Effects of the Food Additive Sulfite on Nitrite-Dependent Nitric Oxide Production under Conditions Simulating the Mixture of Saliva and Gastric Juice

Abstract

The food additive sulfite is mixed with saliva, which contains nitrite, in the oral cavity, and the mixture is mixed with gastric juice in the stomach. In the stomach, salivary nitrite can be transformed to nitric oxide (NO). In this study, the effects of sulfite on nitrite-dependent NO production were investigated using acidified saliva (pH 2.6) and acidic buffer solutions (pH 2.0). Sulfite enhanced NO production in acidified saliva and acidic buffer solutions, and the enhancement increased with the increase in sulfite concentration from 0 to 0.1 mM, whereas suppressed NO production and the suppression increased as the concentration was increased over 0.2 mM. The enhancement was due to the increase in reaction rate between nitrous acid and nitrososulfonate (ONSO(3)(-)) that was formed by the reaction of nitrous acid with hydrogen sulfite, and the suppression was due to the increase in hydrogen sulfite-dependent consumption rate of ONSO(3)(-). A salivary component SCN(-) (1 mM) enhanced and suppressed NO production induced by 1 mM nitrite when sulfite concentrations were lower and higher than 1 mM, respectively. ONSO(3)(-) formed from hydrogen sulfite and nitrosyl thiocyanate (ONSCN), which was produced by the reaction of nitrous acid with SCN(-), seemed to contribute to the enhancement and suppression. NO production induced by nitrite/ascorbic acid systems was suppressed by sulfite, and the suppressive effects were decreased by SCN(-), whereas sulfite-induced suppression of NO production in nitrite/rutin systems was increased by SCN(-). During reactions of nitrite with sulfite in the presence and absence of SCN(-), oxygen was taken up. The oxygen uptake is discussed to be due to autoxidation of NO and radical chain reactions initiated by hydrogen sulfite radicals. The results of the present study suggest that sulfite can enhance and suppress nitrite-dependent NO production. It is discussed that radicals including hydrogen sulfite radicals can be formed through the reactions of nitrite and sulfite in the stomach.