SYNTHESIS OF DEPURINATING DNA-ADDUCTS FORMED BY ONE-ELECTRON OXIDATION OF 7H-DIBENZO[C,G]CARBAZOLE AND IDENTIFICATION OF THESE ADDUCTS AFTER ACTIVATION WITH RAT-LIVER MICROSOMES

It is hypothesized that 7H-dibenzo[c,g]carbazole (DEC) is metabolicall y activated by one-electron oxidation in accordance with its propensit y to be easily oxidized to its radical cation. Iodine oxidation of DEC produces a radical cation that subsequently binds to nucleophilic gro ups of dG or Ade. Oxidation of DEC in the presence of dG produces thre e adducts: DBC-5-N7Gua, DBC-6-N7Gua, and DBC-6-C8Gua, whereas in the p resence of Ade, four adducts are obtained: DBC-5-N7Ade, DBC-5-N3Ade, D BC-5-N1Ade, and DBC-6-N3Ade. Formation of these adducts demonstrates t hat the DEC radical cation reacts at C-5 or C-6 with the reactive nucl eophiles N-7 and C-8 of dG and N-7, N-3, and N-l of Ade. Formation of DNA adducts by DEC was studied by using horseradish peroxidase or 3-me thylcholanthrene-induced rat liver microsomes for activation. Identifi cation of the biologically-formed depurinating adducts was achieved by comparison of their retention times on HPLC in two different solvent systems and by matrix-assisted laser desorption ionization (MALDI) mas s spectrometry. Quantitation of the adducts formed by rat liver micros omes shows that 96% are depurinating adducts, DBC-5-N7Gua (11%), DBC-6 -N7Gua (32%), and DBC-5-N7Ade (53%), and 4% are unidentified stable ad ducts. Activation of DEC by horseradish peroxidase affords 32% stable unidentified adducts and 68% depurinating adducts: 19% DBC-5-N7Gua, 13 % DBC-6-N7Gua, 27% DBC-5-N7Ade, and 9% DBC-5-N3Ade. Thus, activation o f DEC by cytochrome P450 predominantly forms depurinating adducts by o ne-electron oxidation.