Purine excretion describes the series of reactions used to excrete the free purine bases, hypoxanthine and guanine. Either of these bases is then converted into Xanthine, which is then converted to Uric Acid by Xanthine Oxidase. The enzyme Xanthine Oxidase can be inhibited by the drugs Allopurinol and Febuxostat. Lastly, uric acid clearance plays a major role in purine excretion, and clearance is enhanced by the drugs Probenecid and Rasburicase. In contrast, aspirin blocks clearance of uric acid.