A new route to peroxynitrite: a role for xanthine oxidoreductase.

Peroxynitrite, a potent oxidising, nitrating and hydroxylating agent, results from the reaction of nitric oxide with superoxide. We show that peroxynitrite can be produced by the action of a single enzyme, xanthine oxidoreductase (XOR), in the presence of inorganic nitrite, molecular oxygen and a reducing agent, such as pterin. The effects of oxygen concentration on peroxynitrite production have been examined. The physiologically predominant dehydrogenase form of the enzyme is more effective than the oxidase form under aerobic conditions. It is proposed that XOR-derived peroxynitrite fulfils a bactericidal role in milk and in the digestive tract.