1 Overview

1.1 Purpose and Scope of Application

This protocol is used to establish a standardized operating procedure for measuring alkaline phosphatase activity using a diethanolamine (DEA) buffer system. It is suitable for enzymatic determination of alkaline phosphatase products whose activities are calibrated using a diethanolamine assay system.

This method is not applicable to alkaline phosphatase samples whose specific activity is referenced only in glycine units.

1.2 Principle

(1) Under alkaline conditions, alkaline phosphatase catalyzes the hydrolysis of p-nitrophenyl phosphate (PNPP), producing p-nitrophenol and inorganic phosphate.

(2) At 37°C and pH 9.8, the rate of p-nitrophenol formation can be continuously monitored by the rate of increase in absorbance at A405 nm.

(3) Based on the blank-corrected ΔA405/min, together with the total reaction volume, enzyme addition volume, and the millimolar extinction coefficient of p-nitrophenol at 405 nm, the enzyme activity of the sample can be calculated.

Reaction:

p-Nitrophenyl phosphate + H₂O ──alkaline phosphatase──> p-Nitrophenol + Pi

1.3 Terms and Definitions

(1) Purified water: Water from a deionization system, with a resistivity of approximately 18 MΩ·cm at 25°C.

(2) DEA unit: Under the conditions of 37°C and pH 9.8, one DEA unit is defined as the amount of enzyme required to hydrolyze 1.0 μmol of p-nitrophenyl phosphate per minute.

(3) Pi: Inorganic phosphate.

2 Materials and Reagents

2.1 Main Reagents

(1) Diethanolamine.

(2) Hydrochloric acid.

(3) 1 M magnesium chloride solution.

(4) p-Nitrophenyl phosphate (PNPP).

(5) Purified water.

(6) Alkaline phosphatase sample to be tested.

2.2 Instruments and Equipment

(1) Temperature-controlled spectrophotometer.

(2) 1 cm pathlength cuvettes.

(3) 37°C temperature-control device.

(4) Pipettes and compatible tips.

(5) pH meter.

(6) Timer.

2.3 Reaction Conditions

(1) Temperature: 37°C.

(2) pH: 9.8.

(3) Detection wavelength: 405 nm.

(4) Pathlength: 1 cm.

(5) Detection mode: Continuous spectrophotometric rate method.

3 Reagent Preparation

3.1 1.0 M Diethanolamine Buffer Containing 0.50 mM Magnesium Chloride, pH 9.8 at 37°C

(1) Prepare an initial solution of diethanolamine at 140 mg/mL.

(2) Adjust the pH of this solution to 9.8 at 37°C using 5 M hydrochloric acid.

(3) Dilute the pH-adjusted solution with purified water to a final diethanolamine concentration of 105 mg/mL.

(4) Add 0.5 mL/L of 1 M magnesium chloride solution.

(5) The buffer should be freshly prepared and protected from light.

3.2 0.67 M p-Nitrophenyl Phosphate Solution (PNPP)

(1) Prepare a 247 mg/mL solution of p-nitrophenyl phosphate.

(2) The substrate solution should be freshly prepared and protected from light.

3.3 Alkaline Phosphatase Solution

(1) Before use, prepare a working solution containing approximately 0.15 U/mL alkaline phosphatase using prechilled buffer.

(2) For liquid samples, first prepare a concentrated stock solution in prechilled buffer, with a recommended concentration of 2000 U/mL or higher.

(3) Continue stepwise dilution with prechilled buffer to obtain a final working solution concentration of approximately 0.15 U/mL.

4 Experimental Procedure

4.1 Pre-equilibration

(1) Preheat the spectrophotometer and cuvette system to 37°C.

(2) Equilibrate the buffer and PNPP substrate solution to 37°C.

(3) Keep the enzyme solution cold and add it to the reaction system only immediately before use.

4.2 Preparation of the Reaction System

Add buffer and PNPP solution to each cuvette in the order shown below:

(1) Invert to mix after reagent addition.

(2) Equilibrate each cuvette to 37°C.

(3) Monitor absorbance at A405 nm until the baseline is stable.

4.3 Reaction Initiation

After the baseline is stable, add enzyme solution to each reaction cuvette as follows:

(1) Immediately invert to mix after enzyme addition.

(2) Quickly start recording the rate of increase in A405 nm.

4.4 Rate Measurement

(1) Continuously record the absorbance change for approximately 5 min.

(2) Collect at least 4 data points within each 1 min interval.

(3) For each test sample and blank, use the maximum linear rate from the best linear interval as ΔA405/min.

(4) Subsequent calculations should use the net reaction rate after subtracting the blank rate from the sample rate.

