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In This Article

  • Summary
  • Abstract
  • Introduction
  • Protocol
  • Representative Results
  • Discussion
  • Acknowledgements
  • Materials
  • References
  • Reprints and Permissions

Summary

Electron paramagnetic resonance (EPR) spectroscopy is an unambiguous method to measure free radicals. The use of selective spin probes allows for detection of free radicals in different cellular compartments. We present a practical, efficient method to collect biological samples that facilitate treating, storing, and transferring samples for EPR measurements.

Abstract

The accurate and specific detection of reactive oxygen species (ROS) in different cellular and tissue compartments is essential to the study of redox-regulated signaling in biological settings. Electron paramagnetic resonance spectroscopy (EPR) is the only direct method to assess free radicals unambiguously. Its advantage is that it detects physiologic levels of specific species with a high specificity, but it does require specialized technology, careful sample preparation, and appropriate controls to ensure accurate interpretation of the data. Cyclic hydroxylamine spin probes react selectively with superoxide or other radicals to generate a nitroxide signal that can be quantified by EPR spectroscopy. Cell-permeable spin probes and spin probes designed to accumulate rapidly in the mitochondria allow for the determination of superoxide concentration in different cellular compartments.

In cultured cells, the use of cell permeable 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine (CMH) along with and without cell-impermeable superoxide dismutase (SOD) pretreatment, or use of cell-permeable PEG-SOD, allows for the differentiation of extracellular from cytosolic superoxide. The mitochondrial 1-hydroxy-4-[2-triphenylphosphonio)-acetamido]-2,2,6,6-tetramethyl-piperidine,1-hydroxy-2,2,6,6-tetramethyl-4-[2-(triphenylphosphonio)acetamido] piperidinium dichloride (mito-TEMPO-H) allows for measurement of mitochondrial ROS (predominantly superoxide).

Spin probes and EPR spectroscopy can also be applied to in vivo models. Superoxide can be detected in extracellular fluids such as blood and alveolar fluid, as well as tissues such as lung tissue. Several methods are presented to process and store tissue for EPR measurements and deliver intravenous 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) spin probe in vivo. While measurements can be performed at room temperature, samples obtained from in vitro and in vivo models can also be stored at -80 °C and analyzed by EPR at 77 K. The samples can be stored in specialized tubing stable at -80 °C and run at 77 K to enable a practical, efficient, and reproducible method that facilitates storing and transferring samples.

Introduction

While measures of oxidative stress and reactive oxygen species are important to the study of diverse diseases across all organ systems, the detection of reactive oxygen species (ROS) is challenging due to a short half-life and high reactivity. An electron paramagnetic resonance (EPR) technique is the most unambiguous method for detecting free radicals. Spin probes have advantages over the more commonly used fluorescent probes. Though fluorescent probes are relatively inexpensive and easy to use and provide rapid, sensitive detection of ROS, they do have serious limitations due to artifactual signals, an inability to calculate ROS concentrations, and a general lack of ....

Protocol

All animal studies were approved by the University of Colorado Denver Institutional Animal Care and Use Committee.

1. Preparation of Reagents

  1. Diethylenetriaminepentaacetic acid (DTPA) stock (150 mM)
    1. Add 2.95 g of DTPA (393.35 g/mol) to 10 mL of deionized water.
    2. To dissolve DTPA, add 1 M NaOH dropwise and bring to a pH of 7.0.
    3. Bring the volume to 50 mL with water for a final DTPA concentration of 150 mM, and store at 4 °C.
    4. .......

Representative Results

Superoxide detection using CMH was validated using the X/XO superoxide generating system to demonstrate that the nitroxide (CM.) signal was fully inhibited by SOD, while catalase had no effect (Figure 1A). The total, extracellular superoxide was then evaluated in RAW 264.7 cells by incubating cells with the cell-permeable CMH spin probe +/- SOD pretreatment. The nitroxide concentration was measured in both the cell suspension and buffer, which.......

