is comparable for both the BaP ( approximately 40-60%) and BcPh ( approximately 30-40%) diol epoxides. This is in contrast to the partitioning between tetraols and adducts observed with DNA, where the BcPh DEs form adducts much more efficiently than the BaP DEs. Preference for trans versus cis ring opening by the exocyclic amino groups of the free nucleotides in the dAMP/dGMP mixture is greater for the DE diastereomer in which the benzylic hydroxyl group and the epoxide oxygen are trans (DE-2). This is qualitatively similar to the preferences for trans versus cis adduct formation on reaction of these isomers with DNA, as well as trans versus cis tetraol formation on their acid hydrolysis. For the BcPh DE isomers, competitive reaction between dGMP and dAMP gives 40-62% of the total exocyclic amino group adducts as dA adducts. A similar distribution of dG versus dA adducts had previously been observed on reaction of the BcPh DEs with DNA, except in the case of (+)-3(R),4(S)-dihydroxy-1(R),2(S)-epoxy-1,2,3, 4-tetrahydrobenzo[c]phenanthrene, which gives approximately 85% dA adducts on reaction with DNA. With the BaP DEs, 60-77% of the exocyclic amino group adducts formed upon competitive reaction with the free nucleotides are derived from dGMP. The observed dG selectivity of these BaP DEs is much smaller with the nucleotide mixture than it is with DNA, leading to the conclusion that DNA structure has a much larger modifying effect on the base selectivity of the BaP relative to the BcPh DEs.