Inorganic nitrite (NO2−) is emerging as a regulator of physiological functions and tissue responses to ischemia, whereas the more stable nitrate anion (NO3−) is generally considered to be biologically inert. Bacteria express nitrate reductases that produce nitrite, but mammals lack these specific enzymes. Here we report on nitrate reductase activity in rodent and human tissues that results in formation of nitrite and nitric oxide (NO) and is attenuated by the xanthine oxidoreductase inhibitor allopurinol. Nitrate administration to normoxic rats resulted in elevated levels of circulating nitrite that were again attenuated by allopurinol. Similar effects of nitrate were seen in endothelial NO synthase–deficient and germ-free mice, thereby excluding vascular NO synthase activation and bacteria as the source of nitrite. Nitrate pretreatment attenuated the increase in systemic blood pressure caused by NO synthase inhibition and enhanced blood flow during post-ischemic reperfusion. Our findings suggest a role for mammalian nitrate reduction in regulation of nitrite and NO homeostasis.
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We thank the European Union (Eicosanox, LSMH-CT-2004-005033), Vinnova (KIDaT, chronic inflammation), Torsten & Ragnar Söderbergs Foundation, the Swedish Research Council, the Swedish Heart & Lung Foundation and a regional agreement between Stockholm County Council and Karolinska Institutet for generous support. We thank I. Johansson (Department of Physiology and Pharmacology, Karolinska Institutet) for kindly providing us with the human liver samples and J. Sällström (Uppsala University) for the Nos3−/− mice.
J.O.L. and E.W. are named inventors on a patent application for the therapeutic use of inorganic nitrate and nitrite salts.
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Jansson, E., Huang, L., Malkey, R. et al. A mammalian functional nitrate reductase that regulates nitrite and nitric oxide homeostasis. Nat Chem Biol 4, 411–417 (2008) doi:10.1038/nchembio.92
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