Nitric oxide formed by nitrite reductase of Paracoccus denitrificans is sufficiently stable to inhibit cytochrome oxidase activity and is reduced by its reductase under aerobic conditions.

Nitric oxide, generated by the action of purified nitrite reductase, inhibited the oxidase activity of both membrane vesicles from anaerobically grown Paracoccus denitrificans and bovine heart submitochondrial particles. In the former case, the inhibition was relatively short-lived and its duration was reduced either by decreasing the concentration of nitrite or raising the ratio of vesicles to nitrite reductase enzyme. These observations indicate that nitric oxide, at least at low concentrations, was sufficiently stable in the presence of oxygen to allow diffusion between proteins in aqueous solution. The shorter inhibition period with P. denitrificans membrane vesicles implies that the nitric oxide reductase of the vesicles is active in the presence of oxygen and has a sufficiently high affinity for nitric oxide to remove it from oxidase enzymes by competition. These observations are related to previous reports of potent inhibition under certain conditions of oxidase activity of P. denitrificans cells by a molecular species produced from nitrite. The implications of the deduced stability of nitric oxide in aerobic solutions are considered with respect to both the phenomenon of aerobic denitrification and the synthesis of nitric oxide in mammalian cells.