
<ArticleTitle>Spectral and kinetic studies on <prot>eosinophil peroxidase</prot> compounds I and II and their reaction with ascorbate and tyrosine.</ArticleTitle>
<AbstractText><prot>Eosinophil peroxidase</prot>, the major granule protein in eosinophils, is the least studied human peroxidase. Here, we have performed spectral and kinetic measurements to study the nature of <prot>eosinophil peroxidase</prot> intermediates, compounds I and II, and their reduction by the endogenous one-electron donors ascorbate and tyrosine using the sequential-mixing stopped-flow technique. We demonstrate that the peroxidase cycle of <prot>eosinophil peroxidase</prot> involves a ferryl/porphyrin radical compound I and a ferryl compound II. In the absence of electron donors, compound I is shown to be transformed to a species with a compound II-like spectrum. In the presence of ascorbate or tyrosine compound I is reduced to compound II with a second-order rate constant of (1.0+/-0.2)x10(6) M(-1) s(-1) and (3.5+/-0.2)x10(5) M(-1) s(-1), respectively (pH 7.0, 15 degrees C). Compound II is then reduced by ascorbate and tyrosine to native enzyme with a second-order rate constant of (6.7+/-0.06)x10(3) M(-1) s(-1) and (2.7+/-0.06)x10(4) M(-1) s(-1), respectively. This study revealed that <prot>eosinophil peroxidase</prot> compounds I and II are able to react with tyrosine and ascorbate via one-electron oxidations and therefore generate monodehydroascorbate and tyrosyl radicals. The relatively fast rates of the compound I reduction demonstrate that these reactions may take place in vivo and are physiologically relevant.</AbstractText>
