Introduction
- Pyruvate kinase (PK) is a key glycolytic enzyme that catalyzes the conversion of phosphoenolpyruvate (PEP) to pyruvate with the generation of adenosine triphosphate (ATP).
- Since mature red blood cells (RBCs) lack mitochondria, glycolysis is their sole source of energy.
- Therefore, pyruvate kinase plays a crucial role in maintaining RBC survival, membrane integrity, and normal cellular function.
- Deficiency of red cell pyruvate kinase is one of the most common inherited defects of glycolysis and can lead to chronic non-spherocytic hemolytic anemia.
- Measurement of pyruvate kinase activity in erythrocytes is therefore important in the diagnosis of PK deficiency.
Principle
Pyruvate kinase catalyzes the following reaction:
PEP + ADP → Pyruvate + ATP
The pyruvate formed is immediately converted to lactate by lactate dehydrogenase (LDH) in the presence of NADH:
Pyruvate + NADH + H⁺ → Lactate + NAD⁺
NADH absorbs ultraviolet light at 340 nm, whereas NAD⁺ does not. The decrease in absorbance at 340 nm due to oxidation of NADH is directly proportional to pyruvate kinase activity in the red cell hemolysate.
Specimen Required
- Fresh EDTA anticoagulated whole blood
- Red blood cells separated and washed with isotonic saline
- Hemolysate prepared from packed RBCs
Sample Precautions
- Avoid hemolysis during collection.
- Analyze as soon as possible.
- Store at 2–8°C if delay is unavoidable.
- Excessive storage may reduce enzyme activity.
Reagents
- Tris buffer (pH 7.5–7.8)
- Magnesium chloride (MgCl₂)
- Potassium chloride (KCl)
- Adenosine diphosphate (ADP)
- Phosphoenolpyruvate (PEP)
- Nicotinamide adenine dinucleotide, reduced form (NADH)
- Lactate dehydrogenase (LDH)
- Distilled water
Procedure
Preparation of Red Cell Hemolysate
- Collect 2–3 mL of venous blood in an EDTA tube.
- Centrifuge the sample at 1500–2000 rpm for 10 minutes.
- Carefully remove the plasma and buffy coat (white cell layer).
- Wash the packed red blood cells three times with isotonic saline (0.9% NaCl).
- After the final wash, prepare a hemolysate by lysing the erythrocytes with distilled water according to the laboratory protocol.
- Determine the hemoglobin concentration of the hemolysate for subsequent calculation of enzyme activity.
Assay Procedure
- Label a cuvette for the test sample.
- Pipette the required volume of reaction mixture containing:
- Tris buffer
- Magnesium chloride (MgCl₂)
- Potassium chloride (KCl)
- ADP
- NADH
- Lactate dehydrogenase (LDH)
- Add the prepared red cell hemolysate to the reaction mixture and mix gently.
- Incubate the mixture at 37°C for 5 minutes.
- Add phosphoenolpyruvate (PEP) to initiate the reaction.
- Immediately record the absorbance at 340 nm using a UV spectrophotometer.
- Measure the absorbance at 1-minute intervals for 3–5 minutes.
- Calculate the average decrease in absorbance per minute (ΔA/min).
Observation
A gradual decrease in absorbance at 340 nm is observed due to oxidation of NADH to NAD⁺. The rate of decrease is directly proportional to the pyruvate kinase activity present in the red cell hemolysate.
Calculation
Pyruvate kinase activity is generally expressed as:
- Units per gram hemoglobin (U/g Hb)
- Units per 10¹⁰ erythrocytes
General formula:
PK Activity = ΔA/min × Factor
The factor depends on:
- Path length
- Total reaction volume
- Sample volume
- Molar absorptivity of NADH
Reference Values
Typical adult values:
| Parameter | Reference Range |
|---|---|
| Red Cell Pyruvate Kinase | 8–18 U/g Hb |
Reference ranges may vary according to laboratory methods and assay kits.
Interpretation
Normal Result
Indicates adequate glycolytic activity and ATP generation in erythrocytes.
Low Result
Suggests:
- Pyruvate kinase deficiency
- Chronic hemolytic anemia
- Enzyme instability disorders
High Result
May occur due to:
- Increased reticulocyte count
- Compensatory erythropoiesis
Advantages
- Early Detection of Pyruvate Kinase Deficiency
- Helps identify hereditary pyruvate kinase deficiency, one of the most common glycolytic enzyme defects causing chronic hemolytic anemia.
- Accurate Assessment of Enzyme Activity
- Directly measures the functional activity of pyruvate kinase in erythrocytes, providing reliable diagnostic information.
- Useful in the Evaluation of Hemolytic Anemia
- Assists in investigating unexplained chronic hemolytic anemia, particularly when other common causes have been excluded.
