3.5.2. *N*-Nitroso Compounds

*Occurrence*: *N-Nitroso compounds* (Figure 5(2,3)) are formed in food and in vivo at acidic pH by nitrosation of secondary and tertiary amino compounds [68,519–521]. Their formation also occurs in vivo at neutral pH by nitric oxide generated by bacteria converting nitrates and nitrites, or macrophages or endothelial cells metabolizing arginine [522–524]. *N*-Nitroso compounds have been found in over 200 foods, including fruits, vegetables, beverages, meats and cereals, and can be present in drinking water [456,519,521,525,526]. Concentrations of nitrosamines in food depend on the method, time and temperature of cooking or fat composition [527]. Thus, mean levels of *N*-nitroso compounds were 0.5 μmol/kg of fresh meat and over 5.5 μmol/kg of frankfurters and salted, dried fish [526]. Nevertheless, the levels in most foods are generally below 10 ppb [519,528] or less than 10 μg/kg, with exception of fired food, such as bacon, in which an average concentration is 36 μg/kg [521].

*Carcinogenicity*: Various dialkyl and cyclic nitrosamines have been found to produce tumors at multiple sites in a range of species, including rats, mice and hamsters. Target organs of carcinogenicity include the liver, esophagus, nasal and oral mucosa, kidney, pancreas, urinary bladder, lung and thyroid [8,68,521,529–532]. *N-nitrosodiethylamine* (NDEA) (Figure 5(2)) is considered to be an exceptionally potent carcinogen, compared to other nitrosamines [533] (Table 2).

*Genotoxicity/DNA Binding (Adducts)*: As a class, nitrosamines, particularly the volatile nitrosamines, are potent mutagens/genotoxins, both in vitro and in vivo [68,521,529,530]. Many of the nitrosamines act as alkylating agents [534,535] leading to pro-mutagenic lesions, including alkylation at the *N*7 and O6 positions of guanine or O<sup>4</sup> position of thymidine [536–538]. For example, NDEA (Figure 5(2)) and *N-nitrosodiethanolamine* (Figure 5(3)) form poorly repaired O4-ethyldeoxythymidine and O6-2-hydroxyethyldeoxyguanosine DNA adducts [536,539]. Some organ specificity of nitrosamines may arise because, unlike the liver, sensitive tissues such as the brain lack a DNA-repair enzyme, alkyltransferase, that regenerates guanine from O6-alkylguanine [540].

*Biotransformation*: *N*-Nitroso compounds are bioactivated to diazonium ions by hydroxylation involving several CYP isozymes [521,541], in particular CYP2E1 [536,542]. The organ specificity probably stems from tissue specific CYPs that activate the nitrosamines which alkylate DNA in the organ where they are activated.

*MoA:* Formation of alkylated DNA adducts and consequent mutagenesis and genomic instability are likely the most prevalent cancer-related mechanism of nitrosamines [529,533,536,543].

*Human Exposure*: In Europe, EDI of volatile *N*-nitrosamines ranges from 0.001 to 0.02 μg/kg bw/day [521]. In US the average daily intake was calculated to be 1.8 μg/day in a vegetarian diet and 1.9 μg/day in a Western diet [544]. The highest values of nitrosamines, up to 0.531 μg/serving, were found in alcohol, meat and dairy products [525].

*Human Effects*: In humans, indirect evidence exists of the carcinogenic action of nitrosamines through reported associations between gastrointestinal (esophageal, gastric, colorectal), pancreatic, bladder cancers and the consumption of foods containing relatively high concentrations of nitrosamines, nitrites and nitrates [68,521,529,530]. Additionally, nasopharyngeal cancers associated with consumption of salted fish have been attributed to *N*-nitroso compounds (see below) [545].

*Risk*: IARC classified the majority of food-borne nitrosamines as either probable (Group 2A) or possible (Group 2B) human carcinogens [67], although, certain practices known to result in increased cancer risks, including consumption of processed meat and fish, smoking, and betel quid chewing, and certain occupations in the rubber industry, result in exposures to various nitrosamines. The lack of identification of nitrosamines as "known human carcinogens" is largely a consequence of the low levels of human exposure to these compounds. Using the benchmark approach, permissible daily exposures (PDE) for cancer and mutagenicity were calculated to be 6.2 and 0.6 μg/person/day for *N*-nitrosodimethylamine and 2.2 and 0.04 μg/person/day for NDEA, respectively [543].

#### 3.5.3. α,β-Unsaturated Aldehydes

*α,β-Unsaturated aldehydes* (Figure 5(4–8)) compose a wide ranging class of aldehydes which naturally occur in a variety of foods and can be added as flavor ingredients. In addition, they can be formed endogenously through lipid peroxidation [546–549]. They are formed from the polyunsaturated fatty acids (PUFA) in triglycerides, as well as from any free fatty acids, which are susceptible to auto oxidation [548,550].
