[Horseradish peroxidase: a study of the kinetics and the determination of optimal reaction conditions, using hydrogen peroxide and 2,2'-azinobis 3-ethylbenzthiazoline-6-sulfonic acid (ABTS) as substrates (author's transl)].

The reaction of the two substrates hydrogen peroxide and ABTS with horseradish peroxidase was studied kinetically. Enzyme activity was determined as a function of substrate concentration and pH. Michaelis constants were determined for the two substrates at various pH values. It was found that the affinity of the enzyme for ABTS decreases with increasing pH, and that with higher ABTS concentrations the pH optima of the reaction are shifted towards neutrality. Maximal rate is reached at pH 4.2 with an ABTS concentration of 2 mmol/l. For hydrogen peroxide the data show that the dissociated O2H- is the proper substrate, its affinity for the enzyme being independent of pH. The two substrates show competitive binding to the peroxidase, and each therefore influences the binding constant of the other. A procedure is proposed which allows the determination of peroxidase down to a concentration of 10 ng/l or 2.5 x 10(-13) mol/l.