XDH, or Xanthine dehydrogenase, is an enzyme that plays a crucial role in purine metabolism, which involves the breakdown and recycling of purine nucleotides such as adenine and guanine. XDH catalyses 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 eliminating excess purines from the body, helping to regulate levels derived from both diet and cellular turnover.