X-ray absorption and resonance raman spectroscopy of human myeloperoxidase at neutral and acid pH

Peroxidases: a role in the metabolism and side effects of drugs

Current safety screening of drug candidates or new chemical entities for reactive metabolite formation focuses on the role of cytochrome P450. However, peroxidases also have a major role in drug metabolism, and peroxidase-catalyzed drug oxidation could lead to reactive metabolite formation, resulting in oxidative stress and cytotoxicity. Here, the different classes of human peroxidases are summarized and the molecular mechanisms of peroxidase-catalyzed drug metabolism are discussed. In addition, evidence is presented that indicates a role of these enzymes in drug toxicity.

Active site structure of the catalase-peroxidases from Mycobacterium tuberculosis and Escherichia coli by extended X-ray absorption fine structure analysis

The catalase-peroxidase encoded by katG of Mycobacterium tuberculosis is a more effective activator of the antibiotic isoniazid than is the equivalent enzyme from Escherichia coli. The environment of the heme iron was investigated using X-ray absorption spectroscopy to determine if differences in this region were associated with the differences in reactivity. The variation in the distal side Fe-ligand distances between the two enzymes was the same within experimental error indicating that it was not the heme iron environment that produced the differences in reactivity. Analysis of variants of the E. coli catalase-peroxidase containing changes in active site residues Arg102 and His106 revealed small differences in Fe-water ligand distance including a shorter distance for the His106Tyr variant. The Arg102Leu variant was 5-coordinate, but His106Cys and Arg102Cys variants showed no changes within experimental error. These results are compared with those reported for other peroxidases.

Effects of pH and chloride concentration on resonance Raman spectra of human myeloperoxidase and Raman microspectroscopic analysis of enzyme state in azurophilic granules

Resonance Raman spectra of human myeloperoxidase were examined at pH 3.3-10.5 in the absence and presence of chloride ions. Among the porphyrin vibrational bands, the core-size marker bands showed particularly large wavenumber downshifts on going from pH 8.7 to 5.3 with a transition midpoint at pH 6.5 in the absence of chloride ions. The chloride ions did not affect the spectrum at a pH below 5.3 and above 8.7 whereas an increase of chloride concentration at neutral pH caused spectral changes similar to those observed upon pH lowering. Analogous effects were also observed on the Raman intensity. In addition, the stretching mode of the bond between the heme Fe and proximal histidine shifted by -2 cm(-1) on going from pH 8.7 to 5.3. Decomposition of the nu(3) band revealed the presence of two components, which was confirmed by an isosbestic point in the absorption spectra. The observed spectral changes indicated the existence of alkaline and acidic forms of the enzyme. The pK of interconversion was 6.5, and it was increased by binding of chloride ions. The porphyrin core was slightly expanded in the acidic form compared to that in the alkaline form. A molecular mechanism of the porphyrin core expansion was proposed on the basis of the X-ray crystal structure. The pH-spectrum relationships obtained for the isolated enzyme were applied to in situ analysis of the state of myeloperoxidase in azurophilic granules of living neutrophils. The enzyme was stored in the acidic form and kept inactive in catalyzing HOCl production.