Biotransformation of nitric oxide.

Previous investigations into the health effects of nitrogen oxides (NOx) have mostly been conducted with special reference to nitrogen dioxide (NO2) and its direct effects on the respiratory system, while the study of nitric oxide (NO) has been disregarded. We carried out a study on NO by exposing rats and mice to 15NO or administering 15N-nitrite and 15N-nitrate to these animals by IP injection in order to elucidate the metabolic fate of NO. The results of our study and previous findings led us to assume that the major metabolic path of inhaled NO is as follows: inhaled NO reacts with hemoglobin, forming nitrosyl-hemoglobin (NOHb), and from NOHb, nitrite (NO2-) and nitrate (NO3-) are generated. Major quantities of NO3- are discharged into the urine and a certain amount is discharged into the oral cavity through the salivary glands and transformed to NO2-. Part of this NO2- is converted to N2 gas in the stomach. Nitrate in the intestine is partly reduced to ammonia (NH3) through NO2-, reabsorbed into the body, and converted to urea. Most of the metabolites of inhaled NO are excreted rapidly from the body within 48 hr.