N-nitrosation and nitrosylation mechanisms and kinetics in gasto-intestinal conditions

In 2010, the International Agency for Research on Cancer classified nitrites as “probably carcinogenic for humans”. The long-term toxicity of nitrites arises from their reactions with secondary amines to form nitrosamines (N-nitrosation) or with myoglobin iron to form nitrosyl-heme (nitrosylation). Nitrosamines and nitrosyl-heme are mutagenic compounds implicated in digestive cancers. N-nitrosation and nitrosylation that occur during meat processing are well documented but little data is available concerning these reactions during digestion (endogenous process).

This study was performed for the purpose of improving our knowledge about endogenous N-nitrosation and nitrosylation and to offer solutions that could limit their impact.

Results: N-nitrosation and nitrosylation reactions were studied in vitro in physicochemical conditions that mimic the digestive tract. The tryptophan-nitrite reaction was chosen as a model for studying nitrosamine production, and the myoglobin-nitrite reaction was chosen as a model for studying heme iron nitrosylation.
• N-nitrosation: We demonstrated that nitrosamine formation increased with increasing levels of nitrite and with decreasing pH (from 6.5 to 2). In the same study, we demonstrated that non-heminic iron was a powerful catalyst of N-nitrosation. In the case of a diet rich in iron, N-nitrosation is efficient even at neutral pH, as is the case in the intestine. The addition of antioxidants to the model led to conflicting results depending on the type of antioxidant and the ratio of [iron]/[antioxidant].
• Nitrosylation: Concerning nitrosylation, we demonstrated that the two oxidation states of myoglobin (Fe2+/Fe3+) can react with nitrite to form nitrosyl-heme in conditions mimicking the digestive tract. The extent of nitrosylation was dependent on the pH and the pigment oxidation state. The addition of antioxidants to the model revealed a protective effect against nitrosylation.

For future experiments, we plan to use an artificial digestive system with a reactional medium that is more representative of actual meals (mixture of animal products and vegetables).