A Division of OXIS Health Products, Inc.
Colorimetric Determination of Reduced and Oxidized Glutathione
For Research Use Only. Not For Use In Diagnostic Procedures.
Catalog Number 21040
Reduced glutathione (GSH), a tripeptide (γ-glutamylcysteinylglycine) with a free thiol group, is a major antioxidant in human tissues that provides reducing equivalents for the glutathione peroxidase (GPx) catalyzed reduction of hydrogen peroxide and lipid hydroperoxides to water and the respective alcohol. During this process GSH becomes oxidized glutathione (GSSG). The GSSG is then recycled into GSH by gutathione reductase (GR) and â-nicotinamide adenine dinucleotide phosphate (NADPH). When mammalian cells are exposed to increased oxidative stress, the ratio of GSH/GSSG will decrease as a consequence of GSSG accumulation. Measurement of the GSSG level or determination of the GSH/GSSG ratio, is a useful indicator of oxidative stress and can be used to monitor the effectiveness of antioxidant intervention strategies.
PRINCIPLES OF THE PROCEDURE
The accurate measurement of GSSG levels has proved very difficult due to the low amount of GSSG in tissues and because of the absence of effective methods to prevent oxidation of GSH to GSSG during sample preparation. To measure GSSG in tissues, Guntherberg and Rost (2) first introduced N-ethylmaleimide (NEM) to eliminate the GSH. Although NEM can react with GSH to form a stable complex and prevent the participation of the reduced form in the enzymatic assay, NEM also inhibits GR. For this reason, Griffith (3) first introduced 2-vinylpyridine (2-VP), which does not inhibit GR significantly, to derivatize GSH. However, the 2-VP reaction is relatively slow and the reagent has little solubility in an aqueous medium. The GSH/GSSG-412 assay uses the thiol-scavenging reagent, 1-methyl-2-vinylpyridinium trifluoromethanesulfonate1 (M2VP) at a level that rapidly scavenges GSH but does not interfere with the GR assay.
. Reaction of M2VP with a thiol to form 1-Methyl-2-(2-thioethyl)-pyidinium salt.
2-VP at 10 mM, usually takes 60 minutes to remove 70% of the GSH in the sample during which time oxidation of GSH may occur, resulting in significant overestimation of the GSSG concentration. Using M2VP, complete scavenging of GSH is accomplished in less than one minute.
10 mM 2-VP
20 mM 2-VP
GSH Remaining (%)
:GSH was added to three concentrations of 2-VP and M2VP at 3 mM.
The remaining GSH was determined using the GSH/GSSG-412 Assay.
In 1969, Tietze (1) first introduced an enzymatic method for quantitative determination of amounts of total2 (reduced and oxidized or GSHt) glutathione. The method employs Ellman’s reagent (5,5'-dithiobis-2-nitrobenzoic acid or DTNB), which reacts with GSH to form a spectrophotometrically detectable product at 412 nm. GSSG can be determined by the reduction of GSSG to GSH, which is then determined by the reaction with Ellman’s reagent. In brief, the Tietze method (1) utilizes the change in color development during the reaction, and the reaction rate is proportional to the GSH and GSSG concentrations.
2 The definition of total GSH in this context does not include other mixed disulfides of glutathione such as glutathione-cysteine.
Materials Provided (for 200 tests)
PO4 with EDTA, 1 vial Lyophilzed powder.
PO4 with EDTA, 150 mL.
Glutathione reductase (GR) in Na· PO4 with EDTA, 40 mL.
â–Nicotinamide adenine dinucleotide phosphate with Tris base and mannitol, 6 vials lyophilized powder.
1-Methyl-2-vinyl-pyridium trifluoromethane sulfonate (M2VP) in HCl, 2 mL.
5,5’-Dithiobis-(2-nitrobenzoic acid) (DTNB) in Na·
PO4 with EDTA, with
ethanol, 40 mL.
GSSG in KPO4 buffer with EDTA, 2.0 mL each. Each GSSG molecule is equivalent to two GSH molecules; therefore, the values are expressed as µM GSH:
Items Required But Not Provided
• Centrifuge 1000 x g
• Spectrophotometric cuvettes with a 1 cm optical path length (glass or polystyrene).
• Disposable tubes and stoppers (glass or polypropylene). • Pipettes, preferably adjustable cabable of accurately pipetting 10, 50, 100, 200, 700 and
• Balance • Metaphosphoric acid (Sigma M-5043)
Reagent Storage and Handling
• When not in use, place the bottles at 4°C. Do not allow the capped reagent bottles to sit
at room temperature for long periods of time.
• Unopened reagents are stable until the indicated expiration date.
• NADPH: Just prior to use, reconstitute the lyophilized NADPH Reagent with 7.5 mL of
Assay Buffer. The reconstituted NADPH Reagent is stable for 6 hours at room temperature.
• Assay Buffer: Reconstitute the lyophilzed powder with 650 mL of deionized water. The
reconstituted reagent is stable at 4oC for the life of the kit.
