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Phosphoenolpyruvate Carboxykinase (PEPCK, EC 4.1.1.32) is widely present in animals, plants, microorganisms, and cells. It catalyzes the conversion of oxaloacetate to phosphoenolpyruvate and is a key regulatory enzyme in the gluconeogenesis pathway.
Assay Principle
PEPCK catalyzes the conversion of Oxaloacetate to Phosphoenolpyruvate and CO₂. Pyruvate Kinase and Lactate Dehydrogenase subsequently catalyze the sequential oxidation of NADH to NAD⁺. The rate of decrease in NADH absorbance at 340 nm is measured, which reflects PEPCK activity.
| Component | 100T | Storage |
| Extraction Buffer | 100 mL | 2-8℃ |
| Reagent 1 | 18 mL | 2-8℃ |
| Reagent 2 | 16.5 μL | 2-8℃ |
| Reagent 3 | 1EA | -20℃ |
| Reagent 4 | 1EA | -20℃ |
Required Materials and Equipment (Not Provided)
Spectrophotometer / Microplate reader, benchtop centrifuge, adjustable pipettes, micro quartz cuvette / 96-well plate, mortar and pestle, ice, and distilled water.
Sample Preparation:
1.Bacteria or Cultured Cells:
Collect cells by centrifugation and discard the supernatant.
Add Extraction Buffer at a ratio of 1 ml per 5-10 million cells (e.g., 1 ml for 5 million cells).
Sonicate on ice (20% power or 200W, pulse 3s on/10s off, repeat 30 times).
Centrifuge at 8000 g, 4°C for 10 min. Collect the supernatant and keep it on ice for assay.
2.Tissues:
Homogenize tissue on ice in Extraction Buffer at a ratio of 1:5-10 (w/v) (e.g., 0.1 g tissue in 1 ml buffer).
Centrifuge at 8000 g, 4°C for 10 min. Collect the supernatant and keep it on ice for assay.
3.Serum (or Plasma) Samples:
Assay directly.
Assay Procedure:
1.Preheat the spectrophotometer or microplate reader for at least 30 minutes. Set the wavelength to 340 nm. Zero the instrument with distilled water.
2.Preparation of Working Solution: Just before use, transfer and dissolve Reagent 2 and Reagent 3 into Reagent 1. Mix well. Aliquot and store any unused portions at -20°C. Avoid repeated freeze-thaw cycles.
3.Preparation of Reagent 4: Just before use, dissolve the contents of the vial in 1 ml of distilled water. Mix well. Aliquot and store any unused portions at -20°C. Avoid repeated freeze-thaw cycles.
4.Pre-warm the Working Solution and dissolved Reagent 4 at 37°C (for mammalian samples) or 25°C (for other species) for 5 minutes.
5.In a micro quartz cuvette or a well of a 96-well plate, add:
10 μl sample
10 μl dissolved Reagent 4
180 μl pre-warmed Working Solution
Mix immediately and record the initial absorbance (A₁) at 340 nm. Record the absorbance again (A₂) after exactly 1 minute. Calculate ΔA = A₁ - A₂.
Note: For this kit, if ΔA is greater than 0.1, dilute the sample with Extraction Buffer by an appropriate factor (account for this dilution factor 'n' in the calculations) so that ΔA is less than 0.1 to improve detection sensitivity.
PEPCK Activity Calculation:
1. Calculation for Micro Quartz Cuvette (d = 1.0 cm)
General Parameters for Cuvette:
Vₜₒₜₐₗ (Total reaction volume) = 0.0002 L (200 μL)
ε (NADH molar extinction coefficient) = 6220 L/mol/cm
d (Cuvette light path) = 1.0 cm
Vₛₐₘₚₗₑ (Sample volume in reaction) = 0.01 mL (10 μL)
T (Reaction time) = 1 min
Vₛₐₘₚₗₑₜₒₜₐₗ (Total extract volume) = 1 mL (for tissues/cells)
Cpr (Sample protein concentration, mg/mL)
W (Sample mass, g)
500 (Cell/Bacteria count in millions for example calculation: 5 million)
a. For Serum (Plasma):
Definition: One unit of activity is defined as the amount of enzyme that consumes 1 nmol of NADH per minute per ml of serum.
Calculation:
PEPCK Activity (nmol/min/ml) = [ΔA × Vₜₒₜₐₗ ÷ (ε × d) × 10⁹] ÷ Vₛₐₘₚₗₑ ÷ T
Simplified Formula: PEPCK (nmol/min/ml) = 3215 × ΔA
b. For Tissues, Bacteria, or Cells:
Based on Sample Protein Concentration:
Definition: One unit of activity is defined as the amount of enzyme that consumes 1 nmol of NADH per minute per mg of protein.
Calculation:
PEPCK Activity (nmol/min/mg prot) = [ΔA × Vₜₒₜₐₗ ÷ (ε × d) × 10⁹] ÷ (Vₛₐₘₚₗₑ × Cpr) ÷ T
Simplified Formula: PEPCK (nmol/min/mg prot) = 3215 × ΔA ÷ Cpr
Based on Sample Fresh Weight:
Definition: One unit of activity is defined as the amount of enzyme that consumes 1 nmol of NADH per minute per gram of fresh tissue.
Calculation:
PEPCK Activity (nmol/min/g fresh weight) = [ΔA × Vₜₒₜₐₗ ÷ (ε × d) × 10⁹] ÷ (W × Vₛₐₘₚₗₑ / Vₛₐₘₚₗₑₜₒₜₐₗ) ÷ T
Simplified Formula: PEPCK (nmol/min/g fresh weight) = 3215 × ΔA ÷ W
Based on Bacterial or Cell Density:
Definition: One unit of activity is defined as the amount of enzyme that consumes 1 nmol of NADH per minute per 10⁴ cells.
Calculation (example for 5 million cells in 1 ml extract):
PEPCK Activity (nmol/min/10⁴ cell) = [ΔA × Vₜₒₜₐₗ ÷ (ε × d) × 10⁹] ÷ (500 × Vₛₐₘₚₗₑ / Vₛₐₘₚₗₑₜₒₜₐₗ) ÷ T
Simplified Formula: PEPCK (nmol/min/10⁴ cell) = 6.43 × ΔA
2. Calculation for 96-Well Plate (d = 0.5 cm)
General Parameters for 96-Well Plate:
(All parameters remain the same except for the light path 'd')
d (96-well plate light path) = 0.5 cm
a. For Serum (Plasma):
Simplified Formula: PEPCK (nmol/min/ml) = 6430 × ΔA
b. For Tissues, Bacteria, or Cells:
Based on Sample Protein Concentration:
Simplified Formula: PEPCK (nmol/min/mg prot) = 6430 × ΔA ÷ Cpr
Based on Sample Fresh Weight:
Simplified Formula: PEPCK (nmol/min/g fresh weight) = 6430 × ΔA ÷ W
Based on Bacterial or Cell Density:
Simplified Formula: PEPCK (nmol/min/10⁴ cell) = 12.86 × ΔA
Precautions
Before formal assay, it is essential to perform a pilot test with 2-3 samples expected to have significant differences in activity.
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