Quantitative measurement of the nitrate reductase activity in the human oral cavity

To quantitatively characterise the nitrate reductase activity in the human oral cavity, a new assay based on holding 20 ml of 10 mg nitrate-N/L solution in the mouth was developed. The mouth assay appeared to relate primarily to the oral cavity surface rather than to the saliva. Nitrite formation in the assay was 50–100 times higher compared to in vitro incubation. In the proposed assay, the nitrite formation linearly increased over a period of 3 min. The average nitrate reductase activity in the oral cavity of 20 subjects was 2.39±1.52 μg nitrite-N formed/person × min. The nitrate reductase activity measured for two subjects at different hours varied about 15% for the same subject. The average nitrate reductase activity measured in June for 10 subjects (3.43±1.75 μg-N/person×min) was significantly higher than that measured in November for 10 other subjects (1.54±0.46 μg-N/person×min). Therefore, the nitrate reductase activity in the oral cavity appears to be influenced by the seasonal conditions. Although the amounts of nitrite formed in the mouth assay increased with increasing levels of nitrate, the rate of nitrate to nitrite reduction decreased with increasing levels of nitrate. The nitrite formation was also affected by the pH, with an optimal pH about 8. The nitrite formation was not influenced by uptake in the mouth of glucose, l-ascorbic acid and l-arginine.