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Uric Acid Metabolism – Basics and Disorders

Uric acid metabolism describes the breakdown of purines in the body, producing uric acid as a final product. Disruptions can lead to gout or kidney stones.

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Uric acid metabolism describes the breakdown of purines in the body, producing uric acid as a final product. Disruptions can lead to gout or kidney stones.

What is Uric Acid Metabolism?

Uric acid metabolism refers to the biochemical process by which purines – components of the genetic material DNA and RNA – are broken down in the human body. The end product of this process is uric acid (chemically: 2,6,8-trioxypurine). Uric acid is primarily excreted via the kidneys in the urine, with a smaller portion eliminated through the intestines.

Purines are both absorbed from food and produced naturally by the body during cell turnover. A well-functioning uric acid metabolism keeps the concentration of uric acid in the blood (serum uric acid) within a healthy range. Elevated levels are referred to as hyperuricemia and can trigger several medical conditions.

Biochemistry and Metabolic Pathway

The breakdown of purines occurs in several steps:

Purine bases such as adenine and guanine are first converted to hypoxanthine and xanthine, respectively.
The enzyme xanthine oxidase catalyzes the oxidation of hypoxanthine to xanthine and then xanthine to uric acid.
Unlike many other mammals, humans lack a functional uricase enzyme that would further break down uric acid into the more soluble allantoin. This makes humans particularly susceptible to elevated uric acid levels.

Regulation of Uric Acid Levels

The concentration of uric acid in the blood is determined by the balance between production and excretion. Key influencing factors include:

Diet: Purine-rich foods such as organ meats, seafood, red meat, and certain legumes increase uric acid production.
Alcohol: Beer and spirits in particular inhibit renal uric acid excretion and promote its production.
Fluid intake: Adequate hydration supports the excretion of uric acid through the kidneys.
Genetics: Hereditary variants in genes encoding uric acid transport proteins (e.g., URAT1, GLUT9) significantly affect the ability to excrete uric acid.
Medications: Certain diuretics and low-dose aspirin can raise uric acid levels.

Disorders of Uric Acid Metabolism

Hyperuricemia

Hyperuricemia occurs when uric acid levels in the blood are persistently elevated (above 6.8 mg/dL in men and above 6.0 mg/dL in women). It often causes no symptoms but can lead to serious conditions over time.

Gout (Arthritis Urica)

When uric acid crystals (monosodium urate crystals) are deposited in joints and surrounding tissue, gout develops. This is characterized by sudden, very painful joint inflammation, most commonly affecting the big toe. Chronic gout can lead to tophi (uric acid deposits under the skin) and joint destruction.

Urolithiasis (Uric Acid Kidney Stones)

Persistently elevated uric acid levels or excessively acidic urine can promote the formation of uric acid kidney stones, which can cause severe pain and kidney complications.

Lesch-Nyhan Syndrome

Lesch-Nyhan syndrome is a rare, inherited metabolic disorder in which an enzyme defect (deficiency of HGPRT) leads to massive overproduction of uric acid and causes severe neurological symptoms.

Diagnosis

Disorders of uric acid metabolism are diagnosed through:

Blood test: Measurement of serum uric acid levels
Urine test: Assessment of uric acid excretion via 24-hour urine collection
Imaging: Ultrasound or X-ray to detect kidney stones or gout tophi
Joint aspiration: Detection of urate crystals in joint fluid when gout is suspected

Treatment and Prevention

Dietary Changes

A low-purine diet, reduction of alcohol consumption, and adequate fluid intake are fundamental measures to lower uric acid levels. Lean meat in moderate amounts, plenty of vegetables, low-fat dairy products, and coffee (in moderation) are generally recommended.

Medical Treatment

Xanthine oxidase inhibitors (e.g., allopurinol, febuxostat): Inhibit the key enzyme in uric acid production, thereby reducing uric acid levels.
Uricosuric agents (e.g., benzbromarone, probenecid): Promote the excretion of uric acid through the kidneys.
Colchicine and NSAIDs: Used during acute gout attacks to reduce inflammation and relieve pain.

References

Zöllner N., Gröbner W. (eds.) – Purine Metabolism in Man. Springer-Verlag, Berlin (1984).
Richette P., Doherty M. et al. – 2016 updated EULAR evidence-based recommendations for the management of gout. Annals of the Rheumatic Diseases, 76(1):29–42 (2017). PubMed PMID: 27457514.
World Health Organization (WHO) – Diet, Nutrition and the Prevention of Chronic Diseases. WHO Technical Report Series 916, Geneva (2003).

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