Fig. 1.

Measurement of MPO activity by amperometric monitoring of H₂O₂ consumption. Chlorinating activity of MPO (100 nM) was measured in the presence of 100 mM Cl⁻ and (A) Met as HOCl scavenger or indicated native-LDL concentrations, (B) 0.3 mg/ml native LDL or HOCl-LDL (at indicated oxidant/lipoprotein molar ratio), or (C) indicated concentrations of HOCl-LDL (oxidant/lipoprotein molar ratio = 333:1). (D) Peroxidase activity of MPO (100 nM) was measured in the presence of 100 µM Tyr and indicated concentrations of native LDL or 0.3 mg/ml HOCl-LDL (oxidant/lipoprotein molar ratio = 333:1). MPO and lipoproteins were incubated 5 min at 25°C before measurement. MPO or MPO-lipoprotein solutions were added to start the reaction. Values are given as percentages of activity of free MPO (100% activity). One unit of activity is defined as the consumption of 1 µmol H₂O₂ per minute at 25°C in the presence of 100 µM electron donor (Cl⁻ or Tyr). Results represent mean ± SD of at least three experiments. * P < 0.05 (significant difference versus MPO).