XDH also known as XO belongs to the group of molybdenum-containing hydroxylases involved in the oxidative metabolism of purines. XDH plays a crucial role in purine metabolism, specifically in converting hypoxanthine to xanthine and subsequently to uric acid, a process vital for maintaining normal uric acid levels in the body. XDH can be converted to xanthine oxidase by reversible sulfhydryl oxidation or by irreversible proteolytic modification. Defects in xanthine dehydrogenase cause xanthinuria, may contribute to adult respiratory stress syndrome, and may potentiate influenza infection through an oxygen metabolite-dependent mechanism. Elevated XDH levels in serum often indicate various pathological conditions, including chronic liver disease and cholestasis, highlighting XDH's importance as a biomarker for liver dysfunction.