Xanthine oxidase (XO) forms uric acid from xanthine. It is assumed that at the same time oxygen is reduced by the XO to reactive oxygen species (ROS), mainly to ·O2- and to H2O2. Under certain conditions such ROS can be highly damaging to cellular structures. Therefore, XO was frequently used as a model system, in which the impact of ROS on cellular compounds and structures has been investigated. In this in vitro study xanthine oxidases from buttermilk and from microorganisms were compared regarding their ability to generate ROS. It could be shown that both enzymes are able to transform xanthine to uric acid but differ significantly in their reductive properties to oxygen. XO from buttermilk reduces oxygen to both ·O2- and H2O2 whereas XO from microorganisms generates H2O2, but fails to form ·O2-. Since ·O2- are involved in maintaining transition metal-mediated formation of hydroxyl radicals (·OH) from H2O2, we conclude that XO from microorganisms is therefore largely unsuitable in studies investigating just the interaction of ·O2- with other ROS on cellular compounds.

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