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Here we report the structural characterization of the product formed from the reaction between hydroethidine (HE) and superoxide ( \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} ). By using mass spectral and NMR techniques, the chemical structure of this product was determined as 2-hydroxyethidium (2-OH-E + ). By using an authentic standard, we developed an HPLC approach to detect and quantitate the reaction product of HE and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} formed in bovine aortic endothelial cells after treatment with menadione or antimycin A to induce intracellular reactive oxygen species. Concomitantly, we used a spin trap, 5- tert -butoxycarbonyl-5-methyl-1-pyrroline N -oxide (BMPO), to detect and identify the structure of reactive oxygen species formed. BMPO trapped the \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} that formed extracellularly and was detected as the BMPO-OH adduct during use of the EPR technique. BMPO, being cell-permeable, inhibited the intracellular formation of 2-OH-E + . However, the intracellular BMPO spin adduct was not detected. The definitive characterization of the reaction product of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} with HE described here forms the basis of an unambiguous assay for intracellular detection and quantitation of \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} . Analysis of the fluorescence characteristics of ethidium (E + ) and 2-OH-E + strongly suggests that the currently available fluorescence methodology is not suitable for quantitating intracellular \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} . We conclude that the HPLC/fluorescence assay using HE as a probe is more suitable reactive oxygen species for detecting intracellular \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} \begin{equation*}{\mathrm{O}}_{2}^{{\bullet}-}\end{equation*}\end{document} .

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Abstract-We previously showed that hydrogen peroxide (H 2 O 2 ) contributes to flow-induced dilation in human coronary resistance arteries (HCRAs); however, the source of this H 2 O 2 is not known. We hypothesized that the H 2 O 2 is derived from superoxide (O 2 •Ϫ ) generated by mitochondrial respiration. HCRAs were dissected from right atrial appendages obtained from patients during cardiac surgery and cannulated with micropipettes. H 2 O 2 -derived radicals and O 2•Ϫ were detected by electron spin resonance (ESR) using BMPO as the spin trap and by histofluorescence using hydroethidine (HE, 5 mol/L) and dichlorodihydrofluorescein (DCFH, 5 mol/L). Diameter changes to increases in pressure gradients (20 and 100 cm H 2 O) were examined in the absence and the presence of rotenone (1 mol/L), myxothiazol (100 nmol/L), cyanide (1 mol/L), mitochondrial complex I, III, and IV inhibitors, respectively, and apocynin (3 mmol/L), a NADPH oxidase inhibitor. At a pressure gradient of 100 cm H 2 O, ubisemiquinone and hydroxyl radicals were detected from effluents of vessels. Including superoxide dismutase and catalase in the perfusate reduced the ESR signals. Relative ethidium and DCFH fluorescence intensities in HCRAs exposed to flow were enhanced (1.45Ϯ0.15 and 1.57Ϯ0.12, respectively compared with no-flow) and were inhibited by rotenone (0.87Ϯ0.17 and 0.95Ϯ0.07). Videomicroscopic studies showed that rotenone and myxothiazol blocked flow-induced dilation (% max. dilation at 100 cm H 2 O: rotenone, 74Ϯ3% versus 3Ϯ13%; myxothiazol, 67Ϯ3% versus 28Ϯ4%; PϽ0.05). Neither cyanide nor apocynin altered flow-induced dilation. These results suggest that shear stress induced H 2 O 2 formation, and flow-induced dilation is derived from O 2•Ϫ originating from mitochondrial respiration.