Myeloperoxidase: a target for new drug development?

Abstract

Myeloperoxidase (MPO), a member of the haem peroxidase-cyclooxygenase superfamily, is abundantly expressed in neutrophils and to a lesser extent in monocytes and certain type of macrophages. MPO participates in innate immune defence mechanism through formation of microbicidal reactive oxidants and diffusible radical species. A unique activity of MPO is its ability to use chloride as a cosubstrate with hydrogen peroxide to generate chlorinating oxidants such as hypochlorous acid, a potent antimicrobial agent. However, evidence has emerged that MPO-derived oxidants contribute to tissue damage and the initiation and propagation of acute and chronic vascular inflammatory disease. The fact that circulating levels of MPO have been shown to predict risks for major adverse cardiac events and that levels of MPO-derived chlorinated compounds are specific biomarkers for disease progression, has attracted considerable interest in the development of therapeutically useful MPO inhibitors. Today, detailed information on the structure of ferric MPO and its complexes with low- and high-spin ligands is available. This, together with a thorough understanding of reaction mechanisms including redox properties of intermediates, enables a rationale attempt in developing specific MPO inhibitors that still maintain MPO activity during host defence and bacterial killing but interfere with pathophysiologically persistent activation of MPO. The various approaches to inhibit enzyme activity of MPO and to ameliorate adverse effects of MPO-derived oxidants will be discussed. Emphasis will be put on mechanism-based inhibitors and high-throughput screening of compounds as well as the discussion of physiologically useful HOCl scavengers.

Figure 1

Primary and secondary MPO-reaction products and (patho)physiology. Adapted and modified from Malle et al. (2006a). CNS, central nervous system.

Figure 2

General reaction scheme of MPO. In the first step H₂O₂ is used for compound I formation (reaction 1). Compound I is two oxidizing equivalents above the native enzyme with a porphyrin π-cation radical in combination with an oxoiron(IV) centre. Compound I can react with halides (X⁻) reducing the enzyme back to the ferric state (reaction 2, halogenation activity). In the peroxidase reaction, compound I is transformed in the first one-electron reduction to compound II, which contains an oxoiron(IV) centre (reaction 3). Compound II is finally reduced back to ferric peroxidase in a second one-electron reduction (reaction 4). Compound III (oxyperoxidase) is formed either from ferric peroxidase with superoxide anion (reaction 9), from ferrous MPO with O₂ (reaction 8) or from compound II with H₂O₂ (reaction 6). Compound III is a complex of ferrous-dioxygen in resonance with ferric superoxide. Ferrous [Fe(II)]MPO can be formed from ferric [Fe(III)]MPO by reducing radicals produced in the peroxidase cycle (reaction 11).

Figure 3

(a) The hydrogen bonding network and locations of five water molecules (W1–W5) at the distal haem cavity of ferric high-spin MPO. (b) The non-planar porphyrin ring in MPO and its covalent attachments to the protein via two ester bonds (Glu242 and Asp94) and one sulfonium ion linkage (Met243). In addition, the proximal His336 and the distal catalytic residues His95, Arg239, and Gln91 are shown, the latter being important in halide binding. The figure was constructed using the coordinates deposited in the Protein Data Bank (accession code 1CXP).

Figure 4

(a) Structure of the myeloperoxidase-cyanide (MPO–CN⁻) complex (accession code 1D5L), which can be regarded as a model of compound I. In the CN⁻-complex water molecule W1 is displaced by CN⁻. (b) Structure of the MPO–CN⁻–bromide (Br⁻) double complex (accession code 1D7W). CN⁻ and Br⁻ are located at positions of W1 and W5. Note that in the MPO–Br⁻ complex (data not shown), the halide is binding at W2. (c) Structure of the MPO–CN⁻–thiocyanate (SCN⁻) double complex (accession code 1DNW). In this complex CN⁻ displaces W1, whereas SCN⁻ displaces both W2 and W5.

Figure 5

View through the access channel to the active site (complexed with cyanide) showing exposure of the haem pyrrole ring D with its 8-methyl and of the δ-methine bridge (that is, electron transfer site). The figure was constructed using PyMol Viewer (version 0.99).

Figure 6

Docking models for SHA (a) and melatonin (b) bound to MPO. The positions of the three oxygen atoms of SHA are close to the positions occupied by the three water molecules W1–W3 in the native enzyme (see Figure 3a). Both models were calculated using LigandScout 1.03 (Wolber and Langer, 2005) from Inte:Ligand GmbH (www.inteligand.com). SHA, salicylhydroxamic acid.