In vitro DNA modification by the ultimate carcinogen of 4-nitroquinoline-1-oxide: influence of superhelicity

The effect of DNA tertiary structure on in vitro modificationby 4-acetoxy-aminoquinoline-l-oxide (Ac-4-HAQO) was investigated.The reactivity of pAT153 plasmid DNA depended on the conformationalstate of the molecule: it progressively decreased accordingto the decrease of the superhelical tension, being negativelysupercoiled DNA about two times more susceptible than singly-nickedrelaxed DNA. HPLC of the three main Ac-4-HAQO adducts showedthat 3-(deoxyguanosin-N2-yl)-4-aminoquinoline-1-oxide, N-(deoxyguanosin-C8-yl)-4-aminoquinoline-l-oxideand 3-(deoxyadenosin-N6-yl)-4-aminoquinoline-l-oxide accountedfor 50, 25 and 10% of total quinoline DNA base adducts in allDNA conformations tested, except in the negatively super-coiledtopoisomers where they accounted for 80, 15 and 5% respectively.DNA modification by Ac-4-HAQO resulted also in the formationof apurinic/apyrimidinic sites and in strand scissions. Thequantification of these damages revealed that they representan important fraction of all damaging events and that theiryield is also influenced by DNA superstructure. Thus, theselesions must be considered as important DNA damage induced byAc-4-HAQO.