Original text - English - Naturally occurring (xanthine oxidase deficiency)...

Naturally occurring (xanthine oxidase deficiency) or drug-induced (allopurinol) impairment of xanthine oxidase ultimately results in hyperxanthinemia and xanthinuria.
We have observed naturally occurring xanthinuria more commonly in cats than dogs. Although the precise underlying abnormality in cats has not been determined, a familial or congenital defect in xanthine oxidase activity is likely. To date, we have not identified a breed predisposition.