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