Plasma Glucose Measurement by Serum Enzyme Hexokinase

Laboratory Procedure Manual

Measurement of plasma glucose concentration is an essential biochemical test used widely in clinical laboratories to assess glucose metabolism and diagnose disorders such as diabetes mellitus. Among the various methodologies available, the hexokinase enzyme method is recognized for its high specificity and accuracy. This manual outlines the principles, reagents, instruments, sample handling, analytical procedure, calculations, quality control, and troubleshooting related to plasma glucose determination using the hexokinase method in serum.

1. Introduction

Glucose is a crucial energy source in the human body. Accurate quantification of plasma glucose levels aids physicians in the management of diabetes and other metabolic disorders. The hexokinase method employs the enzyme hexokinase to catalyze the phosphorylation of glucose to glucose-6-phosphate, which is subsequently oxidized by glucose-6-phosphate dehydrogenase (G6PD) with concomitant reduction of NADP+ to NADPH. The increase in NADPH concentration is directly proportional to the glucose concentration and is measured spectrophotometrically.

2. Principle of the Hexokinase Method

The hexokinase method involves two enzymatic reactions:

• Step 1: Glucose + ATP → Glucose-6-phosphate + ADP (Catalyzed by hexokinase)
• Step 2: Glucose-6-phosphate + NADP⁺ → 6-Phosphogluconate + NADPH + H⁺ (Catalyzed by glucose-6-phosphate dehydrogenase)

The amount of NADPH formed is stoichiometric with glucose concentration and is measured by absorbance increase at 340 nm.

3. Reagents and Solutions

The following reagents are required:

• Buffer solution: Typically Tris buffer or phosphate buffer at pH 7.5-8.0.
• ATP (Adenosine Triphosphate): Required for phosphorylation.
• NADP+ (Nicotinamide Adenine Dinucleotide Phosphate): Coenzyme for the G6PD reaction.
• Hexokinase enzyme: Catalyzes glucose phosphorylation.
• Glucose-6-phosphate dehydrogenase (G6PD): Catalyzes oxidation of glucose-6-phosphate.
• Glucose standards: Standard solutions of known glucose concentrations for calibration.

Reagents should be prepared fresh or stored as recommended by the manufacturer to maintain enzymatic activity and assay accuracy.

4. Equipment and Instrumentation

• Spectrophotometer: Capable of reading absorbance at 340 nm.
• Micropipettes and tips: For accurate reagent and sample volume measurement.
• Water bath: Maintained at 37C (optional depending on protocol).
• Test tubes or cuvettes: Appropriate for spectrophotometric measurements.
• Timer: To measure reaction time precisely.

5. Sample Collection and Handling

Venous blood is collected into tubes containing anticoagulants such as sodium fluoride and potassium oxalate to inhibit glycolysis and preserve glucose concentration. Plasma is separated by centrifugation promptly (within 30 minutes). Samples should be analyzed as soon as possible or stored at 28°C if delay is unavoidable.

6. Analytical Procedure

The following is a generalized stepwise laboratory procedure for plasma glucose determination by the hexokinase method:

6.1 Preparation of Reagent Mixture

Prepare a reagent mix containing:

• Buffer solution
• ATP
• NADP+
• Hexokinase enzyme
• Glucose-6-phosphate dehydrogenase enzyme

Mix thoroughly and maintain at recommended assay temperature, usually room temperature or 37°C.

6.2 Assay Procedure

1. Label test tubes or cuvettes for blank, standard, and samples.
2. Add defined volume (e.g., 1 mL) of reagent mix into each tube.
3. Add sample (e.g., 10 µL) to the designated tube; add same volume of distilled water to blank.
4. Mix gently and incubate for 5 to 10 minutes at room temperature or 37°C depending on protocol.
5. Measure absorbance against the blank at 340 nm using the spectrophotometer.
6. Calculate plasma glucose concentration using the absorbance of the standard for calibration.

7. Calculation

The plasma glucose concentration (mg/dL or mmol/L) can be calculated using the following formula:

Parameter	Explanation
Abs_sample	Absorbance of the plasma sample at 340 nm
Abs_blank	Absorbance of the reagent blank
Abs_standard	Absorbance of the standard solution
Conc_standard	Concentration of the glucose standard (e.g., 100 mg/dL)

Glucose concentration in sample =
[(Abs_sample - Abs_blank) / (Abs_standard - Abs_blank)] Conc_standard

Results can be converted to mmol/L by dividing mg/dL values by 18.

8. Quality Control

To ensure accuracy and reliability of results, the following quality control measures should be implemented:

• Run controls (normal and pathological) alongside patient samples daily.
• Calibrate the spectrophotometer regularly and check wavelength accuracy.
• Use fresh reagents and verify enzyme activity periodically.
• Perform assay in duplicate or triplicate to confirm precision.
• Maintain proper documentation of assay conditions and results for traceability.

9. Interferences and Limitations

The hexokinase method is noted for its high specificity for glucose. However, certain factors can affect the accuracy of the assay:

• Hemolysis: Free hemoglobin can interfere by absorbing at 340 nm.
• Lipemia: Turbidity may affect accurate spectrophotometric readings.
• Drugs and metabolites: Some compounds like high concentrations of uric acid, bilirubin, or ascorbic acid may interfere with NADPH measurement.
• Sample handling: Delayed separation or improper storage can reduce glucose levels due to glycolysis.

10. Safety Considerations

Handle all blood samples as potentially infectious material, following universal precautions. Use personal protective equipment (PPE) such as gloves and lab coats. Dispose of waste, sharps, and reagents according to institutional biosafety and chemical safety protocols.

11. Summary

The hexokinase serum enzyme method for plasma glucose determination offers a precise and reliable assay widely used in clinical laboratories. Its enzymatic specificity and minimal interference make it the reference method for glucose measurement. Adherence to proper sample collection, reagent preparation, assay procedure, and quality control measures ensures consistent and clinically meaningful results essential for patient care.