Oxidative modification by the combination of MPO and H₂O₂. An HOCl-generating system consisting of 50nM MPO plus 100μM H₂O₂ also completely oxidized Met¹⁶⁰⁶ to methionine sulfoxide in the A2 substrate peptide (■). Hydrogen peroxide alone had no effect. Purified multimeric plasma VWF (100nM, molar concentration of VWF monomers) was also oxidized at Met¹⁶⁰⁶ by HOCl (100μM) and by MPO/H₂O₂, although to a lesser extent than the substrate peptide (□).