Abstract
Foods are mixed with saliva in the oral cavity and swallowed. While staying in the stomach, saliva is contentiously provided to mix with the ingested foods. Because a salivary component of nitrite is protonated to produce active nitrous acid at acidic pH, the redox reactions of nitrous acid with phenolic compounds in foods become possible in the stomach. In the reactions, nitrous acid is reduced to nitric oxide (•NO), producing various products from phenolic compounds. In the products, stable hydroxybezoyl benzofuranone derivatives, which are produced from quercetin and its 7-O-glucoside, are included. Caffeic acid, chlorogenic acid, and rutin are oxidized to quinones and the quinones can react with thiocyanic acid derived from saliva, producing stable oxathiolone derivatives. 6,8-Dinitrosocatechis are produced from catechins by the redox reaction, and the dinitrocatechins are oxidized further by nitrous acid producing the quinones, which can make charge transfer complexes with the dinitrosocatechin and can react with thiocyanic acid producing the stable thiocyanate conjugates. In this way, various products can be produced by the reactions of salivary nitrite with dietary phenolic compounds, and reactive and toxic quinones formed by the reactions are postulated to be removed in the stomach by thiocyanic acid derived from saliva.
Highlights
Phenolic compounds are normally contained in terrestrial plants, and their various functions have been reported
It is well known that nitrous acid can transform amines into carcinogenic compounds nitrosoamines [10], and it has been reported that phenolic compounds can reduce nitrous acid to a Antioxidants 2017, 6, 53; doi:10.3390/antiox6030053
The ascorbic acid-dependent inhibition of their oxidation suggests that ascorbic acid in gastric juice, the concentration of which ranges from 0 to 0.5 mM [39,40,41], can suppress the nitrous acid-induced oxidation of phenolic compounds in the stomach. These results suggest that efficiency of the transport of phenolic compounds, which can react with nitrous acid readily, to the intestine is dependent on the concentrations of both salivary nitrite and gastric ascorbic acid
Summary
Phenolic compounds are normally contained in terrestrial plants, and their various functions have been reported. Accompanying the ingestion of foods that contain phenolic compounds, the compounds are mixed with saliva, which contains 0.05–1 mM nitrite produced by oral nitrate reducing bacteria [8,9]. Salivary nitrite is continuously provided to the swallowed foods. The functions of NO produced in the gastric lumen include the increase in gastric mucosal blood flow, the increase in gastric mucus thickness, and the relaxation of gastric smooth muscle [13,14,15,16]. There are many reports that nitrite/nitrate absorbed from the intestine can contribute to the production of NO in human cells [17,18]. This review deals with the redox reactions of phenolic compounds with nitrite to produce NO and the various reactions of the products of phenolic compounds with NO, nitrite, and a salivary component thiocyanate under the simulated stomach conditions
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