Manganese superoxide dismutase (Mn-SOD), a critical mitochondrial antioxidant enzyme, becomes inactivated and nitrated in vitro and potentially in vivo by peroxynitrite. Since peroxynitrite readily reacts with transition metal centers, we assessed the role of the manganese ion in the reaction between peroxynitrite and Mn-SOD. Peroxynitrite reacts with human recombinant and Escherichia coli Mn-SOD with a second order rate constant of 1.0 ؎ 0.2 ؋ 10 5 and 1.4 ؎ 0.2 ؋ 10 5 M ؊1 s ؊1 at pH 7.47 and

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... 37°C, respectively. The E. coli apoenzyme, obtained by removing the manganese ion from the active site, presents a rate constant <10 4 M ؊1 s ؊1 for the reaction with peroxynitrite, whereas that of the manganese-reconstituted apoenzyme (apo/Mn) was comparable to that of the holoenzyme. Peroxynitrite-dependent nitration of 4-hydroxyphenylacetic acid was increased 21% by Mn-SOD. The apo/Mn also promoted nitration, but the apo and the zinc-substituted apoenzyme (apo/Zn) enzymes did not. The extent of tyrosine nitration in the enzyme was also affected by the presence and nature (i.e. manganese or zinc) of the metal center in the active site. For comparative purposes, we also studied the reaction of peroxynitrite with low molecular weight complexes of manganese and zinc with tetrakis-(4-benzoic acid) porphyrin (tbap). Mn(tbap) reacts with peroxynitrite with a rate constant of 6.8 ؎ 0.1 ؋ 10 4 M ؊1 s ؊1 and maximally increases nitration yields by 350%. Zn(tbap), on the other hand, affords protection against nitration. Our results indicate that the manganese ion in Mn-SOD plays an important role in the decomposition kinetics of peroxynitrite and in peroxynitrite-dependent nitration of self and remote tyrosine residues. Manganese-superoxide dismutase (Mn-SOD) is the SOD iso-form 1 found in the mitochondrial matrix of eukaryotes and in a variety of prokaryotes (1-3). Mn-SODs from different organisms are homologous and have a manganese ion in the active site. Whereas the human mitochondrial enzyme is a homotetramer (ϳ88 kDa) (4), Escherichia coli Mn-SOD (45.8 kDa) is a dimer (3) . Mitochondria are essential organelles where most of the cell superoxide (O 2 . ) is produced (5, 6), and therefore, Mn-