• 5% Metaphosphoric Acid: Prepare fresh daily. Weigh 1 gram MPA and dissolve in 20 mL
• MPA and NADPH reagents are intended for same day use following reconstitution.
Preparing GSSG Standard
Standards are ready to use. Sample Preparation
The sample preparation for whole blood is described below. Please read the appropriate NOTES
sections for further information regarding the whole blood assay as well as procedural guidelines
for other sample types. GSSG Sample
1. Add 10 µL M2VP to a microcentrifuge tube (recommended).
2. Carefully add 100 µL whole blood to the bottom of the centrifuge tube.
3. Mix gently.
4. Freeze the sample at –70°C. (Sample is stable for at least 30 days at –70°C).
5. Thaw the sample and immediately mix, incubate at room temperature for 2-10 minutes.
6. Add 290 µL cold 5% MPA to the tube (1/4 dilution of original sample).
7. Vortex the sample for 15-20 seconds.
8. Centrifuge at 1000 x g
or greater for 10 minutes.
9. Add 50 µL MPA extract to 700 µL GSSG buffer (1/15 dilution of the acid extract).
10. Place the diluted extract on ice until use (Final sample dilution is 1/60). GSSG Blank
1. Add 50 µL MPA to 700 µL GSSG buffer (1/15 dilution of the acid extract). 2. Place the diluted MPA on ice until use (Final sample dilution is 1/60).
1. Carefully add 50 µL of whole blood to the bottom of a microcentrifuge tube (recommended).
2. Freeze the sample at –70°C. (Sample is stable for at least 30 days at –70°C).
3. Thaw the sample and immediately mix.
4. Add 350 µL cold 5% MPA to the tube (1/8 dilution of original sample).
5. Vortex the sample for 15-20 seconds.
6. Centrifuge at 1000 x g
or greater for 10 minutes.
7. Add 50 µL MPA extract to 3 mL Assay Buffer (1/61 dilution of the acid extract).
8. Place diluted extract on ice until use (Final sample dilution is 1/488). Assay
1. Add 200 µL of standards, blank or samples to the cuvettes. 2. Add 200 µL of Chromogen to each cuvette. 3. Add 200 µL of Enzyme to each cuvette. 4. Mix and incubate at room temperature for 5 minutes. 5. Add 200 µL of NADPH to each cuvette. 6. Record the change of absorbance at 412 nm for 3 min.
The calculation of the GSH and GSSG concentrations and the GSH/GSSG ratio requires four
steps: 1) Determination of the reaction rate, 2) Construction of calibration curves, 3) Calculation
of the analyte concentrations, and 4) Calculation of the GSH/GSSG ratio. Rate Determination
The change in absorbance at 412 nm is a linear function of the GSH concentration in the reaction mixture, is described by the following equation of a line:
A412 = slope x Minutes + intercept
where the slope of the regression equation is equal to the rate. Note that the intercepts for these
rate curves are ignored because they are dependent on the DTNB background and the time
interval between the addition of the NADPH (reaction start) and the first recorded A412
In the examples below, linear regression gave the following equation of the line for the GSHt
) and the GSSG (Figure 4
. Reaction rate for an untreated sample. The rate is Figure 4
. Reaction rate for a M2VP treated sample ( ) and the
proportional to the concentration of GSHt.
GSSG Blank (o). The rate is proportional to the concentration of GSSG.
GSHt: A412 = 0.2209 x Minutes + 0.2363 with an r2 value of 1.0000. Therefore, the rate for the GSHt sample is 0.2209 A412/min. GSSG: A412 = 0.05938 x Minutes + 0.1651 with an r2 value of 0.9999. Therefore, the rate for the GSSG sample is 0.05938 A412/min. GSSG BLANK: A412 = 0.04238 x Minutes + 0.1454 with an r2 value of 0.9999. Therefore, the rate for the GSSG Blank is 0.04238 A412/min.
The GSH/GSSG-412 assay uses a six-point standard curve for both GSHt and GSSG determinations. The Net Rate is the difference between the rate at each concentration of GSH and the Blank rate.
Table 1. A typical 6 point calibration of the GSH/GSSG-412 Assay.
µM GSH A412/min Net Rate
Because the concentration of GSSG is much lower in the reaction mixture compared to GSHt, it is
recommended that selected data ranges from the calibration curve be plotted separately. For
GSHt, perform linear regression on a three-point curve using the 0, 1.50 and 3.00 µM GSH data
points, see Figure 5
. In the case of GSSG, use the 0, 0.10, 0.25, and 0.50 µM GSH data points,
see Figure 6
GSH Calibration Curve
GSSG Calibration Curve
. A three point calibration curve of A412/min vs. µM Figure 6
. A four point calibration curve of A 412/min vs. µM
GSH used to determine the concentration of GSHt.
GSH used to determine the concentration of GSSG.
