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Thiopurine metabolism. Both mercaptopurine and thioguanine are methylated by thiopurine methyltransferase (TPMT), as is mercaptopurine nucleotide (thioinosine monophosphate). Nucleotide formation is catalyzed by hypoxanthine phosphoribosyltransferase (HPRT). Oxidation is catalyzed by xanthine oxidase (XO). Thioguanine requires deamination by guanase before catabolic oxidation. The thioguanine nucleotides (TGNs) are the mono-, di- and tri- phosphates of thioguanosine
