Assay Procedure for Xanthine Oxidase Microbial


The appearance of uric acid is measured at 293nm by spectrophotometry.

Unit definition

One unit causes the formation of one micromole of uric acid per minute under the conditions described below.




A. Tris-HCl buffer, pH 7.5
B. Sodium hydroxide solution
C. Xanthine solution :10mM[Dissolve 15.2mg of xanthine (MW=152.11) in 10ml solution (B)]
D. Oxonic acid potassium salt solution :1mM (Dissolve 9.75mg of oxonic acid・K salt in 50ml H2O)
E. Enzyme diluent :50mM Tris-HCl buffer, pH 7.5


  1. Prepare the following reaction mixture in a cuvette (d=1.0cm) and equilibrate at 37℃ for about 5 minutes.

    2.24ml Tris-HCl buffer, pH 7.5 (A)
    0.08ml Xanthine solution          (C)
    0.08ml Oxonic acid solution     (D)
Concentration in assay mixture
Tris-HCl buffer ca.89.6 mM
Xanthine 0.32mM
Oxonic acid 32 μM
  1. Add 0.1ml of the enzyme solution* and mix by gentle inversion.
  2. Record the increase in optical density at 293nm against water for 3 to 4 minutes in a spectrophotometer thermostated at 37℃, and calculate the ΔOD per minute from the initial linear portion of the curve (ΔOD test).

At the same time, measure the blank rate (ΔOD blank) by using the same method as the test except that the enzyme diluent (E) is added instead of the enzyme solution.

* Dissolve the enzyme preparation in ice-cold enzyme diluent (E), dilute to 0.1-0.2U/ml with the same buffer and store on ice.


Activity can be calculated by using the following formula:


Weight activity (U/mg)=(U/ml)×1/C


Vt :Total volume (2.5ml)
Vs :Sample volume (0.1ml)
12.5 :Millimolar extinction coefficient of uric acid under the assay condition (F/micromole)
1.0 :Light path length (cm)
df :Dilution factor
C :Enzyme concentration in dissolution (c mg/ml)


This procedure is for informational purposes. For a current copy of Sigma’s quality control procedure contact our Technical Service Department.


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