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Myeloperoxidase, a heme protein expressed by professional phagocytic cells, generates an array of oxidants which are proposed to contribute to tissue damage during inflammation. We now report that enzymatically active myeloperoxidase and its characteristic amino acid oxidation products are present in human brain. Further, expression of myeloperoxidase is increased in brain tissue showing Alzheimer's neuropathology. Consistent with expression in phagocytic cells, myeloperoxidase immunoreactivity was present in some activated microglia in Alzheimer brains. However, the majority of immunoreactive material in brain localized with amyloid plaques and, surprisingly, neurons including granule and pyramidal neurons of the hippocampus. Confirming neuronal localization of the enzyme, several neuronal cell lines as well as primary neuronal cultures expressed myeloperoxidase protein. Myeloperoxidase mRNA was also detected in neuronal cell lines. These results reveal the unexpected presence of myeloperoxidase in neurons. The increase in neuronal myeloperoxidase expression we observed in Alzheimer disease brains raises the possibility that the enzyme contributes to the oxidative stress implicated in the pathogenesis of the neurodegenerative disorder.

Myeloperoxidase (MPO), a heme enzyme secreted by activated phagocytes, generates an array of oxidants proposed to play critical roles in host defense and local tissue damage. Both MPO and its reaction products are present in human atherosclerotic plaque, and it has been proposed that MPO oxidatively modifies targets in the artery wall. We have now generated MPO-deficient mice, and show here that neutrophils from homozygous mutants lack peroxidase and chlorination activity in vitro and fail to generate chlorotyrosine or to kill Candida albicans in vivo. To examine the potential role of MPO in atherosclerosis, we subjected LDL receptor-deficient mice to lethal irradiation, repopulated their marrow with MPO-deficient or wild-type cells, and provided them a high-fat, high-cholesterol diet for 14 weeks. White cell counts and plasma lipoprotein profiles were similar between the two groups at sacrifice. Cross-sectional analysis of the aorta indicated that lesions in MPO-deficient mice were about 50% larger than controls. Similar results were obtained in a genetic cross with LDL receptor-deficient mice. In contrast to advanced human atherosclerotic lesions, the chlorotyrosine content of aortic lesions from wild-type as well as MPO-deficient mice was essentially undetectable. These data suggest an unexpected, protective role for MPO-generated reactive intermediates in murine atherosclerosis. They also identify an important distinction between murine and human atherosclerosis with regard to the potential involvement of MPO in protein oxidation.

Myeloperoxidase, a heme protein secreted by activated phagocytes, may be a catalyst for lipoprotein oxidation in vivo. Active myeloperoxidase is a component of human atherosclerotic lesions, and atherosclerotic tissue exhibits selective enrichment of protein dityrosine cross-links, a well characterized product of myeloperoxidase. Tyrosylation of lipoproteins with peroxidase-generated tyrosyl radical generates multiple protein-bound tyrosine oxidation products in addition to dityrosine. The structural characterization of these products would thus serve as an important step in determining the role of myeloperoxidase in lipoprotein oxidation in the artery wall. We now report the identification and characterization of four distinct tyrosyl radical addition products generated by human phagocytes. Activated neutrophils synthesized three major fluorescent products from -tyrosine; on reverse phase HPLC, each compound coeluted with fluorescent oxidation products formed by myeloperoxidase. We purified the oxidation products to apparent homogeneity by cation and anion exchange chromatographies and identified the compounds as dityrosine (3,3'-dityrosine), trityrosine (3,3',5',3"-trityrosine) and pulcherosine (5-[4"-(2-carboxy-2-aminoethyl)phenoxy]3, 3'-dityrosine) by high resolution NMR spectroscopy and mass spectrometry. Additionally, we have found that dityrosine is a precursor to trityrosine, but not pulcherosine. In a search for a precursor to pulcherosine, we identified isodityrosine (3-[4'-(2-carboxy-2-aminoethyl)phenoxy]tyrosine), a non-fluorescent product of L-tyrosine oxidation by human phagocytes. Our results represent the first identification of this family of tyrosyl radical addition products in a mammalian system. Moreover, these compounds may serve as markers specific for tyrosyl radical-mediated oxidative damage in atherosclerosis and other inflammatory conditions.