The Inactivation of Human Aldehyde Oxidase 1 by Hydrogen Peroxide and Superoxide

Mammalian aldehyde oxidases (AOX) are molybdo-flavoenzymes of pharmacological and pathophysiologic relevance that are involved in phase I drug metabolism and, as a product of their enzymatic activity, are also involved in the generation of Reactive Oxygen Species. So far, the physiologic role of aldehyde oxidase 1 in the human body remains unknown. The human Enzyme hAOX1 is characterized by a broad substrate specificity, oxidizing aromatic/aliphatic aldehydes into their corresponding carboxylic acids, and hydroxylating various heteroaromatic rings. The Enzyme uses oxygen as terminal electron acceptor to produce hydrogen peroxide and superoxide during turnover. Since hAOX1 and, in particular, some natural variants produce not only H₂O₂ but also high amounts of superoxide, we investigated the effect of both ROS molecules on the enzymatic activity of hAOX1 in more detail. We compared hAOX1 to the high-O₂ ^.--producing natural variant L438V for their time-dependent inactivation with H₂O₂/O₂ ^.- during substrate turnover. We show that the inactivation of the hAOX1 wild-type Enzyme is mainly based on the production of hydrogen peroxide, whereas for the variant L438V, both hydrogen peroxide and superoxide contribute to the time-dependent inactivation of the Enzyme during turnover. Further, the level of inactivation was revealed to be substrate-dependent: using substrates with higher turnover numbers resulted in a faster inactivation of the Enzymes. Analysis of the inactivation site of the Enzyme identified a loss of the terminal sulfido ligand at the molybdenum active site by the produced ROS during turnover. SIGNIFICANCE STATEMENT: This work characterizes the substrate-dependent inactivation of human aldehyde oxidase 1 under turnover by Reactive Oxygen Species and identifies the site of inactivation. The role of ROS in the inhibition of human aldehyde oxidase 1 will have a high impact on future studies.