Hemin potentiates nitric oxide-mediated nitrosation of 2-amino-3- methylimidazo[4,5-f]quinoline (IQ) to 2-nitrosoamino-3-methylimidazo[4,5-f] quinoline

Heme has been reported to be an important contributor to endogenous N-nitrosation within the colon and to the enhanced incidence of colon cancer observed with increased intake of red meat. This study uses the heterocyclic amine 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) as a target to evaluate hemin potentiation of nitric oxide (NO)-mediated nitrosation. Formation of ¹⁴C-2-nitrosoamino-3-methylimidazo[4,5-f]quinoline (N-NO-IQ) was monitored by HPLC following incubation of 10 μM IQ with the NO donor spermine NONOate (1.2 μM NO/min) at pH 7.4 in the presence or absence of hemin. N-NO-IQ formation due to autoxidation of NO was at the limit of detection (0.1 μM) and increased 22-fold in the presence of 10 μM hemin and an in situ system for generating H₂O₂ (glucose oxidase/glucose). A linear increase in N-NO-IQ formation was observed from 1 to 10 μM hemin. Significant nitrosamine formation occurred at fluxes of NO and H ₂O₂ as low as 0.024 and 0.25 μM/min, respectively. Potentiation by hemin was not affected by a 400-fold excess flux of H ₂O₂ over NO or a 4.8-fold excess flux of NO over H ₂O₂. Reactive nitrogen species produced by hemin potentiation had a 46-fold greater affinity for IQ than those produced by autoxidation. Azide inhibited autoxidation, suggesting involvement of the nitrosonium ion, NO⁺. Hemin potentiation was inhibited by NAIDH, but not azide, suggesting oxidative nitrosylation with NO₂^. or a NO₂^.-like species. IQ and 2,3-diaminonaphthylene were much better targets for nitrosation than the secondary amine morpholine. Apc^min mice with dextran sulfate sodium-induced colitis demonstrated increased levels of urinary nitrite and nitrate consistent with increased expression of iNOS and NO synthesis. As reported previously, identical conditions increased fecal N-nitroso compounds. Thus, hemin potentiation of NO-mediated nitrosation of heterocyclic amines provides a testable mechanism by which red meat consumption can generate N-nitroso compounds and initiate colon cancer under inflammatory conditions, such as colitis.