Resistance to infection by microorganisms involves both non-specific mechanisms and adaptive immune responses mediated by B and T lymphocytes. Non-specific mechanisms such as enzyme action, pH and epithelial tissue secretions prevent invasion by many organisms. However, when the first line of defense is breached, phagocytes engulf infectious agents and process them leading to the stimulation of B and T cells. Deficiencies in the phagocytic system, whether hereditary or acquired, have serious consequences, since microorganisms with low pathogenicity for normal individuals can cause recurrent infections in individuals with ineffective phagocytic systems.
Polymorphonuclear leukocytes (PMNs) are of particular importance for combating infections. These cells contain an enzyme, myeloperoxidase, with well documented microbicidal action. PMNs act non-specifically by phagocytosis to engulf microorganisms, incorporate them into vacuoles, termed phagosomes, which fuse with granules containing myeloperoxidase to form phagolysosomes. In phagolysosomes the enzymatic activity of the myeloperoxidase leads to the formation (from hydrogen peroxide and chloride) of hypochlorous acid, a potent bactericidal compound. Hypochlorous acid is oxidizing in itself, and reacts most avidly with thiols and thioethers but also converts amines into chloramines and chlorinates aromatic amino acids. Macrophages are large phagocytic cells which, like PMNs are capable of phagocytosing microorganisms. Macrophages can generate hydrogen peroxide and upon activation also produce myeloperoxidase. In addition, myeloperoxidase in the plasma is taken up by macrophages.
Linkage of myeloperoxidase activity to disease has been implicated in numerous inflammatory diseases including Alzheimer's disease, multiple sclerosis, asthma, atherosclerosis, cancer, cystic fibrosis, chronic obstructive pulmonary disease, inflammatory bowel disease, and rheumatoid arthritis. Assays for identification of inhibitors of myeloperoxidase will be invaluable in demonstrating the contribution of hypochlorous acid to the pathogenesis of such diseases, as well as for diagnostic tools. They will also provide the platform for the development of effective drugs against myeloperoxidase-dependent tissue damage. A suitable screening assay for MPO inhibitors should measure the chlorination activity of myeloperoxidase under realistic physiological conditions. It must be simple, sensitive, and produce a coloured or fluorescent product, and it must not perturb the activity of MPO. The detector will need to be present at high enough concentrations to scavenge all the hypochlorous acid produced but not so as to react directly with the enzyme. Currently, the most commonly used assay is based on the primary formation of taurine chloramine from hypochlorous acid and taurine. Previous attempts to use modified forms of this taurine chloramine assay for measuring the activity of myeloperoxidase have revealed specific problems that negate its use for screening potential inhibitors of the enzyme. Major problems are that dimethylsulphoxide (DMSO), the solvent in which test compounds are most conveniently and usually dissolved, interferes with the assay, and many of the test compounds absorb in the same region as the commonly used chromophore.
Physiologically, PMN-derived myeloperoxidase primarily catalyzes the hydrogen peroxide-dependent oxidation of halides such as chloride ions, but its ability to use electron donors such as O-dianisidine as a substrate forms the basis of an assay to quantify PMNs in inflamed tissue. Adaptations of the method to avoid artifacts caused by tissue/blood constituents, such as the cytosolic enzymes, catalase and glutathione peroxidase, reducing substrates such as glutathione and ascorbic acid, and heme proteins have been described (Grisham et al., Methods in Enzymology, Vol. 186, Ed. Packer et al., San Diego: Academic Press, pp. 729–742 (1990)). However, there is a need for a more reliable analytical method of quantifying the PMN content of inflamed tissues that is not affected by other endogenous tissue constituents.
The present invention describes a simple, sensitive and robust assay that avoids the major problems associated with currently known assays and that is well suited to the identification of inhibitors of myeloperoxidase. In another aspect, the present invention is useful for certain diagnostic tests associated with myeloperoxidase.