Modification of lipoxygenase by hydrogen peroxide and photooxidation.

The kinetic study of fluorescence stopped-flow method suggested that the interaction between lipoxygenase and H2O2 is consistent with a simple irreversible one-step mechanism. The activation energy of the reaction was 7.2 kcal/mol. Participation of an ionizable group with pK about 8.8, possibly a histidine residue, was suggested from the pH-dependence of the rate constant. No further fluorescence quenching of lipoxygenase was observed when the product was added to the lipoxygenase solution before mixing the lipoxygenase and H2O2 solutions. The fluorescence quenching of lipoxygenase by H2O2 was in parallel with the inactivation of the enzyme. Hydroperoxylinoleic acid strongly protects the inactivation of lipoxygenase caused by H2O2. These results are consistent with an interpretation that OH- and/or O- - are produced when the iron of the enzyme is oxidized by H2O2, which in turn will attack some amino acid essential for the enzyme activity. The pH-dependence of the inactivation rate constant of photooxidation of lipoxygenase sensitized by methylene blue indicated that an ionizable group with pK about 8.8 is concerned with the enzymatic activity. In contrast to the inactivation of lipoxygenase by H2O2, the product protected the inactivation of the enzyme by photooxidation only at high concentration.