Protocol for Phosphoglucose Isomerase (PGI) Activity Assay
Protocol for Phosphoglucose Isomerase (PGI) Activity Assay
1 Overview
1.1 Purpose and Scope
This protocol is used to determine phosphoglucose isomerase (PGI) activity in cell or microbial extracts. It is applicable to enzyme activity assays using microbial pellets from fermentation broth, cell lysate supernatants, and other soluble protein samples.
1.2 Method Principle
PGI catalyzes the reversible isomerization between fructose 6-phosphate (F6P) and glucose 6-phosphate (G6P). In the coupled assay system, PGI first converts F6P into G6P. Subsequently, under the action of glucose-6-phosphate dehydrogenase (G6PDH), G6P reacts with NADP⁺ to generate NADPH. NADPH has a characteristic absorption peak at 340 nm; therefore, PGI activity can be calculated based on the rate of increase in absorbance at 340 nm over time.
2 Materials and Reagents
2.1 Samples
(1) Cell pellets obtained by centrifugation of fermentation broth.
(2) Cell lysate supernatant.
(3) Other soluble enzyme extracts.
2.2 Main Reagents
(1) 100 mM phosphate buffer (pH 7.0).
(2) 100 mM Tris-HCl buffer (pH 7.8).
(3) NADP⁺.
(4) MgCl₂.
(5) Fructose 6-phosphate (F6P).
(6) Glucose-6-phosphate dehydrogenase (G6PDH).
(7) Quartz sand, optional, for mechanical assistance in cell disruption.
(8) Deionized water or ultrapure water.
2.3 Instruments
(1) Centrifuge.
(2) Ultrasonic cell disruptor.
(3) UV-Vis spectrophotometer or microplate reader with kinetic detection capability at 340 nm.
(4) Constant-temperature water bath or temperature-controlled module.
(5) 1 cm pathlength quartz cuvette.
3 Sample Preparation
3.1 Cell Collection
(1) Take 25 mL of fermentation broth or cell suspension.
(2) Centrifuge at an appropriate speed, discard the supernatant, and collect the cell pellet.
3.2 Cell Washing
(1) Resuspend the pellet in 100 mM phosphate buffer, pH 7.0.
(2) Centrifuge and discard the supernatant.
(3) Repeat the washing step twice.
(4) Finally, resuspend the cell pellet in 5 mL of phosphate buffer.
3.3 Cell Disruption
(1) Place the cell suspension in an ice bath.
(2) Perform ultrasonic disruption for 10 min. Reference conditions are 40 kHz, 10 s on, and 5 s off.
(3) If the sample is difficult to disrupt, add an appropriate amount of quartz sand to assist mechanical disruption.
(4) After disruption, centrifuge and collect the supernatant as the crude enzyme solution.
4 Enzyme Activity Assay
4.1 Preparation of the Reaction System
The total reaction volume is 1 mL and contains the following components:
(1) 100 mM Tris-HCl (pH 7.8).
(2) 10 mM MgCl₂.
(3) 0.5 mM NADP⁺.
(4) 2 mM F6P.
(5) 1 U G6PDH.
(6) Add an appropriate volume of deionized water to bring the reaction to the final volume.
(7) Finally, add 100 μL of sample to initiate the reaction.
4.2 Blank Controls
At least the following controls are recommended:
(1) Sample blank: replace F6P with buffer to correct for background absorbance changes from the sample.
(2) Reagent blank: replace the sample with buffer to exclude spontaneous reactions in the reagent system.
(3) An inactivated sample control may be included when necessary.
4.3 Reaction Conditions
(1) The detection temperature can be fixed at 25°C. If detection is performed at 37°C, the same temperature should be maintained throughout the protocol and used consistently in calculations.
(2) Pre-equilibrate the reaction mixture without the sample for 3–5 min.
(3) After adding the sample, mix quickly and immediately begin measurement.
5 Absorbance Measurement
5.1 Detection Conditions
(1) Detection wavelength: 340 nm.
(2) Pathlength: 1 cm.
(3) Recording time: continuous monitoring for 3–5 min is recommended.
(4) Sampling interval: record absorbance every 30 s, or use the instrument’s continuous kinetic acquisition mode.
5.2 Result Reading
(1) Select data from the initial linear phase of the reaction.
(2) Calculate ΔA340/min.
(3) Use the blank-corrected value for enzyme activity calculation.
6 Enzyme Activity Calculation
6.1 Calculation Parameter
For calculation, the molar extinction coefficient of NADPH at 340 nm can be expressed as:
ε340 = 6.22 mM⁻¹·cm⁻¹
6.2 Calculation Formula
Enzyme activity (U/mL) = (ΔA340/min × Vt) / (6.22 × l × Vs)
where:
Vt is the total reaction volume (mL)
ε is the molar extinction coefficient of NADPH at 340 nm
l is the optical pathlength (cm)
Vs is the sample volume added (mL)
6.3 Definition of Enzyme Activity
One unit of enzyme activity (U) is defined as the amount of enzyme that catalyzes the formation of 1 μmol NADPH per minute under the specified conditions.
7 Key Points for Result Interpretation
7.1 Determination of the Linear Range
(1) Absorbance should increase linearly during the initial phase of the reaction.
