Oxygen Reduction by Nitric-oxide Synthases

Abstract

Four conclusions derive from the studies exploring NOS-generated O⨪2 and the kinetic data outlined here. 1) When sufficientl-arginine and H4B are present, NOS dimers secrete small amounts of O⨪2 or H2O2and instead couple their heme and O2 reduction to NO⋅ synthesis. 2) Significant O⨪2 production may occur when concentrations of H4B (≤2μm) orl-arginine (≤100μm) fall below levels required to saturate the enzyme. In these circumstances, O⨪2forms by heme-catalyzed O2 reduction. 3) NOS reductase domain flavins are protected from autooxidation and do not secrete large amounts of O⨪2 unless certain redox-active xenobiotics are present. These cause O⨪2 production by catalyzing electron transfer from the NOS reductase domain to O2. One recent example suggests that NOS-derived O⨪2 participates in tissue injury associated with the xenobiotic (54Day B.J. Patel M. Calavetta L. Chang L.-Y. Stamler J.S. Proc. Natl. Acad. Sci. U. S. A. 1999; 96: 12760-12765Crossref PubMed Scopus (169) Google Scholar). 4) NOS may generate both NO⋅ and O⨪2 when concentrations ofl-arginine or H4B are low (41Xia Y. Roman L.J. Masters B.S.S. Zweier J.L. J. Biol. Chem. 1998; 273: 22635-22639Abstract Full Text Full Text PDF PubMed Scopus (354) Google Scholar, 42Vásquez-Vivar J. Kalyanaraman B. Martasek P. Hogg N. Masters B.S. Karoui H. Tordo P. Pritchard K.A.J. Proc. Natl. Acad. Sci. U. S. A. 1998; 95: 9220-9225Crossref PubMed Scopus (1242) Google Scholar, 43Xia Y. Tsai A.-L. Berka V. Zweier J.L. J. Biol. Chem. 1998; 273: 25804-25808Abstract Full Text Full Text PDF PubMed Scopus (616) Google Scholar, 44Pou S. Keaton L. Surichamorn W. Rosen G.M. J. Biol. Chem. 1999; 274: 9573-9580Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar, 45Vásquez-Vivar J. Hogg N. Martásek P. Karoui H. Pritchard Jr., K.A. Kalyanaraman B. J. Biol. Chem. 1999; 274: 26736-26742Abstract Full Text Full Text PDF PubMed Scopus (168) Google Scholar, 51Klatt P. Heinzel B. John M. Kastner M. Bohme E. Mayer B. J. Biol. Chem. 1992; 267: 11374-11378Abstract Full Text PDF PubMed Google Scholar, 59Culcasi M. Lafon-Cazal M. Pietri S. Bockaert J. J. Biol. Chem. 1994; 269: 12589-12593Abstract Full Text PDF PubMed Google Scholar). When the steady-state flux of O⨪2 was high there was evidence for ONOO− formation (57Xia Y. Dawson V.L. Dawson T.M. Snyder S.H. Zweier J.L. Proc. Natl. Acad. Sci. U. S. A. 1996; 93: 6770-6774Crossref PubMed Scopus (657) Google Scholar, 58Xia Y. Zweier J.L. Proc. Natl. Acad. Sci. U. S. A. 1997; 94: 6954-6958Crossref PubMed Scopus (610) Google Scholar). Indeed, certain pathologic states might promote formation of ONOO− such as ischemia/reperfusion injury (67Grisham M.B. Granger D.N. Lefer D.J. Free Radic. Biol. Med. 1998; 25: 404-433Crossref PubMed Scopus (251) Google Scholar). Formation of HO⋅, either through metal ion-catalyzed H2O2 decomposition (68Haber F. Weiss J. Proc. R. Soc. Lond. A. 1934; 147: 332-351Crossref Google Scholar) or from decomposition of ONOO− (69van der Vliet A. O'Neill C.H. Halliwell B. Cross C.E. Kaur H. FEBS Lett. 1994; 339: 89-92Crossref PubMed Scopus (361) Google Scholar, 70Pou S. Nguyen S.Y. Gladwell T. Rosen G.M. Biochim. Biophys. Acta. 1995; 1244: 62-68Crossref PubMed Scopus (84) Google Scholar, 71Richeson C.E. Mulder P. Bowry V.W. Ingold K.U. J. Am. Chem. Soc. 1998; 120: 7211-7219Crossref Scopus (152) Google Scholar), at sensitive cellular sites may also contribute to cytotoxicity. Finally, it is worth noting that sequential formation of NO⋅ and O⨪2can result in differing cell signaling pathways (15Lane P. Gross S.S. Semin. Nephrol. 1999; 19: 215-229PubMed Google Scholar, 16Finkel T. Curr. Opin. Cell Biol. 1998; 10: 248-253Crossref PubMed Scopus (1019) Google Scholar), few of which have been well defined. Therefore, under different conditions a variety of oxidants may derive from NOS that can impact cell function in ways that are significant.