Enzyme-catalyzed side reactions with molecular oxygen may contribute to cell signaling and neurodegenerative diseases

A link between neurodegeneration and well-characterized enzymatic and non-enzymatic reactions that produce reactive oxygen species (ROS) from O-2 is well established. Several enzymes that contain pyridoxal 5'-phosphate (PLP) or thiamine diphosphate (ThDP) catalyze side reactions (paracatalytic reactions) in the presence of ambient O-2. These side reactions produce oxidants such as hydrogen peroxide [H2O2] or extremely reactive peracids [RC(O)OOH]. We hypothesize that although these enzymes normally produce oxidants at low or undetectable levels, changes in substrate levels or disease-induced structural alterations may enhance interactions with O-2, thereby generating higher levels of reactive oxidants. These oxidants may damage the enzymes producing them, alter nearby macromolecules and/or destroy important metabolites/coenzymes. We propose that paracatalytic reactions with O-2 catalyzed by PLP-dependent decarboxylases and by ThDP-dependent enzymes within the alpha-keto acid dehydrogenase complexes may contribute to normal cellular signaling and to cellular damage in neurodegenerative diseases.