32P-postlabeling analysis of IQ, MeIQx and PhIP adducts formed in vitro in DNA and polynucleotides and found in vivo in hepatic DNA from IQ-, MeIQx- and PhIP-treated monkeys.

Abstract

The 32P-postlabeling method was used to examine the adducts in DNA, polynucleotides, and mononucleotides reacted in vitro with the N-hydroxy and N-acetoxy derivatives of 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3, 8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP). Adduct profiles were compared to those found in vivo in liver of cynomolgus monkeys fed IQ, MeIQx or PhIP. The N-acetoxy derivatives of IQ, MeIQx and PhIP (generated in situ from the corresponding N-hydroxylamine in the presence of acetic anhydride) each formed three principal adducts in DNA. Adduct 1 of IQ, MeIQx and PhIP was chromatographically identical to the 32P-labeled bis(phosphate) derivative of N-(deoxyguanosin-8-yl)-IQ, N-(deoxyguanosin-8-yl)-MeIQx, and N-(deoxyguanosin-8-yl)-PhIP respectively, and this adduct comprised approximately 65% of total adduct levels found in DNA in vitro. The C8-guanine adduct and the two minor adducts were also found in poly(dG-dC). poly(dG-dC), suggesting that the two minor adducts of IQ, MeIQx and PhIP are also formed on the guanine base. The N-acetoxy derivatives of IQ, MeIQx, and to a much lesser extent PhIP, also formed adducts with adenine-containing polynucleotides including poly(dA), poly(dA).poly(dT) and poly(dA-dT).poly(dA-dT), but these adenine adducts were chromatographically different from those found in DNA. The three guanine adducts of N-acetoxy-IQ, -MeIQx and -PhIP found in vitro in DNA and in guanine-containing polynucleotides were also found in the liver of monkeys fed IQ, MeIQx or PhIP respectively, indicating that metabolic activation via N-hydroxylation and esterification occurred in vivo in monkeys. With each compound, the C8-guanine adduct was the predominant adduct found in vivo. The results indicate similarities among IQ, MeIQx and PhIP in the DNA adducts formed in vitro and in vivo and substantiate the use of the 32P-postlabeling method for comparative adduct studies.