XDH and major depressive disorder: It has been found that increased activities of adenosine deaminase (ADA) and xanthine oxidase (XO), as well as elevated level of xanthine and hypoxanthine correlate with a higher risk of depression.198 This evidence indicates that enzymes and metabolites involved in purine metabolism, together with purinoceptors, profoundly influence the pathophysiological function of nervous system.