title: The potential role of nitric oxide in substrate switching in eosinophil peroxidase.
abstract: Eosinophil recruitment and enhanced nitric oxide (NO) production are characteristic features of asthma and other airway diseases. Eosinophil peroxidase (EPO), a highly cationic hemoprotein secreted by activation of eosinophils, is believed to play a central role in host defense against invading pathogens. The enzyme uses hydrogen peroxide (H2O2) and bromide (Br-), a preferred cosubstrate of EPO, to generate the cytotoxic oxidant hypobromous acid. The aim of this work was to determine whether NO can compete with plasma levels of Br- and steer the enzyme reaction from a 2e- oxidation to a 1e- oxidation pathway. Rapid kinetic measurements were utilized to measure the rate of EPO compounds I and II formation, duration, and decay at 412 and 432 nm, respectively, at 10 degrees C. An EPO-Fe(III) solution supplemented with increasing Br- concentrations was rapidly mixed with fixed amounts of H2O2 in the absence and in the presence of increasing NO concentrations. In the absence of NO, EPO-Fe(III) primarily converted to compound I and, upon H2O2 exhaustion, it decayed rapidly to the ferric form. NO caused a significant increase in the accumulation of EPO compound II, along with a proportional increase in its rate of formation and duration as determined by the time elapsed during catalysis. The time courses for these events have been incorporated into a comprehensive kinetic model. Computer simulations carried out supported the involvement of a conformational intermediate in the EPO compound II complex decay. Collectively, our results demonstrated that NO displays the potential capacity to promote substrate switching by modulating substrate selectivity of EPO.
authors.affiliation: Department of Obstetrics and Gynecology, The C.S. Mott Center for Human Growth and Development, Children's Hospital of Michigan, Wayne State University School of Medicine, Detroit, Michigan 48201, USA
authors.forename: Semira
authors.initials: S
authors.lastname: Galijasevic
authors.forename: Gheorghe
authors.initials: G
authors.lastname: Proteasa
authors.forename: Ibrahim
authors.initials: I
authors.lastname: Abdulhamid
authors.forename: Husam M
authors.initials: HM
authors.lastname: Abu-Soud
mesh_terms: D000818:Animals; D001965:Bromides / Q000378:metabolism; D047209:Eosinophil Peroxidase / Q000097:blood / Q000737:chemistry* / Q000378:metabolism*; D004804:Eosinophils / Q000201:enzymology; D066298:In Vitro Techniques; D007700:Kinetics; D008958:Models, Molecular; D009569:Nitric Oxide / Q000378:metabolism; D010084:Oxidation-Reduction; D011487:Protein Conformation; D013053:Spectrophotometry; D013379:Substrate Specificity; D013552:Swine
chemical_list: D001965:Bromides; D009569:Nitric Oxide; D047209:Eosinophil Peroxidase