Heterocyclic Amines (HCA)

Heterocyclic amines (HCAs) contain at least one heterocyclic ring (with atoms of at least 2 different elements) and one amine group, attached to the heterocyclic ring. Major groups of HCAs are Carbolines (indoles), Pyrrolidines, Pyrroles (in Hb and vitamin B12), Pyridines (vitamin B3 and B6) and Pyrimidines (vitamin B1), including Purines (adenine, guanine).

Many HCA are harmless, or even beneficial (eg vitamins), whereas others, created by cooking, may cause cancer or influence brain functioning.

Mutagenic HCA in cigarette smoke

Besides carcinogens such as polycyclic aromatic hydrocarbons (eg Benzo(alpha)Pyrene) and nitrosamines (eg N-Nitrosodimethylamine), The following HCA are a few of the HCA found in cigarette smoke:

• Trp-P-1 [1]
• Trp-P-2 [2]
• Glu-P-1 [3]
• Glu-P-2 [4]
• A alpha C [5]
• MeA alpha C [6]
• PhIP [7]

But exactly the same compounds are also found in cooked foods:

• Trp-P-1 [8][9]
• Trp-P-2 [10][11]
• Glu-P-1 [12]
• Glu-P-2 [13]
• A alpha C [14]
• MeA alpha C [15]
• PhIP [16]

Carcinogenic HCA in cooked foods

HCAs form when amino acids are heated, particularly in the presence of creatinine (in meat and fish). HCA concentration is associated with meat doneness [17] and cooking temperature [18][19]. When barbequed, the darker the surface colour of the meat, the higher the HCA concentrations [20]. Antioxidants inhibit HCA formation [21], particularly phenols from olive oil and tea [22]. Enzymes in the human liver (and tongue [23]) activate those HCAs [24]: HCAs are oxidized to hydroxyamino derivatives by cytochrome P450s, and further converted to ester forms (with acetic acid, sulfuric acid, proline) by acetyltransferase and sulfotransferase. Eventually, they produce DNA adducts through the formation of N-C bonds at guanine bases [25], which actually exist in human tissues, and may be involved in human cancer development [26] Adding PhIP to the diet at a concentration of 400 ppm (parts per million) for 1 year induced carcinomas in 47% of female rats. 100 ppm of PhIP for 2 years yielded the same incidence. [27] There is a linear relation between DNA adducts in the liver and doses of MeIQx fed to rats [28]. The carcinogenic effects of HCAs are additive or synergistic, particularly at low doses [29][30][31][32].

These 10 HCA have been shown to be carcinogenic in rats and/or mice when administered in the diet (for 1 to 2 years) at concentrations of 100-800 ng / gram : IQ, MeIQ, MeIQx, PhIP, Trp-P-!, Trp-P-2, Glu-P-1, Glu-P-2, AalphaC and MeAalphaC.

Now consider human daily intake, not for 2 years, but for a lifetime. These amounts were found in cooked foods (in ng / gram cooked food) [33]:

• 0.2 ng IQ
• 0.03 ng MeIQ
• up to 6 ng MeIQx
• up to 69 ng PhIP
• up to 0.2 ng Trp-P-1
• 0.2 ng Trp-P-2
• up to 2.5 ng AalphaC
• 0.2 ng MeAalphaC

Human tissues have been shown to be vulnerable to these HCAs [34][35][36][37]. Even very low doses of HCAs may cause cancer [38]. Epidemiological studies show associations between HCA intake and breast cancer, colon cancer, prostate cancer and pancreatic cancer [39].

Mutagenic HCA in cooked foods

The IQ-type HCAs are derived from amino acids and from creatinine in raw meat and fish, or sugars additionally. Non-IQ-type HCAs are obtained by heating tryptophan (indoles / gamma-carbolines) or glutamic acid (imidazoles).

• 2-amino-3-methylimidazo[4,5-f]quinoline (IQ; in broiled sardines, cooked beef, fried fish) [40]
• 2-amino-3-methylimidazo[4,5-f]quinoxaline (IQx; broiled sardines)[41]
• 2-amino-1-methylimidazo[4,5-b]quinoline (IQ[4,5-b]) [42]
• 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ; in fried fish) [43][44][45]
• 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx; in various cooked foods) [46]
• 2-amino-3,4,7,8-tetramethylimidazo[4,5-f]quinoxaline (TriMeIQx; in griddled bacon) [47]
• 2-amino-3,4,8-trimethylimidazo[4,5-f]quinoxaline (4,8-DiMeIQx; in fried fish [48] and beef extract [49]}
• 2-amino-3,7,8-trimethylimidazo[4,5-f]quinoxaline (7,8-DiMeIQx; in roasted eel) [50] (creatinine + glycine + glucose[51])
• 2-amino-1,7,9-trimethylimidazo[4,5-g]quinoxaline (7,9-DiMeIgQx; in beef extract) [52]
• 2-amino-4-hydroxymethyl-3,8-dimethylimidazo[4,5-f]quinoxaline (4-CH2OH-8-MeIQx; in beef extract) [53]
• 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP; in cooked meat and fish) [54][55]
• 2-amino-(1,6-dimethylfuro[3,2-e]imidazo[4,5-b])pyridine (IFP) [56]
• 2-amino-1-methyl-6-(4-hydroxyphenyl)imidazo[4,5-b]pyridine (4'-OH-PhIP; in broiled beef) [57] (creatine + tyrosine + glucose)
• 2-amino-n,n-dimethylimidazopyridine (DMIP) [58]
• 2-amino-n,n,n-trimethylimidazopyridine (TMIP) [59]
• 2-Amino-5-phenylpyridine (2-APP or Phe-P-1)[60] (its ultimate acetoxy reactive species) [61]