5 Result Calculation

5.1 Enzyme Activity Calculation

Units/mL enzyme are calculated using the following formula:

Units/mL enzyme = [(ΔA405/min test − ΔA405/min blank) × VF] / [18.5 × VE]

Where:

(1) VF: Assay volume, i.e., total reaction volume (mL).

(2) VE: Volume of enzyme solution added (mL).

(3) 18.5: Millimolar extinction coefficient of p-nitrophenol at 405 nm.

If the sample has been additionally diluted before measurement, multiply the result by the dilution factor df.

5.2 Units/mg Solid

Units/mg solid = Units/mL enzyme ÷ mg solid/mL enzyme

5.3 Units/mg Protein

If the protein concentration of the sample is known, calculate as follows:

Units/mg protein = Units/mL enzyme ÷ mg protein/mL enzyme

6 Final Assay Concentrations

In a 3.00 mL reaction mixture, the final concentrations are as follows:

(1) Diethanolamine: 983 mM.

(2) Magnesium chloride: 0.49 mM.

(3) p-Nitrophenyl phosphate: 11.2 mM.

(4) Alkaline phosphatase: approximately 0.0075 U.

7 Key Points for Result Interpretation

7.1 Interpretation of the Linear Range

(1) Absorbance should increase linearly during the measurement period.

(2) If the early part of the curve fluctuates significantly or the later part approaches a plateau, only the most stable linear interval should be selected for rate calculation.

7.2 Blank Correction

(1) The blank is used to correct for spontaneous hydrolysis of the substrate and background absorbance changes in the system.

(2) If the blank rate is significantly elevated, first check PNPP freshness, buffer quality, and temperature stability.

7.3 Consistency Among Replicates

(1) Units/mL results obtained from test samples with different enzyme addition volumes should show good consistency.

(2) If the three test groups differ greatly, first check pipetting error, enzyme solution homogeneity, and linear interval selection.

8 Common Problems and Cause Analysis

8.1 Absorbance Changes Too Slowly

(1) Enzyme concentration is too low.

(2) Sample activity has decreased or the enzyme is inactive.

(3) The substrate or buffer was prepared improperly.

8.2 Absorbance Changes Too Quickly and the Linear Range Is Too Short

(1) Enzyme concentration is too high.

(2) Sample dilution is insufficient.

(3) Increase the predilution factor and repeat the assay.

8.3 High Blank Value

(1) PNPP substrate solution is not fresh or has undergone photodegradation.

(2) Buffer pH deviates significantly.

(3) Unstable temperature control causes background drift in the system.

8.4 Poor Reproducibility Among Replicates

(1) Addition sequence is inconsistent or mixing is insufficient.

(2) Recording is not started promptly after enzyme addition.

(3) The enzyme working solution is not kept cold, causing activity fluctuation.

9 Quality Control and Precautions

9.1 Buffer Control

(1) The diethanolamine buffer should be adjusted to pH 9.8 at 37°C.

(2) The buffer should be freshly prepared and protected from light.

9.2 Substrate Control

(1) PNPP substrate solution should be freshly prepared.

(2) Avoid prolonged exposure to light.

9.3 Enzyme Solution Control

(1) The enzyme working solution should be prepared immediately before use.

(2) The entire process before sample addition should be kept as cold as possible.

9.4 Assay Range Control

(1) The sample working solution should be adjusted to approximately 0.15 U/mL.

(2) The enzyme amount in the final system should be controlled within a range suitable for continuous rate measurement, ensuring an analyzable linear interval within 5 min.

10 Safety and Operating Requirements

10.1 Personal Protection

(1) Wear a laboratory coat and disposable gloves during the experiment.

(2) Diethanolamine, hydrochloric acid, and other chemical reagents should be handled in a well-ventilated environment.

10.2 Safety Data

(1) Refer to the relevant Safety Data Sheets (SDS) for hazard information and handling precautions.

(1) Waste liquids containing diethanolamine, hydrochloric acid, PNPP, and enzyme reaction residues should be collected separately.

(2) All waste liquids and contaminated consumables should be disposed of according to laboratory chemical waste handling regulations.

11 Selection of Related Reagents and Enzyme Products

Table 1 Selection of Key Buffer Components, Substrates, and Basic Reagents

Table 2 Selection of Alkaline Phosphatase Enzyme Products

For more related articles, please see below:

[1] Experiments on secretion and expression of exogenous proteins with alkaline phosphatase promoter and signaling sequences

[2] λ Alkaline phosphatase treatment assay of phage vector DNA

[3] Dephosphorylation of DNA fragments by alkaline phosphatase

[4] Acid phosphatase display assay

[5] Acid phosphatase activity assay