XDH, or Xanthine dehydrogenase, is an enzyme that plays a vital 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 subsequently xanthine to uric acid, the final product of purine degradation in humans. This process is crucial for eliminating excess purines from the body, aiding in the regulation of levels derived from both diet and cellular turnover.