Abstract
Myeloperoxidase, an abundant neutrophil enzyme, promotes oxidative damage during inflammation by generating hypohalous acids and free radicals. Currently, there are no selective drugs to inhibit its adverse activity. This short-coming is partly due to the lack of screening assays that mimic the complex enzymatic activities of myeloperoxidase in vivo. We have developed an assay for myeloperoxidase activity that includes its major physiological substrates - chloride, thiocyanate, tyrosine, and urate. The multi-substrate assay monitors bleaching of 5-thio-2-nitrobenzoic acid and measures total oxidant production when hydrogen peroxide activates the enzyme. Known suicide inhibitors and tight-binders tested positive in the assay, whereas compounds that merely convert myeloperoxidase to reducible enzyme intermediates were poor inhibitors. The new assay revealed that some aromatic compounds, including tryptamine, inhibit myeloperoxidase by binding reversibly to the enzyme. Our multi-substrate assay is selective for physiologically relevant inhibitors and has potential for identifying new classes of myeloperoxidase inhibitors.