Allopurinol inhibits xanthine oxidase, the enzyme that breaks down mercaptopurine. Allopurinol is in the xanthine oxidase inhibitor family of medications. Febuxostat is a non-purine-selective inhibitor of xanthine oxidase. The active metabolite of allopurinol is oxipurinol, which is also an inhibitor of xanthine oxidase. This pathway also is independent of the enzyme activity of xanthine oxidase. Hypoxanthine is produced from adenine, xanthine from guanine, and uracil results from deamination of cytosine. It is caused by a deficiency of the enzyme xanthine oxidase. Their waste consists mostly of insoluble nitrogenous compounds, such as xanthine, guanine and uric acid. Inhibition of xanthine oxidase has been proposed as a mechanism for improving cardiovascular health. Here is a diagram highlighting the pathways catalyzed by xanthine oxidase. Guanine is deaminated to xanthine which in turn is oxidized to uric acid. Theobromine is categorized as a dimethyl xanthine. Some strains also required a combination of uracil, guanine, adenine and xanthine. Hypoxanthine is oxidized to xanthine and finally to uric acid. Xanthosine is a nucleoside derived from xanthine and ribose.