Heterocyclic Aromatic Amines

Potential Human Carcinogens

Robert J. Turesky*    Division of Environmental Health Sciences, Wadsworth Center, New York State Department of Health, Empire State Plaza, Albany, NY, USA
* Corresponding author. Tel.: 518-474-4151; Fax: 518-473-2095 email address: Rturesky@wadsworth.org

1 INTRODUCTION

The diet provides key nutritents and minerals for sustenance and well-being of the body; however, hazardous chemicals present in the diet can also contribute to the development of diseases such as cancer [1]. Heterocyclic aromatic amines (HAAs) are a class of compounds in the diet that are receiving widening attention as a risk factor for human cancer. HAAs were discovered more than 30 years ago by Professor Sugimura and his colleagues in Japan [2], who showed that the charred parts and smoke generated from broiled fish and beef contained potent activity in Salmonella typhimurium-based mutagenicity assays. Since that discovery, more than 20 HAAs have been identified in cooked meats, fish, and poultry [3–6]. Several HAAs have also been identified in cigarette smoke condensate and diesel exhaust [7,8]. HAAs induce cancer of the oral cavity, liver, stomach, lung, colorectum, prostate, and mammary glands of rodents, during long-term feeding studies [5]. It is noteworthy that the colon, prostate, and female mammary glands are common sites of cancer in the Western countries in which well-done cooked meats containing HAAs are frequently consumed [9]; and the rates of cancer in these organs are increasing in Japan and other countries that are adapting western dietary habits [5]. Therefore, questions have been raised about the safety of foods containing HAAs, and much research has already been devoted to the biochemical toxicology of HAAs and their potential role in the etiology of human cancer. The recent Report on Carcinogens, Eleventh Edition, of the National Toxicology Program concluded that several prevalent HAAs are “reasonably anticipated” to be human carcinogens [10].

1.1 Mechanisms of formation of HAAs

There are two major classes of HAAs. One class of compounds is known as “pyrolytic HAAs.” These compounds arise during the high-termperature pyrolysis (>250 °C) of individual amino acids. HAAs are formed during the pyrolysis of tryptophan, glutamic acid, phenylalanine, or ornithine. Several HAAs also arise during the pyrolysis of proteins such as soybean globulin and casein [2,11]. The reactions of these amino acids or proteins at high temperature produce deaminated and decarboxylated products and reactive radical fragments, which can combine to form heterocyclic ring structures. The known pyrolytic HAAs comprise five structurally distinct groups that contain pyridoindoles, pyridoimidazoles, phenylpyridine, tetraazofluoranthene, or benzmidazole moieties (Figure 1). The tryptophan pyrolysates 2-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) and 2-amino-1-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-2), and the soybean globulin pyrolysates 2-amino-9H-pyrido[2,3-b]indole (AαC) and 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeAαC) each contain an indole moiety as a part of their structure, which may be derived from tryptophan. The high temperature of burning cigarettes catalyzes the formation of several pyrolytic HAAs [12]. AαC and MeAαC are the two most abundant HAAs that arise in mainstream cigarette smoke [13], with levels reported up to 258 and 37 ng per cigarette, respectively. These levels are considerably higher than the levels reported for many polycyclic aromatic hydrocarbons and aromatic amines, which are established human carcinogens [14]. Several pyrolytic HAAs are formed at low nanogram per gram concentrations in some broiled or grilled fish and meats that are cooked very well-done or burnt [15–21]. However, concentrations of pyrolytic HAAs are typically formed below the nanogram per gram range in cooked meat prepared under common household cooking practices, because the temperatures of the cooking surfaces are below 250 °C: this heat level is insufficient to catalyze the production of pyrolytic HAAs.

The second class of HAAs, named aminoimidazoarenes (AIAs) (Figure 1), are formed in meats cooked at temperatures (150–250 °C) commonly used in the household kitchen. The Maillard reaction is thought to play an important role in the formation of AIAs. Jägerstad and coworkers employed a model system that contained creatine, free amino acids, and hexoses, constituents present in uncooked meats, and they refluxed these components at temperatures ≥130 °C to produce 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3-methylmidazo[4,5-f]quinoxaline (IQx), 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx), and 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx) [22,23]. The proposed pathways of AIA formation are shown in Figure 2A and B. The N-methyl-2-aminoimidazole portion of the molecule is derived from creatine, and the remaining parts of the IQ and IQx skeleton are assumed to arise from Strecker degradation products (e.g., pyridines or pyrazines), formed in the Maillard reaction between hexoses and amino acids [22,24]. An aldol condensation is thought to link the two molecules through an aldehyde or related Schiff base, to form IQ- and IQx-ring-structured HAAs. 2-Amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) can form in a model system containing phenylalanine, creatinine, and glucose [25]; however, PhIP can also form in the absence of sugar. Phenylalanine and creatine were shown, by dry heating of 13C-labeled phenylalanine and creatine, to be precursors of PhIP [3]; the Strecker aldehyde phenylacetaldehyde was identified as a critical intermediate [26] (Figure 2B). Free radical intermediates have also been suggested to be involved in AIA formation [24,27,28]. IQx-type compounds arise in model systems containing glucose, glycine, and creatine through formation of pyrazine cation radical and carbon-centered radicals [28]; evidence for this pathway was obtained by heating of creatinine with 2,5-dimethylpyrazine or 2-methylpyridine and acetaldehyde, to produce 4,8-DiMeIQx or IQ, respectively [24].

1.2 Endogenous formation of HAAs

The β-carboline compounds 9H-pyrido[3,4-b]indole (norharman) and 1-methyl-9H-pyrido[3,4-b]indole (harman) are formed at considerably higher levels in tobacco condensates and cooked foods than are other HAAs (Figure 1) [29]. Norharman and harman are not mutagenic in S. typhimurium in the presence or absence of liver S9 fraction mixture; however, a mutagenic effect is observed when these compounds are coincubated with aniline or o-toluidine [30]. This comutagenic effect is attributed to the formation of novel, mutagenic HAAs [31]. The structures of the compounds formed are 9-(4′-aminophenyl)-9H-pyrido[3,4-b]indole (amino-phenylnorharman, APNH), 9-(4′-amino-3-methyl-phenyl)-9H-pyrido[3,4-b]indole (aminomethyl-phenylnorharman, AMPNH), and...