Xanthinuria

Definition

Xanthinuria is a rare autosomal recessive disorder associated with a deficiency in xanthine dehydrogenase (XDH - also referred to as xanthine oxidoreductase, XOR), which normally catalyses the conversion of hypoxanthine and xanthine to uric acid. In humans NAD+ is the electron acceptor and significant activity is confined to liver and intestinal mucosa. Irreversible conversion to oxidase occurs during ischaemia. The preferential accumulation/excretion of xanthine in plasma and urine results from extensive hypoxanthine recycling by the salvage pathway for which xanthine is not a substrate in humans: excess xanthine deriving from guanine via guanine deaminase. Classical xanthinuria has two types - an isolated deficiency (XDH type I), a dual deficiency with aldehyde oxidase (XDH/AOX: type II). Additionally xanthiuria occurs in Molybdenum cofactor deficiency, where sulphite oxidase (SO) is also inactive. More than 150 cases have been described from 22 countries, indicating that the disorder is not confined to specific ethnic groups. Although xanthinuria is a rare disorder the number of cases found is certainly an underestimate. Clinical symptoms in classical XDH deficiency include xanthine calculi, crystalluria, or acute renal failure and unrecognized can lead to end-stage renal disease, nephrectomy, or death. All symptoms relate to the extreme insolubility and high renal clearance of xanthine and can manifest from birth to the 80's, 50% of cases being children. Duodenal ulcers, myopathy, or arthropathy have been noted in 10%. Treatment involves high fluid intake and dietary purine restriction. Twenty percent of type 1 and 2 patients (and occasional cofactor patients), have been asymptomatic.

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