Kinetics and thermodynamics of halide and nitrite oxidation by mammalian heme peroxidases

The human heme peroxidases myeloperoxidase (MPO), eosinophil peroxidase (EPO) and lactoperoxidase (LPO) are able to oxidise (pseudo)halides and nitrite to reactive species that participate in host defence against foreign microorganisms as well as in immunomodulation and tissue degradation in certain pathologies. The heme in EPO and LPO is covalently linked to the apoprotein by two ester bonds, whereas in MPO it is additionally linked by a unique sulfonium ion bond to a methionine residue. As a consequence, the prosthetic group in MPO is significantly distorted from a planar conformation. These structural differences are reflected by distinct spectral and redox properties as well as reactivities toward chloride, bromide, iodide, thiocyanate and nitrite, which function as endogenous two- and one-electron donors for these enzymes in vivo. Standard reduction potentials at pH 7 have been determined for all redox couples involved in the halogenation and peroxidase cycle of MPO and LPO and partially of EPO. A detailed thermodynamic analysis of the formation of reactive halide species by MPO and EPO was also performed. Thus, for the first time, a comprehensive analysis of reactions catalysed by human heme peroxidases is presented that allows a better understanding of their role in physiological and pathophysiological processes.