DNA ADDUCT FORMATION OF THE FOOD CARCINOGEN 2-AMINO-3-METHYLIMIDAZO[4,5-F]QUINOLINE AT THE C-8 AND N-2 ATOMS OF GUANINE

DNA adduct formation of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) h as been investigated by P-32-postlabeling. Similar adduct profiles wer e observed from calf thymus DNA modified in vitro with the putative ca rcinogenic metabolite N-2-acetoxyamino-3-methylimidazo[4,5-f]quinoline (N-acetoxy-IQ) and from hepatic DNA of rats treated with IQ. guanosin -8-yl)-2-amino-3-methylimidazo[4,5-f]quino (dG-C8-IQ) accounted for ap proximately 90% of the total adducts observed in calf thymus DNA under postlabeling conditions where ATP was limiting; however, in-N-2-yl)-2 -amino-3-methylimidazo[4,5-f]quinoline (dG-N-2-IQ) was detected only w hen DNA was labeled with excess ATP. Under these labeling conditions, dG-C8-IQ and dG-N-2-IQ accounted for approximately 75% and 7% of the t otal adducts, respectively. Five other spots accounted for the remaini ng radioactivity. Comparable results were obtained from rat liver DNA. Following DNA adduct enrichment by solid phase extraction, dG-C8-IQ a nd dG-N-2-IQ accounted for 60-76% and 10-13%, respectively, of the tot al adducts in rat liver. The adduct profiles obtained from reaction of 2'-deoxyguanosine 3'-monophosphate (dG-3'-PO4-) with the photoactivat ed azide derivative of IQ, 2-azido-3-methylimidazo[4,5-f]quinoline (N- 3-IQ), were qualitatively similar to those obtained by reaction with N -acetoxy-IQ. The C-8 and N-2 adducts were the only reaction products d etected. The reactivity and sites of adduct substitution were dependen t upon solvent conditions and pH, with increasing adduct formation und er alkaline pH. The chemical reactivity of photoactivated N-3-IQ with dG-3'-PO4- was significantly greater than that of N-acetoxy-IQ when re actions were conducted in water, in citrate buffer (pH 5.0), or in pho sphate buffer (pH 7.4). Increased reactivity was attributed to increas ed levels of dG-C8-IQ adduct formation, except for reactions conducted in citrate buffer (pH 5.0), where there was a proportional increase i n both C-8 and N-2 guanine adducts. However, the chemical reactivity o f these two IQ derivatives and their sites of dG substitution were ide ntical when the reactions were conducted in phosphate buffer (pH 9.0). The ratio of the dG-N-2-IQ adduct to the total adducts increased at a lkaline pH in reactions involving N-3-IQ, but the ratio was not affect ed by a change in the pH of the medium for reactions with N-acetoxy-IQ . The ratio of the dG-N-2-IQ adduct to the total adducts also increase d as a function of phosphate concentration for reactions involving bot h N-acetoxy-IQ and N-3-IQ. Formation of the ring substituted dG-N-2-IQ adduct indicates that nitrenium and carbenium ion formation occurred for both N-acetoxy-IQ and photoactivated N-3-IQ. The reactivity and th e effects of pH and solvent on sites of adduct substitution are distin ct for these two chemically reactive derivatives of IQ and suggest tha t the mechanism of adduct formation is not identical.