Kinetic characterization of phenol and aniline derivates as substrates of peroxidase

The kinetic characterization of horseradish peroxidase (HRPC) substrates is difficult because the reaction products are free radicals. The application of a spectrophotometrical method, which is based on determining the time necessary for a given quantity of Lascorbic acid to be consumed (lag period) during its reaction with the free radicals generated by the enzyme acting on the reducing substrate, makes it possible to obtain the initial steadystate rates (v(0)). From the kinetic study of a series of derivates of phenol and aniline, the following parameters were determined for the first time: the global catalytic constant (k(cat)), the Michaelis constant of HRPC for H2O2 in the presence of each reducing substrate (K-m(H2O2)), the Michaelis constant of HRPC for the reducing substrate (K-m(S)), the binding constant of the reducing substrate with HRPC compound II (k(5)) and the rate constant of substrate oxidation by HRPC compound II (k(6)). The values obtained are disccussed.