- Sensitive and Specific Method
- The coupled enzymatic assay using NADH oxidation is highly sensitive and capable of detecting even moderate reductions in enzyme activity.
- Quantitative Results
- Provides precise numerical values that can be compared with reference ranges and used for clinical interpretation.
- Requires Only a Small Blood Sample
- Can be performed using a relatively small volume of EDTA-anticoagulated blood.
- Rapid and Reproducible
- The spectrophotometric method is simple, rapid, and produces reproducible results when performed under standardized conditions.
- Useful for Family Screening
- Can aid in the investigation of family members of affected individuals in inherited pyruvate kinase deficiency.
- Supports Clinical Decision-Making
- Helps clinicians differentiate pyruvate kinase deficiency from other causes of hereditary hemolytic anemia.
- Suitable for Routine Clinical Laboratories
- Can be performed using standard UV-visible spectrophotometric equipment available in most clinical biochemistry laboratories.
Limitations
- Effect of Recent Blood Transfusion
- Transfused donor red blood cells may contain normal pyruvate kinase activity, leading to falsely normal or elevated results and masking an underlying deficiency.
- Influence of Reticulocytosis
- Reticulocytes contain higher levels of pyruvate kinase than mature erythrocytes.
- Patients with increased reticulocyte counts may show falsely elevated enzyme activity.
- Sample Stability Issues
- Pyruvate kinase activity decreases during prolonged storage.
- Delayed analysis or improper sample handling can result in falsely low values.
- Cannot Identify Genetic Mutations
- The enzyme assay measures functional activity but does not determine the specific genetic mutation responsible for pyruvate kinase deficiency.
- Molecular testing may be required for confirmation.
- Overlap Between Normal and Carrier States
- Some heterozygous carriers may have enzyme activities within the normal range, making carrier detection difficult.
- Interference from Leukocyte Contamination
- Incomplete removal of white blood cells during sample preparation may affect enzyme measurements and lead to inaccurate results.
- Requires Fresh Blood Samples
- Freshly collected EDTA blood is preferred because enzyme activity may decline during storage.
- Limited Diagnostic Value Alone
- Results should always be interpreted along with clinical findings, peripheral smear examination, reticulocyte count, and other hematological investigations.
- Specialized Equipment Required
- The assay requires a UV spectrophotometer capable of measuring absorbance at 340 nm, which may not be available in all laboratories.
- May Miss Mild Deficiency
- Patients with mild enzyme deficiency may have enzyme activities near the lower limit of normal, making diagnosis challenging without additional testing.
Clinical Applications
1. Diagnosis of Hereditary Pyruvate Kinase Deficiency
- The principal application of the test is the diagnosis of hereditary pyruvate kinase deficiency, an autosomal recessive disorder characterized by reduced ATP production in red blood cells.
- It helps confirm the diagnosis in patients presenting with chronic hemolytic anemia, jaundice, and splenomegaly.
2. Evaluation of Chronic Hemolytic Anemia
- Useful in investigating patients with unexplained chronic hemolytic anemia when common causes such as autoimmune hemolytic anemia, hemoglobinopathies, and membrane defects have been excluded.
3. Investigation of Neonatal Jaundice
- Pyruvate kinase deficiency may present in newborns with severe jaundice and anemia.
- Measurement of enzyme activity aids in identifying an underlying enzymatic defect.
4. Differential Diagnosis of Hereditary Hemolytic Disorders
- Helps differentiate pyruvate kinase deficiency from other inherited red cell disorders such as:
- G6PD deficiency
- Hereditary spherocytosis
- Other erythrocyte enzyme deficiencies
5. Family Screening and Genetic Counseling
- Family members of affected individuals may be screened for enzyme abnormalities.
- Results can support genetic counseling in families with a history of pyruvate kinase deficiency.
6. Assessment of Erythrocyte Metabolic Disorders
- The test is valuable in evaluating rare disorders of red cell glycolysis and energy metabolism.
7. Pre-Molecular Diagnostic Evaluation
- Enzyme activity measurement is often performed before genetic testing and helps identify patients who may benefit from molecular analysis of the PKLR gene.
8. Monitoring Patients with Known Enzyme Deficiency
- In selected cases, enzyme activity assessment may be used as part of the clinical evaluation and follow-up of patients with diagnosed pyruvate kinase deficiency.
9. Research Applications
- Widely used in hematology and biochemical research to study erythrocyte metabolism, glycolytic pathways, and inherited enzyme disorders.
10. Educational and Laboratory Training Purposes
- Serves as an important practical assay for teaching enzyme kinetics, coupled enzyme reactions, and spectrophotometric methods in clinical biochemistry laboratories.