Kinetics and Mechanism of Tetrahydrobiopterin-induced Oxidation of Nitric Oxide

A Clark-type nitric oxide-sensitive electrode was used for electrochemical determination of NO oxidation kinetics. Reaction with molecular oxygen followed second-order rate law with respect to NO with an overall rate constant of 9.2 +/- 0.33 x 10(6) M-2 s-1. Tetrahydrobiopterin, an essential cofactor of NO synthases, was found to induce rapid oxidation of NO in a 1:1 stoichiometry. The reaction required the presence of oxygen, was zero order with respect to NO and first order with respect to tetrahydrobiopterin, completely blocked by 5,000 units/ml superoxide dismutase, and mimicked by a superoxide-generating system. Purified brain NO synthase produced no detectable NO unless high amounts of superoxide dismutase were present. NO synthase-catalyzed citrulline formation was inhibited by superoxide dismutase (5,000 units/ml) in an oxyhemoglobin-sensitive manner, indicating that NO induces feedback inhibition of NO synthase. NO-stimulated soluble guanylyl cyclase was inhibited by tetrahydrobiopterin at half-maximally active concentrations of 2 microM. The present data suggest that NO is inactivated to peroxynitrite by superoxide generated in the course of tetrahydrobiopterin autoxidation.