Assays of d-Amino Acid Oxidases

d -Amino acid oxidase and d -aspartate oxidase are two well-known FAD-containing flavooxidases that catalyze the same reaction (the oxidative deamination) on different d -amino acids. d -aspartate oxidase is specific for acidic d -amino acids (i.e., d -aspartate and d -glutamate) and d -amino acid oxidase is active on neutral and polar d -amino acids (a low activity is also detected on basic d -amino acids). The assay of these flavoenzymes is of utmost importance in different fields because d -amino acids are common constituents of bacterial cell walls, are present in foods and because free d -serine and d -aspartic acid were identified in brain and peripheral tissues of mammals. In this chapter, we report on the most used methods employed to assay the activity of d -amino acid oxidase and d -aspartate oxidase. Interestingly, their activity can be followed using different assays, namely d -amino acid or oxygen consumption, α-keto acid or ammonia production, or using artificial dyes as final indicator of the flavin redox reaction.