title: Catalytic scavenging of peroxynitrite by isomeric Mn(III) N-methylpyridylporphyrins in the presence of reductants.
abstract: Three isomers of manganese(III) 5,10,15, 20-tetrakis(N-methylpyridyl)porphyrin (MnTMPyP) were evaluated for their reaction with peroxynitrite. The Mn(III) complexes reacted with peroxynitrite anion with rate constants of 1.85 x 10(7), 3.82 x 10(6), and 4.33 x 10(6) M(-1) s(-1) at 37 degrees C for MnTM-2-PyP, MnTM-3-PyP, and MnTM-4-PyP, respectively, to yield the corresponding oxo-Mn(IV) complexes. Throughout the pH range from 5 to 8.5, MnTM-2-PyP reacted 5-fold faster than the other two isomers. The oxo-Mn(IV) complexes could in turn be reduced by glutathione, ascorbate, urate, or oxidize tyrosine. The rate constants for the reduction of the oxo-Mn(IV) complexes ranged from >10(7) M(-1) s(-1) for ascorbate to 10(3)-10(4) M(-1) s(-1) for tyrosine and glutathione. Cyclic voltammetry experiments show that there is no significant difference in the E1/2 of the Mn(IV)/Mn(III) couple; thus, the differential reactivity of the three isomeric complexes is interpreted in terms of electrostatic and steric effects. Micromolar concentrations of MnTM-2-PyP compete well with millimolar CO2 at reacting with ONOO-, and it can even scavenge a fraction of the ONOOCO2- that is formed. By being rapidly oxidized by ONOO- and ONOOCO2- and reduced by antioxidants such as ascorbate, urate, and glutathione, these manganese porphyrins, and especially MnTM-2-PyP, can redirect the oxidative potential of peroxynitrite toward natural antioxidants, thus protecting more critical targets such as proteins and nucleic acids.
mesh_terms: D001205:Ascorbic Acid / Q000737:chemistry; D002384:Catalysis; D016166:Free Radical Scavengers / Q000737:chemistry*; D005978:Glutathione / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D007700:Kinetics; D008665:Metalloporphyrins / Q000737:chemistry*; D009566:Nitrates / Q000737:chemistry*; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D011166:Porphyrins / Q000737:chemistry; D019163:Reducing Agents; D014443:Tyrosine / Q000737:chemistry; D014527:Uric Acid / Q000737:chemistry