Interaction of lactoperoxidase with hydrogen peroxide. Formation of enzyme intermediates and generation of free radicals.

Peroxidases belong to a group of enzymes which catalyze the oxidation of numerous organic and inorganic substrates by hydrogen peroxide. Most peroxidases, including lactoperoxidase (LPO), contain ferriprotoporphyrin IX as a prosthetic group. A characteristic feature of hemoprotein peroxidases is their ability to exist in various oxidation states. There are five known enzyme intermediates. In increasing order of their oxidative equivalents these are ferrous enzyme, ferric or native enzyme, Compound II, Compound I, and Compound III (sections 5, 7). They are readily distinguished from each other by their absorbance in the Soret region (380-450 nm) and visible range (450-650 nm). In the course of Compound III and Compound II conversion back to the native peroxidase, oxygen derived free radicals such as O2-, HO.2, and .OH are generated. Simultaneously the enzyme is irreversibly damaged. In the presence of an exogenous electron donor, such as iodide, the interconversion between the various oxidation states of the peroxidase is markedly affected. Compound II and/or Compound III formation is inhibited, depending on the H2O2 concentration. In addition, the enzyme is largely protected from irreversible inactivation. These effects of iodide are readily explained by 1) the two-electron oxidation of iodide to Iox by Compound I, which bypasses Compound II as an intermediate, and 2) the rapid oxidation of H2O2 to O2 by the oxidized species of iodide which prevents the generation of oxygen derived free radicals.