Transformation of Nitrite and Nitric Oxide Produced by Oral Bacteria to Reactive Nitrogen Oxide Species in the Oral Cavity

Nitrate and nitrite are present in the human oral cavity. The main origin of these components is nitrate that is contained in leafy vegetables such as lettuce and spinach (Tamme et al., 2006). Ingested nitrate is absorbed into human body by the intestine, and part of the absorbed nitrate is secreted into the oral cavity as a component of saliva. In the oral cavity, nitrate is reduced to nitrite by nitrate-reducing bacteria (Doel et al., 2004, 2005; Zetterquist et al., 1999), and nitrite is reduced to nitric oxide (NO) by nitrite-reducing bacteria (Palmerini et al., 2003). If NO is produced from nitrite by nitrite-reducing bacteria, NO can autoxidize to dinitrogen trioxide (N2O3) and can react with superoxide anion radical (O2 ) to produce a strong oxidant peroxynitrite (ONOO/ONOOH, pKa = 6.8) (ONOOH/NO2, E ’ = 2.10 V at pH 7) (Halliwell & Gutteridge, 1999). In the oral cavity, two peroxidases are present; one is salivary peroxidase that is derived from saliva and the other is myeloperoxidase that is derived from leukocytes migrated into the oral cavity. Nitrite can be oxidized by these peroxidases producing nitrogen dioxide (NO2) (van der Vliet et al., 1997). The E’ of NO2/NO2 – at pH 7 is 0.99 V (Halliwell & Gutteridge, 1999). If the pH in the oral cavity decreased around 5, nitrite ion is protonated to produce nitrous acid (pKa = 3.3) that can be transformed to NO, N2O3, NO2, and NO + (NO/NO, E’ = 1.21 V at pH 7) by selfdecomposition (Oldrreive & Rice-Evabs, 2001). In this way, the formation of nitrite and NO by oral bacteria results in the production of reactive nitrogen oxide species (RNOS) by various reactions. This chapter deals with the mechanisms of production of RNOS in the human oral cavity under neutral and acidic conditions. Taking the mechanism into consideration, we discuss that the decrease in pH in the oral cavity results in the injury of oral tissue cells.