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MOCOS gene
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Gene

MOCOS gene

The MOCOS gene provides instructions for an enzyme that activates other enzymes involved in the breakdown of purines and various other compounds.

Normal Function

The MOCOS gene provides instructions for making an enzyme called molybdenum cofactor sulfurase. This enzyme adds sulfur to the molybdenum cofactor, which is required to activate two other enzymes: xanthine dehydrogenase and aldehyde oxidase.

  • Xanthine dehydrogenase: Involved in the breakdown of purines (DNA and RNA building blocks). It converts hypoxanthine to xanthine, and xanthine to uric acid, which is then excreted.
  • Aldehyde oxidase: Appears to play a role in the metabolism of many different compounds.

Health Conditions Related to Genetic Changes

Hereditary xanthinuria

At least four mutations in the MOCOS gene cause hereditary xanthinuria type II. These mutations typically alter the shape and function of molybdenum cofactor sulfurase, preventing it from adding sulfur to the molybdenum cofactor.

  • Enzyme Inactivation: Without the sulfur addition, xanthine dehydrogenase and aldehyde oxidase remain inactive.
  • Xanthine Accumulation: The loss of xanthine dehydrogenase activity prevents the conversion of xanthine to uric acid.
  • Clinical Impact: Excess xanthine can form crystals in the kidneys, potentially leading to stones, impaired kidney function, or kidney failure. In some cases, crystals accumulate in muscles, causing pain and cramping. Some individuals with this condition remain asymptomatic.

Other Names for This Gene

  • FLJ20733
  • HMCS
  • MCS
  • MOS

Genomic Location

The MOCOS gene is found on chromosome 18.

Frequently Asked Questions

Management focuses on keeping urinary xanthine concentrations below the precipitation threshold. Patients are advised to maintain high, consistent fluid intake day and night to produce dilute urine. A low-purine diet restricting red meat, organ meats, seafood, and yeast extracts reduces xanthine generation. Unlike uric acid stones, xanthine does not become more soluble in alkaline urine, so urinary alkalinization agents like potassium citrate are generally ineffective.
While many individuals remain asymptomatic, excess insoluble xanthine can precipitate outside the urinary tract. In severe or chronic cases, xanthine crystals deposit directly into skeletal muscle tissue. This causes microtrauma, localized inflammation, exertional muscle pain (myalgia), and persistent cramping, which often requires evaluation to differentiate from primary neuromuscular disorders.
Diagnosis is suspected when routine blood tests demonstrate profoundly low serum uric acid concentrations alongside normal or elevated urinary purine excretion. Xanthine stones are radiolucent, meaning they do not appear on standard X-rays; renal ultrasound or non-contrast abdominal CT scans are necessary to visualize them. Differentiating type II from type I requires either genetic sequencing of the MOCOS gene or an allopurinol loading test to evaluate aldehyde oxidase activity.
No, allopurinol and febuxostat are xanthine oxidase inhibitors and are contraindicated. In MOCOS deficiency, the xanthine dehydrogenase enzyme is already non-functional, causing high xanthine accumulation. Administering xanthine oxidase inhibitors provides no benefit and can worsen metabolic balance or trigger idiosyncratic toxicities without reducing stone formation.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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