Xanthine oxidase enzyme

Xanthine Oxidase - Enzyme

1. Xanthine Oxidase Deepak (IIT Delhi) 2015-2017 Department of Chemistry c hemistrytoday.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.   Intro…

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. Cont….

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

5. FeS -cluster (orange) bounded FAD (red) molybdopterin cofactor with molydenium (yellow). Crystallographic structure (monomer) of bovine xanthine oxidase

6. hypoxanthine + H 2 O + O 2  ⇌ xanthine + H 2 O 2 xanthine + H 2 O + O 2   ⇌ uric acid + H 2 O 2 Xanthine oxidase can also act on certain other purines and aldehydes . it can also produce superoxide ion . RH + H 2 O + 2 O 2   ⇌ ROH + 2 O 2 −  + 2 H + Reactions catalyzed by xanthine oxidase 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 Catalytic Mechanism

8. H ₂O

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 + H 2 O + O 2  ⇌ xanthine + H 2 O 2 xanthine + H 2 O + O 2   ⇌ uric acid + H 2 O 2 Enzymatic Assay

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. Clinical significance

11. What is 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. How are Gout attack prevented ?

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 . Clinical significance