• 3-amino-1,4-dimethyl-5H-pyrido[4,3-b]indole (Trp-P-1) [62]
• 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) [63]
• 2-amino-9H-pyrido[2,3-b]indole (A alpha C; soybean globulin and cooked meat) [64]
• 2-amino-3-methyl-9H-pyrido[2,3-b]indole (MeA alpha C; soybean globulin, cooked meat, fried fish) [65][66][67]

• 2-amino-6-methyldipyrido[1,2-a:3',2'-d]imidazole (Glu-P-1 (a delta-carboline); in roasted mackerel and pork) [68]
• 2-aminodipyrido[1,2-a:3',2'-d]imidazole (Glu-P-2 (a delta-carboline); in Worcestershire sauce) [69]

• methyl-2-methylamino-IH,6H-pyrrolo[3,4-f]benzimidazole-5,7-dione (Cre-P-1) [70]

Neuro-active HCAs; natural ones and in cooked foods

In the human body, various endogenous beta-carbolines (tetrahydro-beta-carboline, 6-methoxytetrahydro-beta-carboline and harmane) act on the Benzodiazepine receptors as neurotransmitters, and are monoamine oxidase (MAO) inhibitors [71]. beta-Carbolines are also often associated with (acetyl)cholinesterase inhibition [72][73]

Cooked foods also contain various beta-carbolines (indoles produced by heating L-tryptophan [74]), such as harman and norharman. [75] Studies show long-term retention of these specific neurotoxic beta-carbolines in brain neuromelanin [76]. Nitrosation (in the presence of nitrite) of beta-carbolines generally produces mutagenics [77], but also reaction with aniline may produce mutagenics [78][79].

beta-Carbolines in cooked foods

• Norharman(e) (9H-pyrido[3,4-b]indole = β-carboline) in cigarette smoke, cooked meat [80][81][82], cooked fish, toasted bread [83] and coffee [84] is a neurotoxin [85] has synergistic effects with Trp-P-2 [86] and may contribute to idiopathic Parkinson's disease [87]). Daily total dietary exposure is estimated at max 4 mcg/kg bodyweight (daily endogenous formation 50-100 nng/kg).[88]
• Harman(e) (1-methyl-β-carboline) in cigarette smoke, cooked meat [89], particularly chicken[90], cooked fish and toasted bread [91][92]; is a a tremor-producing neurotoxin. Meat consumption is higher in men with essential tremor [93]. Harman also impairs learning [94][95] through the nicotinic cholinergic system [96] and alters behaviour [97]). Daily total dietary exposure is estimated at max 1 mcg/kg bodyweight (daily endogenous formation 20 nng/kg)[98]
• Harmine (in smoked salmon and soft cheese [99]; inhibts the enzymes DYRK1A, CLK1, CLK2 [100] and phosphodiesterase [101])
• Harmaline (induces nitric oxide release and inhibits phosphodiesterase [102])
• Harmalol
• ethyl-β-carboline-3-carboxylate (impairs learning through modulating GABA [103])
• methyl-β-carboline-3-carboxylate (modulates GABA [104])
• 1-acetyl-β-carboline-3-carboxylic acid (in ketchup and heated tomato concentrate[105])
• 3,4-dinitro-1-methyl-β-carboline-3-carboxylic acid in soy sauce and beer [106]
• Tryptoline (1,2,3,4-tetrahydro-β-carboline) in sausages[107]; a MAO-A inhibitor [108])
• 1,2,3,4-tetrahydro-1-(5’-hydroxymethylfuryl)-β-carboline-3-carboxylic acid in bean paste [109]
• 1,2,3,4-tetrahydro-1-furyl-β-carboline-3-carboxylic acid in soy sauce and bean paste [110]
• Flazin (in soy sauce; induces quinone reductase (QR) activity [111])
• Perlolyrin (in soy sauce; induce quinones reductase (QR) activity [112])

beta-Carbolines in fruits, herbs

• 1-methyltryptoline (1-methyl-1,2,3,4-tetrahydro-β-carboline) in sausages [113] tomato and kiwi; antioxidant [114]and MAO-A inhibitor [115])
• MTCA (1-methyl-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid) acting as antioxidant [116] in orange juice [117], soy sauce [118], vinegar [119] and fermented garlic. [120] MTCA is a xanthine oxidase (XO) inhibitor, thus inhibiting uric acid formation [121]
• MTCdiC (1-methyl-1,2,3,4-tetrahydro-β-carboline-1,3-dicarboxylic acid) in aged garlic (not raw[122]); a superoxide scavenger [123])
• 6-hydroxy-1-methyl-1,2,3,4-tetrahydro-beta-carboline in bananas, pineapple and tomato; acting as antioxidants [124]