(2) If the early section of the curve fluctuates significantly, check mixing, temperature control, and sample clarity.
(3) If the overall increase is too rapid and exceeds the linear range, dilute the sample appropriately and repeat the assay.
7.2 Blank Control
(1) Blank controls should not show a clear continuous increase in absorbance.
(2) If the blank absorbance increases significantly, reagent contamination, substrate degradation, or other interfering enzymes in the system may be present.
8 Critical Control Points
8.1 Sample Processing
(1) Cell disruption should be performed under low-temperature conditions as far as possible.
(2) The crude enzyme solution should be assayed as soon as possible after preparation, and repeated freeze-thaw cycles should be avoided.
(3) If the sample contains a large number of particles, centrifuge thoroughly to avoid interference with the 340 nm reading.
8.2 Reaction System
(1) NADP⁺ and G6PDH should be used together in the coupled assay system.
(2) The F6P concentration should not be too low, otherwise the reaction rate may be limited.
(3) MgCl₂ helps stabilize the coupled enzyme system and is recommended to be retained.
8.3 Consistency of Detection Conditions
(1) Temperature, pathlength, total volume, and sample input volume should remain consistent.
(2) If a microplate reader is used, effective pathlength correction should be considered.
9 Common Problems and Cause Analysis
9.1 No Increase in Absorbance
(1) PGI activity in the sample is too low.
(2) G6PDH is inactive.
(3) NADP⁺ or F6P was omitted.
(4) Cell disruption was insufficient.
9.2 High Blank Value
(1) The reagent system is contaminated.
(2) Other enzymes capable of generating NADPH are present in the sample.
(3) The substrate or coenzyme was not freshly prepared.
9.3 Poor Reproducibility
(1) Ultrasonic disruption conditions are unstable.
(2) Sample mixing is insufficient.
(3) Differences in sample addition time are too large.
(4) Temperature control is inconsistent.
10 Safety and Operating Standards
10.1 Personal Protection
(1) Wear a lab coat and disposable gloves during the experiment.
(2) Pay attention to protection during ultrasonic disruption and avoid overheating the sample.
10.2 Waste Disposal
(1) Reaction waste containing enzymes, coenzymes, and substrates should be collected separately.
(2) Sample waste and contaminated consumables should be handled according to laboratory biochemical waste disposal regulations.
11 Related Reagent and Material Selection
Table 1 Reagent and material selection for PGI activity assay
Cat. No. | Product Name | Grade and Specification | Corresponding Step | Use |
Tris-HCl | Injection grade | Reaction buffer preparation | Used to prepare 100 mM Tris-HCl reaction buffer (pH 7.8) | |
β-Nicotinamide adenine dinucleotide phosphate(NADP) hydrate | Moligand™, ≥95% | Coenzyme | Used as oxidized coenzyme II in the coupled reaction system | |
β-Nicotinamide adenine dinucleotide phosphate sodium salt hydrate(NADP) | ≥97% | Coenzyme | Can be used as a high-purity NADP⁺ alternative | |
β-NADP-Na(NADP) | ≥93% | Coenzyme | Can be used as a routine NADP⁺ alternative | |
NADP, Disodium Salt | ≥97% | Coenzyme | Directly corresponds to NADP⁺ in the method | |
NADP, Disodium Salt | 10mM in Water | Coenzyme | Suitable for direct preparation or condition optimization | |
Magnesium chloride solution | UltraBio™, Suitable for molecular biology, Ultra pure, ~1 M in H2O | Reaction system preparation | Suitable for direct addition to prepare the final MgCl₂ concentration in the reaction system | |
Magnesium chloride solution | Suitable for molecular biology, 1.00 M±0.01 M | Reaction system preparation | Can be used as an alternative MgCl₂ working solution | |
Magnesium chloride hexahydrate | Suitable for molecular biology, ≥99%(T) | Reaction system preparation | Suitable for preparing MgCl₂ stock solution | |
Magnesium chloride hexahydrate | BioReagent, for cell culture, suitable for insect cell culture | Reaction system preparation | Can be used as a routine MgCl₂ raw material alternative | |
D-Fructose 6-phosphate disodium salt hydrate | ≥95% | Substrate | Directly corresponds to fructose 6-phosphate (F6P) in the method | |
D-Fructose 6-phosphate dipotassium salt | ≥97% | Substrate | Can be used as a high-purity F6P alternative | |
Glucose-6-Phosphate Dehydrogenase (G6PD) | ActiBioPure™, Bioactive, High Performance, EnzymoPure™, ≥95%(SDS-PAGE), ≥600 U/mg protein | Coupling enzyme | Directly corresponds to the PGI coupled assay system | |
Glucose-6-phosphate Dehydrogenase from Leuconostoc mesenteroides | EnzymoPure™, ≥200 NADP units/mg protein | Coupling enzyme | Suitable for NADP-based coupled detection systems | |
Glucose-6-phosphate Dehydrogenase from Leuconostoc mesenteroides(Suspension) | EnzymoPure™, ≥200 NADP units/mg protein | Coupling enzyme | Suitable for use in liquid enzyme systems | |
Sand | 8-16 mesh/1-2mm | Cell disruption | Can be used for mechanical assistance in cell disruption | |
Water | Ultra pure | Solution preparation | Suitable for buffer and reaction solution preparation |