The reaction of catalase catalyzed decomposition of H_2O_2 was studied by thermokinetic method in 0.1 mol·L~1 Na2HP04-NaH2P04 buffer solution at 310.15 K and pH 7.0 in the presence of L-ascorbic acid or Cu~(2+) or of the both. There is no apparent inhibition effect on catalase when L-ascorbic acid or Cu~(2+) is present solely. However, when they are present simultaneously, a non-linear inhibition of the reaction can be observed. At definite enzyme and substrate concentrations, L-ascorbic acid and Cu~(2+) have no effect on the form of kinetic equation of the overall reaction. In other words, the overall reaction of HjC^ decomposition catalyzed by catalase is according to pseudo-first order rate equation when L-ascorbic and Cu~ (2+) are present or absent, but the rate constant decrease when the concentrations of L-ascorbic and Cu~(2+) increase. Dose relation of the enzyme activity with the concentrations of L-ascorbic and Cu~(2+) is a sigmoidal curve. Combined the results of this experiment with the conclusions of literatures, a possible mechanism of L-ascorbic acid and Cu~(2+) cooperative inhibition on catalase was proposed.

Key words: PEROXIDASE, ASCORBIC ACID, THERMODYNAMICS, COOPERATIVITY, HYDROGEN PEROXIDE, DEPRESSING