Reduction of Nitrous Acid to Nitric Oxide by Coffee Melanoidins and Enhancement of the Reduction by Thiocyanate: Possibility of Its Occurrence in the Stomach

摘要

Reactions of nitrous acid with freeze-dried instant coffee and its methanol-insoluble melanoidin fractions were studied at pH 2 in the presence and absence of thiocyanate (SCN~), simulating the mixture of coffee, saliva, and gastric juice. Coffee contained stable radicals, and the radical concentration increased by ferricyanide and decreased by ascorbic acid. This result indicates that the radical concentration was affected by the redox state of coffee and that the nature of the radical was due to quinhydrone structure that might be included in coffee melanoidins. Nitrite also increased the electron spin resonance (ESR) signal intensity at pH 2, suggesting that nitrite oxidized melanoidins producing nitric oxide (NO). The formation of NO could be detected by oxygen uptake due to the autoxidation of NO and using an NO-trapping agent. SCN~- largely enhanced NO formation in coffee and methanol-insoluble melanoidin fractions but only slightly in a methanol-soluble fraction, and the enhancement accompanied the consumption of SCN~- but did not accompany the formation of a stable ESR signal. The enhancement was explained by the reduction of NOSCN by melanoidins in methanol-insoluble fractions and that the consumption was due to binding of SCN~- to melanoidins during their oxidation by nitrous acid. The result obtained in this study suggests that when coffee is ingested, in addition to chlorogenic acid and its isomers, melanoidins can also react with salivary nitrite and SCN~- in the gastric lumen, producing NO.