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PubMed This is a summary of 20 peer-reviewed journal articles Updated
Genetics

The Biology of Xanthinuria: Type I vs Type II

At a Glance

Hereditary xanthinuria is a genetic condition that prevents the body from converting xanthine into uric acid. Type I involves an XDH gene mutation, while Type II involves a MOCOS gene mutation that also affects how the body breaks down certain medications, making genetic testing vital for safety.

To understand hereditary xanthinuria, it helps to think of your body as a factory that processes waste. One of the main waste materials your body handles is called purines, which are found in many foods and are naturally produced when cells break down [1].

In a typical factory, purines are sent through an assembly line where they are turned into uric acid. This uric acid is then safely shipped out of the body through the kidneys [2]. In hereditary xanthinuria, a specific worker on this assembly line is missing or cannot do their job, causing the raw material (xanthine) to pile up and the final product (uric acid) to disappear [3][2].

The Role of Enzymes as Biological Workers

Your body uses specialized proteins called enzymes to perform chemical reactions. You can think of enzymes as the workers on the purine assembly line. The most important worker in this story is Xanthine Dehydrogenase (XDH) [4].

XDH has one main job: converting xanthine into uric acid [5][6]. When this worker is absent or inactive, uric acid levels in your blood and urine drop to nearly zero, while xanthine levels skyrocket [2][7].

Type I vs. Type II: What’s the Difference?

While the result (low uric acid and high xanthine) is the same, the reason for the factory breakdown depends on which gene is affected. The clinical presentation and day-to-day dietary management are largely identical for both types, but understanding the genetic difference is important for medication safety.

  • Type I Xanthinuria: This is caused by mutations in the XDH gene [2]. In this type, only the XDH enzyme is missing [8]. The instruction manual for the XDH worker is broken.
  • Type II Xanthinuria: This is caused by mutations in the MOCOS gene [9]. In this type, both XDH and another enzyme called Aldehyde Oxidase (AO) are missing [2]. The MOCOS gene provides the ‘tools’ to both workers, so a defect here knocks both of them out [10].

Aldehyde Oxidase (AO) is another worker that doesn’t help with purines but is very important for breaking down certain medications [11][12]. Because Type II patients lack both workers, they may be much more sensitive to specific drugs, such as certain chemotherapy agents or sleep medications [13][9].

Similar-Looking Conditions

Because low uric acid is the main clue for xanthinuria, it can sometimes be confused with other conditions:

  • Molybdenum Cofactor Deficiency (MoCD): This is a much more severe condition. In MoCD, a third worker called Sulfite Oxidase is also missing, which leads to serious neurological issues early in life [14][15]. If you are a healthy adult with low uric acid, you almost certainly do not have MoCD, but your doctor may check your sulfite levels just to be sure [16].
  • Renal Hypouricemia: In this condition, the assembly line works fine, but the loading dock (the kidneys) is leaky. The body makes uric acid, but the kidneys can’t hold onto it [17]. Unlike xanthinuria, these patients have normal or low xanthine levels [18].
  • Medication Effects: Drugs like allopurinol, used to treat gout, intentionally shut down the XDH worker [19]. This creates a state that looks exactly like hereditary xanthinuria on a blood test [17].

Knowing your genetic type is vital for medication safety [9]. Because your body lacks the AO worker in Type II, common doses of certain drugs can become toxic [13]. Genetic testing is the most accurate way to tell these types apart [20].

Common questions in this guide

What is the exact difference between Type I and Type II xanthinuria?
Type I is caused by an XDH gene mutation that only affects the XDH enzyme. Type II is caused by a MOCOS gene mutation that disables both the XDH enzyme and the Aldehyde Oxidase (AO) enzyme. While symptoms are similar, the lack of AO in Type II affects how the body processes certain medications.
Why does it matter which type of xanthinuria I have?
Knowing your specific type is critical for medication safety. People with Type II xanthinuria lack the Aldehyde Oxidase enzyme, which means common doses of certain chemotherapy drugs or sleep medications can build up in the body and become dangerously toxic.
Why are my uric acid levels so low with this condition?
In xanthinuria, your body is missing the specific enzyme worker needed to convert a waste product called xanthine into uric acid. Because the conversion stops halfway, your xanthine levels build up while your blood and urine uric acid levels drop to almost zero.
How can my doctor find out which type of xanthinuria I have?
Genetic testing is the most accurate method to distinguish between Type I and Type II xanthinuria. By checking for mutations in the XDH and MOCOS genes, your doctor can pinpoint your exact diagnosis and ensure your medication plan is safe.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.Do I have Type I or Type II xanthinuria, and has my MOCOS gene been tested?
  2. 2.Can we confirm my 'Sulfite Oxidase' function is normal to make sure I don't have the more severe MoCD condition?
  3. 3.How often should we monitor my xanthine levels to ensure my stone risk is under control?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (20)
  1. 1

    Multiple variants in XDH and MOCOS underlie xanthine urolithiasis in dogs.

    Tate NM, Minor KM, Lulich JP, et al.

    Molecular genetics and metabolism reports 2021; (29()):100792 doi:10.1016/j.ymgmr.2021.100792.

    PMID: 34584846
  2. 2

    A case of xanthinuria type I with a novel mutation in xanthine dehydrogenase.

    Iguchi A, Sato T, Yamazaki M, et al.

    CEN case reports 2016; (5(2)):158-162 doi:10.1007/s13730-016-0216-3.

    PMID: 28508967
  3. 3

    Potential Opportunities for Pharmacogenetic-Based Therapeutic Exploitation of Xanthine Dehydrogenase in Cardiovascular Disease.

    Massimo G, Dyson N, Olotu F, et al.

