Interaction of peroxynitrite with mitochondrial cytochrome oxidase. Catalytic production of nitric oxide and irreversible inhibition of enzyme activity

Purified mitochondrial cytochrome c oxidase catalyzes the conversion of peroxynitrite to nitric oxide (NO). This reaction is cyanide-sensitive, indicating that the binuclear heme a₃/Cu(B) center is the catalytic site. NO production causes a reversible inhibition of turnover, characterized by formation of the cytochrome as nitrosyl complex. In addition, peroxynitrite causes irreversible inhibition of cytochrome oxidase, characterized by a decreased V(max) and a raised K(m) for oxygen. Under these conditions, the redox state of cytochrome a is elevated, indicating inhibition of electron transfer and/or oxygen reduction reactions subsequent to this center. The lipid bilayer is no barrier to these peroxynitrite effects, as NO production and irreversible enzyme inhibition were also observed in cytochrome oxidase proteoliposomes. Addition of 50 μM peroxynitrite to 10 μM fully oxidized enzyme induced spectral changes characteristic of the formation of ferryl cytochrome a₃, partial reduction of cytochrome a, and irreversible damage to the Cu(A) site. Higher concentrations of peroxynitrite (250 μM) cause heme degradation. In the fully reduced enzyme, peroxynitrite causes a red shift in the optical spectrum of both cytochromes a and a₃, resulting in a symmetrical peak in the visible region. Therefore, peroxynitrite can both modify and degrade the metal centers of cytochrome oxidase.