Mechanisms of Iron Porphyrin Reactions with Peroxynitrite

Peroxynitrite (ONOO - ) is a major cytotoxic agent that has been implicated in a host of pathophysiological conditions; it is therefore important to develop therapeutic agents to detoxify this potent biological oxidant, and to understand the modes of action of these agents. Water-soluble iron porphyrins, such as 5,10,15,20-tetrakis( N -methyl-4‘-pyridyl)porphinatoiron(III) [Fe(III)TMPyP] and 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5-sulfonatophenyl)porphinatoiron(III) [Fe(III)TMPS], have been shown to catalyze the efficient decomposition of ONOO - to NO 3 - and NO 2 - under physiological conditions. However, the mechanisms of ONOO - decomposition catalyzed by these water-soluble iron porphyrins have not yet been elucidated. We have shown that there are two different pathways operating in the catalytic decomposition of ONOO - by FeTMPyP. Fe(III)TMPyP reacts rapidly with ONOO - to produce oxoFe(IV)TMPyP and NO 2 ( k ≈ 5 × 10 7 M - 1 s - 1 ). The oxoFe(IV) porphyrin, which persisted throughout the catalytic decomposition of ONOO -, was shown to be relatively unreactive toward NO 2 and NO 2 - . This oxoFe(IV) porphyrin was also shown to react with ONOO - ( k = 1.8 × 10 6 M - 1 s - 1 ), and it was this oxoFe(IV)-ONOO - reaction pathway that predominated under conditions of excess ONOO - with respect to Fe(III)TMPyP. The competition between the two pathways explains the highly nonlinear relationship observed for k cat with respect to ONOO - concentration. Fe(III)TMPyP is also known to catalyze the dismutation of the ONOO - precursor superoxide (O 2 - • ), and using stopped-flow spectrophotometry, the rate of Fe(III)TMPyP-catalyzed O 2 - • dismutation has been determined to be 1.9 × 10 7 M - 1 s - 1 by direct measurement. A detailed mechanistic understanding of how iron porphyrins function in the catalytic decomposition of both ONOO - and O 2 - • may prove essential in the exploration of the chemistry and biology of these reactive oxygen species, and in understanding the biological activity of these metalloporphyrins.