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J. Biol. Chem., Vol. 280, Issue 28, 26129-26136, July 15, 2005

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Thiocyanate Modulates the Catalytic Activity of Mammalian Peroxidases^*

Yahya R. Tahboub, Semira Galijasevic, Michael P. Diamond, and Husam M. Abu-Soud¶

From the Department of Obstetrics and Gynecology, The C. S. Mott Center for Human Growth and Development and the Department of Biochemistry and Molecular Biology, Wayne State University, Detroit, Michigan 48201

We investigated the potential role of the co-substrate, thiocyanate (SCN^–), in modulating the catalytic activity of myeloperoxidase (MPO) and other members of the mammalian peroxidase superfamily (lactoperoxidase (LPO) and eosinophil peroxidase (EPO)). Pre-incubation of SCN^– with MPO generates a more complex biological setting, because SCN^– serves as either a substrate or inhibitor, causing diverse impacts on the MPO heme iron microenvironment. Consistent with this hypothesis, the relationship between the association rate constant of nitric oxide binding to MPO-Fe(III) as a function of SCN^– concentration is bell-shaped, with a trough comparable with normal SCN^– plasma levels. Rapid kinetic measurements indicate that MPO, EPO, and LPO Compound I formation occur at rates slower than complex decay, and its formation serves to simultaneously catalyze SCN^– via 1e^– and 2e^– oxidation pathways. For the three enzymes, Compound II formation is a fundamental feature of catalysis and allows the enzymes to operate at a fraction of their possible maximum activities. MPO and EPO Compound II is relatively stable and decays gradually within minutes to ground state upon H₂O₂ exhaustion. In contrast, LPO Compound II is unstable and decays within seconds to ground state, suggesting that SCN^– may serve as a substrate for Compound II. Compound II formation can be partially or completely prevented by increasing SCN^– concentration, depending on the experimental conditions. Collectively, these results illustrate for the first time the potential mechanistic differences of these three enzymes. A modified kinetic model, which incorporates our current findings with the mammalian peroxidases classic cycle, is presented.

Received for publication, March 18, 2005 , and in revised form, May 11, 2005.

^* This work was supported by National Institutes of Health Grant HL066367 to (H. M. A.-S.). The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

^¶ To whom correspondence should be addressed: Dept. of Obstetrics and Gynecology, C. S. Mott Center for Human Growth and Development, Wayne State University, 275 E. Hancock, Detroit, MI 48201. Tel.: 313-577-6178; Fax: 313-577-8554; E-mail: habusoud{at}med.wayne.edu.

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