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Papers In Press, published online ahead of print April 16, 2003
Bioquímica, Facultad de Medicina, Montevideo, Montevideo 11800
Corresponding Author: rradi{at}fmed.edu.uy
We have studied the reaction kinetics of ten manganese porphyrins, differing in their meso substituents, with peroxynitrite (ONOO–) and carbonate radical anion (CO3·–) using stopped-flow and pulse radiolysis, respectively. Rate constants for the reactions of Mn(III) porphyrins with ONOO– ranged from 1 x105 to 3.4 x107 M-1s-1, and correlated well with previously reported kinetic and thermodynamic data that reflect the resonance and inductive effects of the substituents on the porphyrin ring. Rate constants for the reactions of Mn(III) porphyrins with CO3·– ranged from 2 x108 to 1.2 x109 M-1s-1 at pH 8.5, and increased with pH as a consequence of the ionization of the complexes. Mn(II) porphyrins reacted with CO3·– with rate constants ranging from 1 x109 to 5 x109 M-1s-1 at pH 10.4. Hence, the fast scavenging of ONOO– and CO3·– by Mn porphyrins in vivo could be due to the catalytic reduction of the oxidized porphyrins by a number of cellular reductants. Additionally, we determined the pKa of the axial water molecules of the Mn(III) complexes at pH 7.5-13.2 by spectrophotometric titration. Results were consistent with two acid-base equilibria for most of the complexes studied. The pKa values also correlated with the resonance and inductive effects of the substituents. The correlations of E½ with the rate constants with ONOO– and with the pKa values display a deviation from linearity when N-alkylpyridinium substituents included N-alkyl moieties longer than ethyl, which is interpreted in terms of a decrease in the local dielectric constant.
J. Biol. Chem, 10.1074/jbc.M213302200
Submitted on December 31, 2002
Revised on April 15, 2003
Accepted on April 16, 2003
Reactions of manganese porphyrins with peroxynitrite and carbonate radical anion
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