Dual-function of thiocyanate on nitrite-induced formation of reactive nitrogen oxide species in human oral cavity: inhibition under neutral and enhancement under acidic conditions

Salivary nitrate is reduced to nitric oxide (NO) via nitrite in the human oral cavity. The nitrite and NO formed can be transformed to reactive nitrogen oxide species (RNOS). In this investigation, RNOS formed in mixed whole saliva and its fractions were detected by the oxidation of aminophenyl fluorescein (APF) and the transformation of 3-amino-4-monomethylamino-2',7'-difluorofluorecein (DAF-FM) to its triazol form (DAF-FMT). Nitrite-induced oxidation of APF and formation of DAF-FMT increased as pH was decreased from 7 to 5 and SCN^- inhibited the oxidation of APF and the formation of DAF-FMT around neutral pH and enhanced at pH about 5. The SCN^--dependent inhibition was due to the suppression of salivary peroxidase and the enhancement was due to the formation of NOSCN from HNO₂ and SCN^-. It is deduced that the increase in the concentrations of nitrite and H⁺ in the oral cavity may result in the enhanced formation of RNOS.