Discussion

The assessment of free radical production in biological settings is important in understanding redox regulated signaling in health and disease, but the measure of these species is highly challenging due to the short half-life of free radical species and technical limitations with commonly used methods. EPR is a valuable and powerful tool in redox biology, as it is the only unambiguous method for detecting free radicals. In this project, we demonstrate practical EPR methods for designing experiments and preparing samples .......

Acknowledgements

This work was supported by the University of Colorado School of Medicine Dean's Strategic Research Infrastructure award, R01 HL086680-09 and 1R35HL139726-01, to E.N.G. and UCD CFReT fellowship award (HE). The authors thank Dr. Sandra Eaton and Dr. Gareth Eaton (University of Denver), Dr. Gerald Rosen and Dr. Joseph P. Kao (University of Maryland), and Dr. Sujatha Venkataraman (University of Colorado Denver) for helpful discussions, and Joanne Maltzahn, Ashley Trumpie and Ivy McDermott (University of Colorado Denver) for technical support.

....

Materials

NameCompanyCatalog NumberComments
DMEMLifeTech10566-016cell culture media
Diethylenetriaminepentaacetic acid (DTPA)Sigma AldrichD6518-5G
sodium chloride (NaCl) Fisher Scientific  BP358-212used to prepare 50 mM phosphate saline buffer  according to Sigma aldrish  
potassium phosphate dibasic (HK2PO4 )Fisher Scientific  BP363-500used to prepare 50 mM phosphate saline buffer  according to Sigma aldrish  
potassium phosphate monobasic (KH2PO4 )Sigma AldrichP-5379used to prepare 50 mM phosphate saline buffer  according to Sigma aldrish  
Krebs-Henseleit buffer (KHB) (Alfa Aesar, Hill)J67820
Bovine erythrocyte superoxide dismutase (SOD)Sigma Aldrich S7571-30KU
Phorbol 12-myristate 13-acetate (PMA) Sigma AldrichP1585-1MGDissolve in DMSO
Antimycin A (AA)Sigma AldrichA8674-25MGDissolve in Ethanol and store in glass vials(MW used is the averaged molecular weights for four lots)
1-Hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine . HCl (CMH)Enzo Life SciencesALX-430-117-M050
1-Hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine . HCl (CPH)Enzo Life SciencesALX-430-078-M250
1-Hydroxy-4-[2-triphenylphosphonio)-acetamido]-2,2,6,6-tetramethylpiperidine, 1-Hydroxy-2,2,6,6-tetramethyl-4-[2-(triphenylphosphonio)acetamido]piperidinium dichloride ( mito-TEMPO-H)Enzo Life SciencesALX-430-171-M005
1-Hydroxy-2,2,6,6-tetramethylpiperidin-4-yl-trimethylammonium chloride . HCl (CAT1H)Enzo Life SciencesALX-430-131-M250
Heparin Sagent PharmaceuticalsNDC 25021-400-10
Diphenyliodonium chloride Sigma Aldrich43088
Deferoxamin mesylate saltSigma AldrichD9533-1G
CritosealLeica39215003
BRAND disposable BLAUBRAND micropipettes, intraMarkSigma Aldrich708733Capillaries
PTFE FRACTIONAL FLUOROPOLYMER TUBING
3/16” OD x 1/8” ID
NORELL1598774ATeflon tubing 
SILICONE RUBBER STOPPERS FOR NMR SAMPLE TUBES  FOR THIN WALL TUBES HAVING AN OD OF 4mm-5mm (3.2mm TO 4.2mm ID) TS-4-5-SRNORELL94987
EMXnano Bench-Top EPR spectrometer Bruker BioSpin GmbHE7004002
EMX NANO TISSUE CELLBruker BioSpin GmbHE7004542

References

  1. Kalyanaraman, B., et al. Measuring reactive oxygen and nitrogen species with fluorescent probes: challenges and limitations. Free Radical Biology and Medicine. 52 (1), 1-6 (2012).
  2. Bobko, A. A., et al.

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Electron Paramagnetic ResonanceEPR SpectroscopyRedox BiologyReactive Oxygen SpeciesSpin ProbesSuperoxide Detection264 7 CellsKrebs Henseleit BufferDTPASuperoxide DismutaseCMHPMARoom Temperature77 K

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