The peroxidatic reaction catalyzed by horse liver alcohol dehydrogenase. 2. Steady-state kinetics and inactivation.

The results of steady-state kinetic measurements on the initial rate of the peroxidatic reaction between beta-NAD+ and hydrogen peroxide, catalyzed by horse liver alcohol dehydrogenase, at pH 7 are described. A simple sequential mechanism is deduced from graphical analysis of the data plotted according to Eadie-Augustinsson-Hofstee primary plots and the values of the true kinetic parameters KmNAD, KmH2O2 and V are estimated from the corresponding secondary plots. Ethanol has been found to compete with hydrogen peroxide for the same enzyme active site. During the catalytic process a progressive inactivation of the enzyme occurs caused by H2O2. The rate law of this process is quantitatively described at pH 7 both in the absence and in the presence of NAD+. The coenzyme has been found to protect the enzyme against inactivation by H2O2, which oxidized essential cysteine residues. The results obtained from the study of both catalytic and inactivating processes are finally rationalized on the basis of a general mechanistic scheme.