FORMATION OF DNA-ADDUCTS BY THE FOOD MUTAGEN 2-AMINO-3,4,8-TRIMETHYL-3H-IMIDAZO[4,5-F]QUINOXALINE (4,8-DIMEIQX) IN-VITRO AND IN-VIVO - IDENTIFICATION OF A N-2-(2'-DEOXYGUANOSIN-8-YL)-4,8-DIMEIQX ADDUCT

The covalent binding of the mutagenic N-2-hydroxy metabolite of the food mutagen 2-amino-3,4,8-trimethyl-3H-imidazo[4,5-f]quinoxaline (4,8-DiMeIQx) to 2'-deoxynucleosides and DNA was investigated in vitro and in vivo. N-2-Hydroxy-4,8-DiMeIQx reacted to a small extent spontaneously with 2-deoxyguanosine. However acetylation of N-2-hydroxy-4,8-DiMeIQx, with acetic anhydride to form the N-2-acetoxy derivative prior to reaction with 2-deoxyguanosine resulted in much higher yield of adduct. N-2-Acetoxy-4,8-DiMeIQx did not form adducts with 2'-deoxyadenosine, 2'-deoxycytidine or 2'-deoxythymidine. The adduct formed between the N-2-OH metabolite of 4,8-DiMeIQx and 2-deoxyguanosine was analysed by mass spectrometry and NMR spectroscopy and the structure of the adduct was shown to be N-2-(2'-deoxyguanosin-8-yl)-4,8-DiMeIQx. N-2-Acetoxy-4,8-DiMeIQx reacted with calf thymus DNA and formed a covalently bound 4,8-DiMeIQx residue, which could not be removed by repeated precipitations or solvent extractions. The 4,8-DiMeIQx-DNA was hydrolysed enzymatically with nuclease P1/acid phosphatase and HPLC analysis showed that 70% of the bound mutagen was recovered as N-2-(2'-deoxyguanosin-8-yl)-4,8-DiMeIQx. An additional minor adduct accounting for similar to 15% of the bound mutagen showed UV spectral characteristics similar to N-2-(2'-deoxyguanosin-8-yl)-4,8-DiMeIQx and is probably an undigested oligomer. P-32-Postlabelling analysis of calf thymus DNA modified with 4,8-DiMeIQx in vitro and liver DNA from rats dosed with 50 mg/kg 4,8-DiMeIQx showed a similar adduct pattern. In both samples N-2-(2'-deoxyguanosin-8-yl)-4,8-DiMeIQx accounted for 60-70% of the bound mutagen. Thus, these results show that 4,8-DiMeIQx similar to other heterocyclic amines form adducts with C-8 of guanine both in vitro and in vivo via its N-2-OH metabolite.