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)TMyP] 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 NO3- and NO2- 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)TMyP reacts rapidly with ONOO- to produce oxoFe(IV)TMPyP and NO2 (k ~ 5 x 107 M-1 s-1). The oxoFe(IV) porphyrin, which persisted throughout the catalytic decomposition of ONOO-, was shown to be relatively unreactive toward NO2 and NO2-. This oxoFe(IV) porphyrin was also shown to react with ONOO- (k = 1.8 x 106 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 (O2-.), and using stopped-flow spectrophotometry, the rate of Fe(III)TMPyP-catalyzed O2-. dismutation has been determined to be 1.9 x 107 M-1 s-1 by direct measurement. A detailed mechanistic understanding of how iron porphyrins function in the catalytic decomposition of both ONOO- and O2-. 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.
|Original language||English (US)|
|Number of pages||9|
|Journal||Journal of the American Chemical Society|
|State||Published - Aug 5 1998|
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry