Xanthine Oxidase – Function, Gout and Therapy

What is Xanthine Oxidase?

Xanthine oxidase (also known as xanthine-oxidase or xanthine oxidoreductase) is an enzyme that plays a central role in purine metabolism. It catalyzes the oxidation of hypoxanthine to xanthine, and subsequently from xanthine to uric acid (urate). These reactions occur primarily in the liver and small intestine.

The enzyme belongs to the family of molybdoflavoproteins and requires the cofactors molybdenum, iron, and FAD (flavin adenine dinucleotide). It exists in two functional forms: the oxidase form (xanthine oxidase) and the dehydrogenase form (xanthine dehydrogenase).

Mechanism of Action

Xanthine oxidase catalyzes the following reactions in the purine degradation pathway:

• Hypoxanthine → Xanthine (first oxidation step)
• Xanthine → Uric acid (second oxidation step)

As byproducts of these reactions, reactive oxygen species (ROS) are generated, particularly superoxide anions and hydrogen peroxide. These free radicals can cause oxidative stress in tissues and have been linked to various diseases, including cardiovascular disorders and inflammatory conditions.

Medical Significance

Gout and Hyperuricemia

Excessively high xanthine oxidase activity leads to increased production of uric acid. When uric acid concentrations in the blood become too elevated (hyperuricemia), uric acid crystals can deposit in joints and soft tissues, causing the painful condition known as gout. Uric acid crystals can also accumulate in the kidneys, forming kidney stones.

Oxidative Stress and Cardiovascular Disease

The reactive oxygen species generated by xanthine oxidase contribute to oxidative stress. Elevated enzyme activity has been associated with the development and progression of atherosclerosis, heart failure, and other cardiovascular diseases.

Ischemia-Reperfusion Injury

During temporary interruption of blood supply (ischemia) followed by restoration (reperfusion), xanthine oxidase becomes particularly activated. The resulting free radicals can cause significant tissue damage, such as that occurring after a heart attack or stroke.

Therapeutic Inhibition of Xanthine Oxidase

Because of its central role in purine metabolism, xanthine oxidase is an important therapeutic target. Xanthine oxidase inhibitors are used to reduce uric acid production:

• Allopurinol: The classic xanthine oxidase inhibitor, used for decades in the treatment of gout and hyperuricemia. Allopurinol is converted in the body to oxipurinol, which persistently inhibits the enzyme.
• Febuxostat: A newer, selective xanthine oxidase inhibitor used as an alternative to allopurinol, particularly in cases of intolerance.

By inhibiting this enzyme, both uric acid production and the generation of reactive oxygen species are reduced, which explains the therapeutic benefits of these medications.

Diagnostic Relevance

Xanthine oxidase activity can be measured in blood and tissue samples. Elevated enzyme activity is found in various conditions such as gout, liver disease, and inflammatory states. Serum uric acid levels serve as an indirect marker of xanthine oxidase activity and are routinely measured in clinical practice.

References

1. Harrison's Principles of Internal Medicine, 21st Edition. McGraw-Hill Education, 2022.
2. Nishino T, Okamoto K, Eger BT, Pai EF. Mammalian xanthine oxidoreductase - mechanism of transition from xanthine dehydrogenase to xanthine oxidase. FEBS Journal. 2008;275(13):3278-3289. PubMed.
3. World Health Organization (WHO). Gout and Hyperuricaemia - Guidelines on Management. WHO Press, Geneva.