Xanthine oxidase is an enzyme that generates reactive oxygen species by catalyzing the oxidation of hypoxanthine to xanthine and further catalyzing the oxidation of xanthine to uric acid. It plays an important role in purine catabolism in humans. The protein is a homodimer with three domains - an N-terminal FeS cluster domain, an FAD-binding domain, and a molybdenum cofactor domain. It can exist as xanthine dehydrogenase or be converted to xanthine oxidase via oxidation or proteolysis.

1. Deepak (IIT Delhi)
2015-2017
Department of Chemistry
chemistrytoday.in

2.  Xanthine oxidase (XO), a type of enzyme that
generates reactive oxygen species.
 Other names: xanthine oxidoreductase, hypoxanthine oxidase,
hypoxanthine - xanthine oxidase
 These enzymes catalyze the oxidation of hypoxanthine to
xanthine and further catalyze the oxidation of xanthine to uric
acid.
 These enzymes play an important role in the catabolism
of purines in some species, including humans.

3.  Source - Milk
 EC 1.17.3.2
 The same protein, which in humans has gene
symbol XDH, can also have xanthine dehydrogenase
activity.
 Most of the protein in the liver exists in a form with
xanthine dehydrogenase activity, but it can be converted
to xanthine oxidase by reversible sulfhydryl oxidation
or by irreversible proteolytic modification.

4.  The protein is homodimer, having a molecular
weight of 270 kDa.
 Each monomer has three –
(1) N-terminal domain - FeS-cluster
(2) FAD-binding domain
(3) The third domain - one molybdenum
 Fe-S convert XDH (xanthine dehydrogenase) to XO.
 FAD reduction of O2
 Molybdenum oxidative hydroxylation of purine.

5.  FeS-cluster (orange)
 bounded FAD (red)
 molybdopterin cofactor
with molydenium (yellow).

6.  hypoxanthine + H2O + O2 ⇌ xanthine + H2O2
 xanthine + H2O + O2 ⇌ uric acid + H2O2
 Xanthine oxidase can also act on certain other purines
and aldehydes.
 it can also produce superoxide ion .
RH + H2O + 2 O2 ⇌ ROH + 2 O2
− + 2 H+
Hypoxanthine Xanthine Uric acid

7.  The active site of XO is composed of a molybdopterin
unit with the molybdenum atom also coordinated by
terminal oxygen (oxo), sulfur atoms and a
terminal hydroxide.
 In the reaction with xanthine to form uric acid, an
oxygen atom is transferred from molybdenum to
xanthine.
 The reformation of the active molybdenum centre
occurs by the addition of water

8. H₂O

9.  Spectrophotometric determination of XO activity is
based on measuring uric acid production from xanthine
or hypoxanthine substrate at around 295 nm.
 The assay mixture always contains xanthine as a
substrate and sample.
hypoxanthine + H2O + O2 ⇌ xanthine + H2O2
xanthine + H2O + O2 ⇌ uric acid + H2O2

10.  Xanthine oxidase is a superoxide-producing enzyme
found normally in serum and the lungs, and its activity
is increased during influenza A infection.
 During severe liver damage, xanthine oxidase is
released into the blood, so a blood assay for XO is a
way to determine if liver damage has happened.
 Because xanthine oxidase is a metabolic pathway
for uric acid formation, the xanthine oxidase
inhibitor allopurinol is used in the treatment of gout.

12.  Maintaining adequate fluid intake helps prevent acute
gout attacks and decreases the risk of kidney stone
formation in people with gout.
 Alcohol is known to have diuretic effects that can
contribute to dehydration and precipitate acute gout
attacks. Alcohol can also affect uric acid metabolism
and cause hyperuricemia. It causes gout by slowing
down the excretion of uric acid from the kidneys as well
as by causing dehydration, which precipitates the
crystals in the joints.

13.  Xanthinuria is a rare genetic disorder where the lack of
xanthine oxidase leads to high concentration of xanthine
in blood and can cause health problems such as renal
failure. There is no specific treatment, sufferers are
advised by doctors to avoid foods high in purine and to
maintain a high fluid intake.