GSH t and GSSG Concentrations
The general form of the regression equation describing the calibration curve is:
Net Rate = slope x GSH + intercept
Therefore, to calculate the analyte concentration from the GSH calibration curve:
For example, from Figure 3
, the net rate of change for the GSHt sample is 0.2209 – 0.0423 or
0.1786 A412/min. Using the calibration curve parameters from Figure 5
, the GSHt can be
calculated as follows:
, Similarly, from the rate of change for the GSSG sample is 0.05938 – 0.04238 or 0.0170
A412/min. Using the calibration curve parameters from Figure 6
, the GSSG concentration can be
calculated as shown below. Note that the dilution factor correction accounts for the conversion of
GSH to GSSG; 60-fold dilution divided by 2 GSH molecules per GSSG molecule.
The GSH/GSSH Ratio is then calculated by dividing the difference between the GSHt and GSSG concentrations (reduced GSH) by the concentration of GSSG.
Continuing the above example, the GSH/GSSG ratio is:
GSSG was diluted in Assay Buffer and tested using the GSH/GSSG-412 assay. The results show that the assay is linear from 0 to 5 µM GSH in the reaction mixture which was equivalent to 0 – 2.5 µM GSH in the original sample.
Linearity on dilution
Rate at 412 nm
GSHt and GSSG were measured in a whole blood (WB) sample, with known concentrations of GSH or GSSG added to determine the addition recovery for the assay. The recovery of the GSH assay was 98% and GSSG assay was 96%.
The precision of the GSH/GSSG-412 assay was determined using a modification of the NCCLS EP5-T2 guideline. A sample containing a high and low concentration of GSSG was assayed twice per day for 13 days. At the same time, the GSHt and GSSG were determined from whole blood samples stored at –70°C. Each precision parameter was expressed as the mean, standard deviation, and coefficient of variation of the net rate were observed in each sample set.
GSSG in Buffer
The enzymatic assay is for total GSH, including both GSH and GSSG. The Lower Limit of Detection (LLD) is a measure of the lowest GSSG concentration based on the differences of blank rates, which will be defined as 3.29 standard deviations above zero.
The GSH analogues were tested in buffer solution and the interference was calculated by comparing with the slope of the GSSG standard. The values are expressed as a percent of the GSSG standard. All tested analogues have no interfering effect on the GSH/GSSG-412 assay. Analogues
Sample Correlation With Reference Assay
A comparison between the enzymatic assay and the HPLC-electrochemical detection assay has
been reported by Richie et al
. (4). They reported that there was no significant difference between
these two methods in the measurement of human whole blood GSH. NOTES
There are no known interferents for the GSH/GSSG-412 Assay. Sample Preparation
Tissues have not been tested with the GSH/GSSG-412 Assay. The following issues need to be considered when preparing tissue samples for analysis: • GSH oxidation in vitro
likely occurs rapidly in disrupted tissues. The M2VP reagent should be
• GSSG diffusion from the cell may result in underestimating the GSSG. This is likely to be only
γ-Glutamyltranspeptidase will metabolize GSH. This membrane bound enzyme is especially high in kidney, pancreas, ciliary body, choroids plexus, intestinal epithelia, bile duct cells, lymphoid cells and many tumor cells (5).
Pipetting. Whole blood is difficult to pipette with precision. Using positive displacement pipetting techniques will improve the precision of the GSH and GSSG assays with whole blood. Freezing Step. The freezing step serves to lyse the red blood cell and maximize the concentration of GSSG in the sample. Frozen Samples. Blood samples that have been frozen without prior treatment with the Scavenging Reagent are not suitable for the GSSG assay. GSH Linearity. Because GSH is at high concentrations in whole blood, approximately 1 mM of the whole blood sample should be diluted at least 244 times in order to maintain linearity of the reaction rate.
Sample Stability. Glutathione and oxidized glutathione are relatively stable in intact “resting” cells
for up to 24 hr at 4°C, Figure 7
. The stability of “elevated” GSSG in the intact red blood cell has
not been determined. It is recommended that blood samples be treated with M2VP as soon as
possible and frozen immediately.
GSSG Stability in Intact Red Blood Cells at 4°C
Stability of GSSG, from a rested subject, in the intact red blood
cell at 4°C. (GSH-159).
Upon disruption of the cell, GSH is rapidly oxidized as shown in Figure 8
. To minimize the in vitro
oxidation of GSH to GSSG, the Scavenger Reagent should be added to the sample prior to lysis
Stability of GSSG in Red Blood Cell Lysate
. Stability of GSSG in the red blood cell lysate at 4°C. (GSH-
GSSG in normal resting plasma is at or below the lowest level of detection for the assay.
GSSG is not detectable in urine. Limitations
This assay has been validated only for whole blood samples containing EDTA as the anticoagulant.
The rate curves must be linear. If not, generally this means the GSH concentration is too high. Dilute the sample and re-run the assay.
Calibration curves must be linear. REFERENCES
1. Tietze F (1969) Analytical Chemistry 27
2. Guntherberg H and Rost J (1966) Analytical Biochemistry 15
3. Griffith OW (1980) Analytical Biochemistry 106
4. Richie JP Jr., et al (1996) Clinical Chemistry 42
5. Anderson, M. (1996) Glutathione in Free Radicals, A Practical Approach, ed. N.A. Punchard
and F.J. Kelly Oxford University Press, New York, p 213.
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