D-amino acids are present in the plants and cell walls of microorganisms and are not used in the biosynthesis of mammalian proteins. They are present in the diet and are metabolised by liver. D-amino acid is an FAD-dependent enzyme that catalyses the oxidative deamination of these unnatural isomers of amino acids. The resulting α-keto acid can enter the general pathways of amino acid metabolism. Thus, D-amino acid oxidase is important as it initiates the first step for the conversion of unnatural D-amino acids to α-amino acids in the body.