Ester derivatives of the hydroxylamine metabolic oxidation products of heterocyclic amines (HCAs) are the ultimate mutagenic and carcinogenic compounds derived from this ubiquitous class of chemical carcinogens that are produced during the cooking of protein‐containing foods. Considerable work has been done on the mechanism of formation of the HCAs during cooking processes, their detection at ppb levels in food products, the metabolism of the HCAs and the detection and characterization of DNA adducts of the HCA metabolites, but until recently very little work had been done to characterize the chemistry of the carcinogenic/mutagenic metabolites themselves. This paper reviews our recent work on the chemistry of model carcinogens from this class. The kinetics of their decomposition in aqueous solution, identification of reaction products and the characterization of the reactivity and selectivity of heterocyclic nitrenium ions generated during their reactions are presented. The implications of these results with respect to mutagenicity and carcinogenicity are discussed. Copyright © 2004 John Wiley & Sons, Ltd.

The model ultimate carcinogens 1a-d, related to the metabolites of the food-derived carcinogenic heterocyclic amines Glu-P-1, Glu-P-2, MeIQx, and IQx, spontaneously decompose in neutral aqueous solution to generate the heterocyclic nitrenium ions, 2a-d. The less reactive esters 1a and 1b also undergo acid-catalyzed ester hydrolysis to generate the corresponding hydroxamic acids at pH <2, while the more reactive 2c and 2d are prone to rearrangement in nonaqueous solvents. The reactions of the nitrenium ions with AcO(-), HPO(4)(2-), N(3)(-), and 2'-deoxyguanosine (d-G) were characterized in aqueous solution by using a combination of competitive trapping methods and product isolation and identification. The reactions with N(3)(-) and d-G generally follow patterns previously established for carbocyclic nitrenium ions, but the reactions with AcO(-) and HPO(4)(2-) are unusual. Similar reactions have previously only been reported for heterocyclic 1-alkyl-2-imidazolium ions. The N(3)(-)/solvent selectivities of these ions (5.1 x 10(6) M(-1) for 2a, 2.3 x 10(6) M(-1) for 2b, 1.2 x 10(5) M(-1) for 2c, and 5.2 x 10(4) M(-1) for 2d) are comparable to those of highly selective carbocyclic nitrenium ions. If k(az) for these ions is diffusion limited at ca. 5 x 10(9) M(-1) s(-1) the aqueous solution lifetimes of these ions range from 10 micros (2d) to 1 ms (2a). These ions are also highly selective for trapping by d-G, but comparisons to other nitrenium ions show that they are 10- to 50-fold less selective for trapping by d-G than they would be if both the N(3)(-) and d-G reactions were diffusion limited. This is not a consequence of their heterocyclic structures. Several carbocyclic ions show similar behavior. The relatively inefficient trapping of 2c and 2d by d-G may account for the observation of the unusual minor N-2 d-G adduct that is isolated for both of these nitrenium ions, but has not previously been observed for the reactions of other nitrenium ions with monomeric d-G.

The hydrolysis reactions of the title compounds N-sulfonatooxy-N-acetyl-1-aminonaphthalene, 2d, and N-sulfonatooxy-N-acetyl-2-aminonaphthalene, 2e, in 5% CH 3 CN-H 2 O (20 °C, pH 5.7-7.5, µ ) 0.5) appear to involve nitrenium ion intermediates that exhibit very small azide/solvent trapping efficiencies. The azide/solvent selectivities, S, were estimated from fitting azide-and solvent-derived product yields as a function of [N 3 -]. The derived values of S for the N-acetyl-N-(1-naphthyl)nitrenium ion (3d) of 0.7 ( 0.1 M -1 and the N-acetyl-N-(2-naphthyl)nitrenium ion (3e) of 1.5 ( 0.2 M -1 show that these ions have short lifetimes (ca. 10 -10 s) in aqueous solution. In turn, these results suggest that the hydrolysis reactions of 2d and 2e should proceed, in part, through ion-pair and/or preassociation pathways. The decrease in the yield of the rearrangement products N-acetyl-1-amino-2-sulfonatooxynaphthalene, 6, and N-acetyl-2-amino-1-sulfonatooxynaphthalene, 11, with increasing [N 3 -] indicates that this is the case. Plots of log(mutagenicity) toward Salmonella typhimurium TA 98 and TA 100 for a series of polycyclic and monocyclic aromatic amines vs log(S) for ArNAc + show that there is no general correlation of mutagenicity with nitrenium ion selectivity, but there does appear to be a limited correlation of these two quantities for five polycyclic amines for which there are reliable mutagenicity and nitrenium ion selectivity data. These results suggest that nitrenium ion selectivity is one of several factors that determines the mutagenicity of aromatic amines.

Hydrolysis of the carcinogenic title compound 1a in 5 vol % CH3CN−H2O, μ = 0.5, 20 °C at pH 7.2 in 0.02 M phosphate buffer, yields the rearranged material 3-(sulfonatooxy)-N-acetyl-4-aminostilbene (4) (23 ± 1%), threo-1,2-dihydroxy-1-phenyl-2-(4-acetamidophenyl)ethane (5) (57 ± 2%), and erythro-1,2-dihydroxy-1-phenyl-2-(4-acetamidophenyl)ethane (6) (20 ± 2%) in the absence of added nucleophiles. Addition of N3 - has no effect on the rate constant for decomposition of 1a (ca. 1.9 × 10-2 s-1), but generates a number of adducts that result from trapping of three different electrophilic intermediates. The ortho-N3 adduct 3-azido-N-acetyl-4-aminostilbene (7) is produced from trapping of the nitrenium ion 2. A fit of the product yield data as a function of [N3 -] provides the ratio k az/k s of 280 ± 10 M-1 for competitive trapping of 2 by N3 - and H2O. The nucleophilic aromatic substitution product 7 is a minor reaction product. The predominant site of attack by N3 - on 2 (ca. 85%) is at the β-vinyl carbon to produce the quinone imide methide 3b. Attack of H2O at the same site produces the analogous intermediate 3a. Both of these electrophilic species are competitively trapped by N3 - and H2O with trapping ratios k az‘/k s‘ for 3b of 107 ± 8 M-1 and k az‘‘/k s‘‘ for 3a of 39 ± 2 M-1. The reactivity patterns of 2 are unlike those of other N-arylnitrenium ions that undergo predominant nucleophilic aromatic substitution with nucleophiles such as N3 -. The quinone imide methides that are produced by nucleophilic attack on the β-carbon of 2 react selectively enough with nonsolvent nucleophiles that they may be physiologically relevant.

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