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

Abstract

Peroxidases belong to a group of enzymes which catalyze the oxidation of numerous organic and inorganic substrates by hydrogen peroxide. Most peroxidaes, 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 equivalents these are ferrous enzyme, ferric or native enzyme, Compound II, Compound I, and Compound III (section 5, 7). They are readily distinguished from each other by their absorbance in the Soret region (380–450 nm) and visible range (450–460 nm). In the course of Compound III and Compound II conversion back to the native peroxidase, oxygen derived free radicals such as O 2 −, 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 H 2O 2 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 toI OX by Compound I, which bypasses Compound II as an intermediate, and 2) the rapid oxidation of H 2O 2 to O 2 by the oxidized species of iodide which prevents the generation of oxygen derived free radicals.