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.