The function of DAO (D-amino acid oxidase, Ensembl gene identifier ENSG00000110887) is as follows. Catalyzes the oxidative deamination of D-amino acids with broad substrate specificity (PubMed:16616139, PubMed:17088322, PubMed:17303072, PubMed:18544534, PubMed:20368421, PubMed:20567862, PubMed:20603179, PubMed:22203986, PubMed:23219954, PubMed:23391306, PubMed:25030849, PubMed:25701391, PubMed:29274788, PubMed:29326945, PubMed:30938755, PubMed:31799256, PubMed:32730563, PubMed:33484270, PubMed:34041270, PubMed:37558109, PubMed:38035964). Required to catabolize D-amino acids synthesized endogenously, of gastrointestinal bacterial origin or obtained from the diet, and to use these as nutrients (By similarity). Regulates the level of D-amino acid neurotransmitters in the brain, such as D-serine, a co-agonist of N- methyl D-aspartate (NMDA) receptors, and may modulate synaptic transmission (PubMed:17303072). Catalyzes the first step of the racemization of D-DOPA to L-DOPA, for possible use in an alternative dopamine biosynthesis pathway (PubMed:17303072). Also catalyzes the first step of the chiral inversion of N(gamma)-nitro-D-arginine methyl ester (D-NNA) to its L-enantiomer L-NNA that acts as a nitric oxide synthase inhibitor (By similarity). The hydrogen peroxide produced in the reaction provides protection against microbial infection; it contributes to the oxidative killing activity of phagocytic leukocytes and protects against bacterial colonization of the small intestine (By similarity). Enzyme secreted into the lumen of the intestine may not be catalytically active and could instead be proteolytically cleaved into peptides with antimicrobial activity (By similarity). The hydrogen peroxide produced in the reaction may also play a role in promoting cellular senescence in response to DNA damage (PubMed:30659069). Could act as a detoxifying agent which removes D-amino acids accumulated during aging (PubMed:17303072). {ECO:0000250|UniProtKB:P18894, ECO:0000269|PubMed:16616139, ECO:0000269|PubMed:17088322, ECO:0000269|PubMed:17303072, ECO:0000269|PubMed:18544534, ECO:0000269|PubMed:20368421, ECO:0000269|PubMed:20567862, ECO:0000269|PubMed:20603179, ECO:0000269|PubMed:22203986, ECO:0000269|PubMed:23219954, ECO:0000269|PubMed:23391306, ECO:0000269|PubMed:25030849, ECO:0000269|PubMed:25701391, ECO:0000269|PubMed:29274788, ECO:0000269|PubMed:29326945, ECO:0000269|PubMed:30659069, ECO:0000269|PubMed:30938755, ECO:0000269|PubMed:31799256, ECO:0000269|PubMed:32730563, ECO:0000269|PubMed:33484270, ECO:0000269|PubMed:34041270, ECO:0000269|PubMed:37558109, ECO:0000269|PubMed:38035964}.