Biochemical Properties of Human D-Amino Acid Oxidase

Abstract

D-amino acid oxidase catalyzes the oxidative deamination of D-amino acids. In the brain, the NMDA receptor coagonist D-serine has been proposed as its physiological substrate. In order to shed light on the mechanisms regulating D-serine concentration at the cellular level, we biochemically characterized human DAAO (hDAAO) in greater depth. In addition to clarify the physical-chemical properties of the enzyme, we demonstrated that divalent ions and nucleotides do not affect flavoenzyme function. Moreover, the definition of hDAAO substrate specificity demonstrated that D-cysteine is the best substrate, which made it possible to propose it as a putative physiological substrate in selected tissues. Indeed, the flavoenzyme shows a preference for hydrophobic amino acids, some of which are molecules relevant in neurotransmission, i.e., D-kynurenine, D-DOPA, and D-tryptophan. hDAAO shows a very low affinity for the flavin cofactor. The apoprotein form exists in solution in equilibrium between two alternative conformations: the one at higher affinity for FAD is favored in the presence of an active site ligand. This may represent a mechanism to finely modulate hDAAO activity by substrate/inhibitor presence. Taken together, the peculiar properties of hDAAO seem to have evolved in order to use this flavoenzyme in different tissues to meet different physiological needs related to D-amino acids.

Development of a cognitive function marker based on D-amino acid proportions using new chiral tandem LC-MS/MS systems

“…Moreover, we anticipate that the proportion of D-Pro may become a new brain health index in connection with cognitive function. Furthermore, with regard to chiral metabolism, D-Pro is decomposed and discharged by D-amino acid oxidase (DAO, EC 1.4.3.3) 34 . DAO catalyses the stereospecific oxidative deamination of the D-form of neutral amino acids, generating neurotoxic metabolites.…”

Advances in D-Amino Acids in Neurological Research

“…Free D-serine is primarily localized in the mammalian forebrain, along with the highest concentrations of NMDA receptor [ 104 , 105 , 106 ]. D-Serine is created by serine racemase by converting L-serine into D-serine, which it is metabolized by DAAO [ 42 , 43 , 83 ]. D-Serine has a beneficial role in the brain and recent studies have broadened this story by showing how D-serine levels are regulated to protect the brain from damage or become dysregulated, causing disease [ 107 , 108 , 109 ].…”

“…There has been recent work to show that D-cysteine promotes the dendritic development of cerebellar Purkinje cells by a H2S mechanism [ 147 ]. Furthermore, it has been recently shown that human DAAO shows the greatest affinity for D-cysteine in spite of D-serine being expressed in the brain at much higher levels [ 42 ]. There has also been some dissenting work in this field, which hints at a more complicated story.…”

“…Neurologically active D-amino acids are tightly regulated within the brain by the D-amino acid oxidase (DAAO) and D-aspartate oxidase (DDO) [ 1 , 35 ]. These two oxidases catalyze the oxidative deamination of their respective substrates and allow them to be excreted by the kidneys [ 35 , 42 ]. Most importantly D-Serine and D-Alanine are metabolized by human DAAO, while D-aspartate and D-glutamate are metabolized by DDO [ 42 , 92 ].…”

“…These two oxidases catalyze the oxidative deamination of their respective substrates and allow them to be excreted by the kidneys [ 35 , 42 ]. Most importantly D-Serine and D-Alanine are metabolized by human DAAO, while D-aspartate and D-glutamate are metabolized by DDO [ 42 , 92 ]. As all four of these amino acids modulate the NMDA receptor, there has been significant interest in their regulation, which will be discussed in detail later in this review.…”