Accessibility of the Active Site in the Enzyme Myeloperoxidase as Measured by Magnetic Resonance Studies of Solvent Proton Spin Relaxation

Objective. This study was conducted to determine whether there is rapid solvent accessibility to the active site in the enzyme myeloperoxidase and whether these effects are dependent upon pH and ligand binding near the heme. Background. The enzyme myeloperoxidase, which is found in mammalian neutrophils, plays an important beneficial role in phagocytosis because of its antimicrobial and cytotoxic activities. These activities proceed via peroxidation of chloride ion to hypochlorite, chlorination of amino acids by hypochlorite, and by decarboxylation of the resulting amino acid chloramines. Deleterious effects in human cardiovascular disease have also been attributed to chlorotyrosine, nitrotyrosine, and tyrosyl radical products which can be generated by myeloperoxidase reactions. The kinetics and mechanisms of some of these reactions have been clarified although not all details of the reaction mechanisms are known. Methods. In this study, solvent proton spin relaxation enhancement properties of the enzyme solutions are explored as a means of probing accessibility of the heme site. Longitudinal (spin-lattice) relaxation times were measured for the enzyme solutions under various conditions. Conclusion. The heme site is accessible, and there is solvent exchange with water bound near the heme iron. The solvent proton relaxation rate depends upon electronic state of the heme iron, exchange rate of bulk solvent with that bound near the heme, temperature, magnetic resonance frequency, pH and chloride concentration