XDH, or Xanthine dehydrogenase, is an enzyme that plays a key role in purine metabolism, which involves the breakdown and recycling of purine nucleotides like adenine and guanine. XDH catalyzes the conversion of hypoxanthine to xanthine and then xanthine to uric acid, the final product of purine degradation in humans. This process is essential for removing excess purines from the body, helping regulate levels derived from both diet and cellular turnover.