    Antioxidants (Basel, Switzerland) 2024; (13(12)) doi:10.3390/antiox13121439.

    PMID: 39765766
  4. 4

    Hepatic Accumulation of Hypoxanthine: A Link Between Hyperuricemia and Nonalcoholic Fatty Liver Disease.

    Toledo-Ibelles P, Gutiérrez-Vidal R, Calixto-Tlacomulco S, et al.

    Archives of medical research 2021; (52(7)):692-702 doi:10.1016/j.arcmed.2021.04.005.

    PMID: 33966916
  5. 5

    XDH-1 inactivation causes xanthine stone formation in Caenorhabditis elegans which is inhibited by SULP-4-mediated anion exchange in the excretory cell.

    Snoozy J, Bhattacharya S, Johnson B, et al.

    PLoS biology 2025; (23(9)):e3003410 doi:10.1371/journal.pbio.3003410.

    PMID: 40991662
  6. 6

    Functional Food Potential of Chrysanthemum morifolium, Perilla frutescens, and Sophora japonica in Managing Hyperuricemia through Dual Enzyme Inhibition.

    Chen KL, Xie DD, Luo MP, et al.

    Journal of agricultural and food chemistry 2024; (72(46)):25879-25894 doi:10.1021/acs.jafc.4c05845.

    PMID: 39526475
  7. 7

    Identification of a new mutation in the human xanthine dehydrogenase responsible for xanthinuria type I.

    Collazo Abal C, Romero Santos S, González Mao C, et al.

    Advances in laboratory medicine 2021; (2(4)):567-574 doi:10.1515/almed-2021-0018.

    PMID: 37360896
  8. 8

    Kidney Failure Secondary to Hereditary Xanthinuria due to a Homozygous Deletion of the XDH Gene in the Absence of Overt Kidney Stone Disease.

    Gonçalves PL, Diniz H, Tavares I, et al.

    Nephron 2024; (148(8)):578-583 doi:10.1159/000536248.

    PMID: 38527446
  9. 9

    Clinical and genetic analysis of MOCOS gene-related hypouricemia.

    Peng H, Tu P, Ma Q, et al.

    Frontiers in genetics 2025; (16()):1636032 doi:10.3389/fgene.2025.1636032.

    PMID: 41164817
  10. 10

    OsABA3 is Crucial for Plant Survival and Resistance to Multiple Stresses in Rice.

    Ni H, Wu W, Yan Y, et al.

    Rice (New York, N.Y.) 2024; (17(1)):46 doi:10.1186/s12284-024-00724-w.

    PMID: 39083143
  11. 11

    Non-cytochrome P450 enzyme aldehyde oxidase is involved in the oxidative metabolic pathway of diquat and its detoxification effect.

    Mao Z, Yu Y, Ba G, et al.

    Pesticide biochemistry and physiology 2024; (199()):105805 doi:10.1016/j.pestbp.2024.105805.

    PMID: 38458670
  12. 12

    Molybdenum.

    Novotny JA, Peterson CA

    Advances in nutrition (Bethesda, Md.) 2018; (9(3)):272-273 doi:10.1093/advances/nmx001.

    PMID: 29767695
  13. 13

    Thiopurine-induced toxicity is associated with dysfunction variant of the human molybdenum cofactor sulfurase gene (xanthinuria type II).

    Stiburkova B, Pavelcova K, Petru L, Krijt J

    Toxicology and applied pharmacology 2018; (353()):102-108 doi:10.1016/j.taap.2018.06.015.

    PMID: 29935280
  14. 14

    Early postnatal hepatocyte transplantation in a child with molybdenum cofactor deficiency type B.

    Selvanathan A, Kankananarachchi I, Bansal S, et al.

    Molecular genetics and metabolism 2025; (145(1)):109079 doi:10.1016/j.ymgme.2025.109079.

    PMID: 40121797
  15. 15

    Molybdenum - a scoping review for Nordic Nutrition Recommendations 2023.

    Oskarsson A, Kippler M

    Food & nutrition research 2023; (67()) doi:10.29219/fnr.v67.10326.

    PMID: 38187804
  16. 16

    A prevalent MOCS2 variant in the Roma population is associated with a novel mild form of molybdenum cofactor deficiency.

    Cho SK, Schwarz G, Tasic V, et al.

    European journal of pediatrics 2025; (184(8)):499 doi:10.1007/s00431-025-06335-x.

    PMID: 40707723
  17. 17

    Xanthinuria type I with a novel mutation of xanthine dehydrogenase.

    Tanaka K, Kanazawa I, Yamasaki H, et al.

    The American journal of the medical sciences 2015; (350(2)):155-6 doi:10.1097/MAJ.0000000000000498.

    PMID: 26110747
  18. 18

    Hereditary xanthinuria is not so rare disorder of purine metabolism.

    Sebesta I, Stiburkova B, Krijt J

    Nucleosides, nucleotides & nucleic acids 2018; (37(6)):324-328 doi:10.1080/15257770.2018.1460478.

    PMID: 29723117
  19. 19

    Drug-Induced Hypouricemia.

    Ben Salem C, Agrebi M, Sahnoun D, et al.

    Drug safety 2025; (48(2)):129-142 doi:10.1007/s40264-024-01485-7.

    PMID: 39289314
  20. 20

    Urolithiasis due to Hereditary Xanthinuria Type II: A Long-term Follow-up report.

    Kubihal S, Goyal A, Singla R, Khadgawat R

    Indian pediatrics 2020; (57(5)):468-469.

    PMID: 32444521

This page explains the biological and genetic mechanisms of hereditary xanthinuria for educational purposes only. It does not replace professional medical advice. Always consult your healthcare provider or genetic counselor regarding your diagnosis and medication safety.

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