Nitration of the salivary component 4-hydroxyphenylacetic acid in the human oral cavity: enhancement of nitration under acidic conditions.

4-Hydroxyphenylacetic acid (HPA) and nitrite are present in human mixed whole saliva, and HPA can be nitrated by peroxidase/hydrogen peroxide (H(2)O(2))/nitrite systems in the oral cavity. Thus, the objectives of the present study were to estimate the concentrations of HPA, nitrated HPA [4-hydroxy-3-nitrophenylacetic acid (NO(2)HPA)], nitrite, and thiocyanate (SCN(-)) in saliva from 73 patients with periodontal diseases and to elucidate the conditions necessary to induce nitration of HPA. High concentrations of HPA, nitrite, and SCN(-) were found in the saliva of patients older than 50 yr of age. NO(2)HPA was detected in seven patients who were older than 60 yr of age. Nitrite-dependent formation of NO(2)HPA by a bacterial fraction prepared from mixed whole saliva was faster at pH 5.3 than at pH 7, and increased as the rate of H(2)O(2) formation increased. The formation of NO(2)HPA was inhibited by SCN(-) and by salivary antioxidants such as uric acid, ascorbic acid, and glutathione. These results suggest that nitration can proceed at an acidic site in the oral cavity where H(2)O(2) is produced under conditions of decreased concentrations of SCN(-) and of antioxidants.