Mammalian aldehyde oxidases (AOXs) are molybdo-flavoenzymes which are present in many tissues in various mammalian species, including humans and rodents. Different species contain a different number of AOX isoforms. So far, the physiological function of AOX for mammals is unknown. However, human AOX1 is of increasing pharmacological interest due to its recognized role in phase I drug metabolism. In particular, the reasons why mammals other than humans express a multiplicity of tissue-specific AOX enzymes is unknown. The problem can be approached by defining the physiological function of human AOX1 by comparing its active site with the isoforms from other organisms. We established a codon-optimized heterologous expression system for human AOX1 in Escherichia coli. This expression system now enables us to obtain sufficient amounts of active protein for kinetic characterizations and sets the basis for site-directed mutagenesis and structure-function studies. Our studies are particularly important to define the physiopathological functions of AOX and will help to understand the drug metabolizing role of AOX for humans in general.