CDNB GSH: The Substrate Pair Behind the Glutathione S-Transferase Assay

CDNB GSH refers to the pairing of 1-chloro-2,4-dinitrobenzene (CDNB) with glutathione (GSH), the substrate combination used in the most common spectrophotometric assay for glutathione S-transferase (GST) activity. In the reaction, GST enzymes conjugate GSH to CDNB and form a product that absorbs light at 340 nm. The rate at which that absorbance rises tells researchers how much active enzyme is present in a sample.

What CDNB and GSH Actually Are

CDNB is a synthetic aromatic electrophile that chemists use as a model substrate. It is not a drug, a nutrient, or a supplement ingredient. Suppliers sell it as a crystalline powder, and it is handled only in laboratory settings.

GSH is a tripeptide assembled from glutamate, cysteine, and glycine. For anyone asking what is gsh glutathione, the simplest answer is that it is the body's most abundant small-molecule antioxidant. It also acts as a cofactor for detoxification enzymes, which is why it shows up in so many biochemical assays.

Glutathione exists in a reduced form (GSH) and an oxidized form (GSSG). The gsh gssg ratio is a widely used marker of oxidative stress, but it is measured with different techniques than the CDNB reaction.

How the CDNB GSH Assay Works

The classic protocol, adapted from Habig, Pabst, and Jakoby (1974), is a continuous kinetic assay. All reagents are combined in a cuvette, and the instrument records absorbance every few seconds for two to five minutes.

1. Prepare 0.1 M potassium phosphate buffer at pH 6.5.
2. Add reduced GSH to a final concentration of 1 mM.
3. Add CDNB dissolved in ethanol to a final concentration of 1 mM.
4. Add the sample (cell lysate, tissue homogenate, or purified enzyme) last to start the reaction.
5. Monitor absorbance at 340 nm at a constant temperature, usually 25 °C or 37 °C.

Component	Typical Final Concentration	Role
Potassium phosphate buffer, pH 6.5	0.1 M	Maintains optimal pH
Reduced glutathione (GSH)	1 mM	Nucleophilic substrate
CDNB in ethanol	1 mM	Electrophilic substrate
Sample	1-50 µg protein	Source of GST enzyme

The CDNB-GSH conjugate has a molar extinction coefficient of about 9.6 mM⁻¹ cm⁻¹ at 340 nm. That value converts raw absorbance readings into product concentration.

Why the CDNB Assay Remains a Laboratory Standard

• It is inexpensive and needs only a basic UV-Vis spectrophotometer.
• It runs in minutes and produces real-time kinetic data.
• It works with crude lysates, so no purification or antibodies are required.
• It captures the activity of several GST classes at once, including alpha, mu, and pi.

These practical advantages explain why the CDNB GSH reaction is still taught in undergraduate biochemistry labs decades after it was introduced.

CDNB GSH Versus Other Glutathione Methods

Choosing a method depends on whether the question is about enzyme activity, total glutathione, or redox balance. These are not interchangeable measurements.

Method	What It Measures	Readout	Key Limitation
CDNB GSH assay	GST enzyme activity	Absorbance change at 340 nm	Does not report total GSH levels
DTNB (Ellman's) assay	Free thiols, including GSH	Absorbance at 412 nm	Other thiols interfere
GSSG/GSH ratio kits	Redox balance	Absorbance or fluorescence	Requires a GSH masking step
LC-MS/MS	Exact GSH and GSSG concentrations	Mass signal	Costly and slower to run
Fluorescent probes	Cellular GSH in live cells	Fluorescence	Probe specificity varies

Units, Controls, and Common Pitfalls

Activity is reported in units. One unit equals 1 micromole of CDNB-GSH conjugate formed per minute at the stated temperature. Results are normally normalized per milligram of protein in the sample.

GSH reacts slowly with CDNB even without any enzyme present. A non-enzymatic blank without sample must be run and subtracted, or activity will be overestimated.

Because the assay reports enzyme activity, it does not measure total gsh content in a sample. Labs that need absolute glutathione concentrations should pair it with a thiol quantification method.

Ethanol or DMSO carried over from substrate stocks can inhibit GST at high concentrations. Keep solvent below 2 percent of the total reaction volume.

Limitations, Safety, and Research Context

CDNB is a broad substrate, which is both its strength and its weakness. Theta- and zeta-class GSTs react with CDNB poorly, so the assay can miss their contribution entirely. Investigators who need isoenzyme-specific data often add alternative substrates such as DCNB or ethacrynic acid.

CDNB is toxic and irritating to skin, eyes, and airways, and it is a known skin sensitizer. It should be weighed and handled in a fume hood with gloves, a lab coat, and eye protection, and disposed of as hazardous waste.

The CDNB GSH assay is a research tool only. It is not validated for clinical diagnosis or for evaluating consumer supplements.

Some investigators use the assay to screen natural compounds that influence GST activity, such as quercetin gsh studies that examine how flavonoids alter enzyme kinetics. Outside the lab, interest in the sublingual form of gsh as a way to raise body glutathione levels is a separate area of discussion with its own evidence base, and it does not involve CDNB at all.

Anyone using glutathione-related products for personal health should talk with a healthcare professional rather than relying on laboratory assay data.

COMMON QUESTIONS

Frequently Asked Questions

What does CDNB GSH mean in a lab protocol?

CDNB GSH refers to the reaction between 1-chloro-2,4-dinitrobenzene and glutathione. It is the standard substrate pair for measuring glutathione S-transferase activity in a spectrophotometer at 340 nm. The abbreviation appears constantly in enzyme kinetics protocols and Methods sections.

How is GST activity calculated from the CDNB GSH assay?

First, measure the change in absorbance at 340 nm per minute during the linear phase of the reaction. Divide that rate by the extinction coefficient of 9.6 mM⁻¹ cm⁻¹, then adjust for the reaction volume and the amount of protein in the sample. The result is expressed as micromoles of CDNB-GSH conjugate formed per minute per milligram of protein.

Why is the CDNB GSH assay run at pH 6.5?

A pH of 6.5 is close to the optimum for most glutathione S-transferase enzymes acting on CDNB. It also slows the spontaneous, non-enzymatic reaction between GSH and CDNB and keeps glutathione stable during the run. Running the assay at a higher pH typically increases background noise and lowers the signal-to-noise ratio.