Reaction of nitric oxide with the turnover intermediates of cytochrome c oxidase: Reaction pathway and functional effects

The reactions of nitric oxide (NO) with the turnover intermediates of cytochrome c oxidase were investigated by combining amperometric and spectroscopic techniques. We show that the complex of nitrite with the oxidized enzyme (O) is obtained by reaction of both the 'peroxy' (P) and 'ferryl' (F) intermediates with stoichiometric NO, following a common reaction pathway consistent with P being an oxo-ferryl adduct. Similarly to chloride-free O, NO reacted with P and F more slowly [k ? (2-8) x 10 4 M -1 s -1] than with the reduced enzyme (k ? 1 x 10 8 M -1 s -1). Recovery of activity of the nitrite-inhibited oxidase, either during turnover or after a reduction-oxygenation cycle, was much more rapid than nitrite dissociation from the fully oxidized enzyme (t( 1/2 ) ? 80 min). The anaerobic reduction of nitrite-inhibited oxidase produced the fully reduced but uncomplexed enzyme, suggesting that reversal of inhibition occurs in turnover via nitrite dissociation from the cytochrome a 3-Cu(B) site: this finding supports the hypothesis that oxidase may have a physiological role in the degradation of NO into nitrite. Kinetic simulations suggest that the probability for NO to be transformed into nitrite is greater at low electron flux through oxidase, while at high flux the fully reduced (photosensitive) NO-bound oxidase is formed; this is fully consistent with our recent finding that light releases the inhibition of oxidase by NO only at higher reductant pressure [Sarti, P., et al. (2000) Biochem. Biophys. Res. Commun. 274, 183].