Quercetin and the Glucosides Inhibit Nitration of a Salivary Component 4-Hydroxyphenylacetic Acid Catalyzed by Salivary Polymorphonuclear Leukocytes

Mixed whole human saliva contains 4-hydroxyphenylacetic acid (HPA), nitrite and polymorphonuclear leukocytes. Salivary leukocytes nitrated HPA to 4-hydroxy-3-nitrophenylacetic acid in the presence of nitrite, and phorbol myristate acetate stimulated the nitration. Quercetin and the glucosides, which are found in the oral cavity after ingestion of quercetin-rich foods, inhibited the leukocyte-dependent nitration. The inhibition by quercetin and the glucosides was in part due to the flavonol-dependent scavenging of nitrogen dioxide which was formed by myeloperoxidase-dependent oxidation of nitrite. Salivary components, SCN and uric acid, also inhibited the nitration. The above re-sults suggest that quercetin can cooperate with SCN and uric acid to prevent nitration in the oral cavity.