Reaction of cytochrome c with nitrite and nitric oxide. A model of dissimilatory nitrite reductase.

The reaction of bovine heart ferrocytochrome c with nitrite was studied under various conditions. The reaction product was ferricytochrome c at around pH 5, whereas at around pH 3 it was Compound I, characterized by twin peaks at 529 and 563 nm of equal intensity. However, ferrocytochrome c decreased obeying first-order kinetics over the pH range examined, irrespective of the presence or absence of molecular oxygen. The apparent first-order rate constant was proportional to the square of the nitrite concentration at pH 4.4 and it increased as the pH was lowered. At pH 3 the reaction was so rapid that it had to be followed by stopped-flow and rapid-scanning techniques. The apparent rate constant at this pH was found to increase linearly with the nitrite concentration. Based on these results the active species of nitrite was concluded to be dinitrogen trioxide at pH 4.4 and nitrosonium ion, no+, at pH 3. Compound II was formed by reaction of ferrocytochrome c and NO gas at acidic and alkaline pH values. The absorption peaks were at 533 and 563 nm at pH 3, and at 538 and 567 nm at pH 12.9. This compound was also formed by reducing Compound I with reductants. Compound I prepared from ferricytochrome c and NO was stable below pH 6. However, appreciable absorption peaks for ferrocytochrome c appeared between pH 8 and 10, because Compound I was dissociated into ferrocytochrome c and NO+, and because ferrocytochrome c thus formed reacted with NO very slowly in this pH region. Saccharomyces ferricytochrome c under NO gas behaved differently from mammalian cytochrome, indicating the significance of the nature of the heme environment in determing the reactivity. Only at extreme pH values was Compound II formed exclusively and persisted. A model system for dissimilatory nitrite reductase was constructed by using bovine heart cytochrome c, nitrite and NADH plus PMS at pH 3.3, and a scheme involving cyclic turnover of ferrocytochrome c, Compound I and Compound II is presented, with kinetic parameters.