Stopped-flow analysis of the effect of increasing mesna concentrations on the formation, duration, and decay of MPO Compounds I and II. A solution containing phosphate buffer (200 mM, pH 7.4) supplemented with 1–1.2 μM (final) MPO-Fe(III) and increasing concentrations of mesna (0, 10, 25, 50, 100, and 250 μM final) was rapidly mixed with an equal volume of buffer containing fixed concentrations of H2O2 (20 μM, final) at 10 °C. Formation, duration and decay of MPO Compounds I and II were monitored at 430 nm. The final concentration of mesna in mixtures is indicated.