Myeloperoxidase metabolizes thiocyanate in a reaction driven by nitric oxide

We examined the potential physiological relevance of myeloperoxidase (MPO)-nitric oxide (NO) interactions as they may relate to the cosubstrate, pseudo halide thiocyanate (SCN(-)), and substrate switching. Direct spectroscopic and rapid kinetics studies revealed that SCN(-) interaction with MPO facilitates formation of the MPO catalytic intermediate Compound II, limiting overall activity. However, a physiological NO concentration (2 microM or less) dramatically influences the build-up, duration, and decay of the steady-state level of MPO Compound II during the metabolism of SCN(-), allowing the enzyme to function at full capacity. At higher NO concentrations, we observed significant increases in the rate of MPO Compound II formation, along with proportional increases in its duration as determined by the time elapsed during catalysis. Surprisingly, the decay rate of MPO Compound II remained unaltered as NO concentrations were increased. Computer simulations were carried out to model the kinetics of MPO Compound II formation, duration, and decay during the metabolism of SCN(-) as a function of NO concentration. These simulation traces closely approximate what was observed experimentally and support the involvement of a conformational intermediate of MPO Compound II complex decay, altering the overall capacity of MPO to promote two electrons versus one-electron oxidation reactions during steady-state catalysis. Collectively, the present studies reveal that (patho)physiologically relevant levels of NO have significant effects on MPO Compound II accumulation. Thus, NO affects the overall rate of peroxidation of substrates and the overall ability of the peroxidase to execute one- versus two-electron oxidation reactions.