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. Author manuscript; available in PMC: 2014 Jun 16.
Published in final edited form as: Biochem J. 2009 Jun 12;421(1):79–86. doi: 10.1042/BJ20090309

Scheme 1. HOCl production by MPO and its proposed modulation by nitroxides.

Scheme 1

Reaction of H2O2 with native MPO yields compound I, which oxidizes Cl− to yield HOCl and regenerate native MPO (reaction A) (the halogenation cycle). Competing reactions of compound I with nitroxides (R2N-O•) divert the catalytic activity of MPO from HOCl production to the peroxidase cycle and yield compound II and oxoammonium cations (R2N+=O) (reaction B). Reduction of compound II by nitroxides (reaction C) is slow and accumulation of compound II limits HOCl production. Rapid reduction of compound II by O2•− (reaction D), which is generated by activated neutrophils and other cells, stimulates HOCl production by regenerating native MPO, but nitroxides may prevent this process via their SOD-mimetic activity (reaction E).