In Vitro Study of 6-mercaptopurine Oxidation Catalysed by Aldehyde Oxidase and Xanthine Oxidase

In spite of over 40 years of clinical use of 6-mercaptopurine, many aspects of complex pharmacology and metabolism of this drug remain unclear. It is thought that 6-mercaptopurine is oxidized to 6-thiouric acid through 6-thioxanthine or 8-oxo-6-mercaptopurine by one of two molybdenum hydroxylases, xanthine oxidase (XO), however, the role of other molybdenum hydroxylase, aldehyde oxidase (AO), in the oxidation of 6-mercaptopurine and possible interactions of AO substrates and inhibitors has not been investigated in more details. In the present study, the role of AO and XO in the oxidation of 6- mercaptopurine has been investigated. 6-Mercaptopurine was incubated with bovine milk xanthine oxidase or partially purified guinea pig liver molybdenum hydroxylase fractions in the absence and presence of XO and AO inhibitor/substrates, and the reactions were monitored by spectrophotometric and HPLC methods. According to the results obtained from the inhibition sudies, it is more likely that 6-mercaptopurine is oxidized to 6-thiouric acid via6-thioxanthine rather than 8-oxo-6-mercaptopurine. The first step which is the rate limiting step is catalized solely by XO, whereas both XO and AO are involved in the oxidation of 6-thioxanthine to 6-thiouric acid.