Stopped-flow analysis of the effect of increasing mesna concentrations on the formation, duration, and decay of MPO Compound 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, 5, 10, 25, 50, 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 Compound II were monitored at 450 nm. The final concentration of mesna in mixtures is indicated.