The peroxynitrite reductase activity of cytochrome c oxidase involves a two-electron redox reaction at the heme a(3)-Cu-B site

Fully and partially reduced forms of isolated bovine cytochrome c oxidase u ndergo a two-electron oxidation-reduction process with added peroxynitrite, leading to catalytic oxidation of ferrocytochrome c to ferricytochrome c, The other major reaction product is nitrite ion, 86% of the added peroxynit rite being measurably converted to this species. The reaction is inhibited in the presence of cyanide, implicating the heme alpha(3)-Cu-B binuclear pa ir as the active site. Moreover, provided peroxynitrite is not added to exc ess, the reductase activity of the enzyme toward this oxidant efficiently p rotects other protein and detergent molecules in vitro from nitration of ty rosine residues and oxidative damage. If the enzyme is exposed to similar t o 10(2)-fold excesses of peroxynitrite, then significant irreversible loss of electron transfer activity results, and the heme alpha(3)-Cu-B binuclear pair no longer undergo a characteristic carbon monoxide-driven reduction. The accompanying rather small changes in the observed electronic absorption spectrum are suggestive of a modification in the vicinity of one or both h emes but probably not to the cofactors themselves.