Determination of GPO and catalase activity in a mixture of purified enzymes. (A) After the addition of enzyme mixture (GPO, 0.01 U/mL; catalase, k_cat = 0.034 s⁻¹) only the catalase-mediated H₂O₂ decay is observed. (B) Inhibition of catalase by 1 mmol/L NaN₃ . (C) Second addition of 10⁻⁵ mol/L H₂O₂ shows complete inhibition of catalase. (D) Addition of 2 mmol/L GSH and recalibration of the measuring system (see Mueller et al25 ). (E) The GPO-mediated decomposition of H₂O₂ is observed after the addition of 10⁻⁴ mol/L H₂O₂ . These experiments were performed with GPO activities higher than those in erythrocytes with respect to catalase to improve the kinetic characterization of both enzymes.