Expression Of D-amino Acid Oxidase Research Articles

Investigating D-Amino Acid Oxidase Expression and Interaction Network Analyses in Pathways Associated With Cellular Stress: Implications in the Biology of Aging.

D-amino acid oxidase (DAO) is a flavoenzyme that metabolizes D-amino acids by oxidative deamination, producing hydrogen peroxide (H2O2) as a by-product. The generation of intracellular H2O2 may alter the redox-homeostasis mechanism of cells and increase the oxidative stress levels in tissues, associated with the pathogenesis of age-related diseases and organ decline. This study investigates the effect of DAO knockdown using clustered regularly interspaced short palindromic repeats (CRISPR) through an in silico approach on its protein-protein interactions (PPIs) and their potential roles in the process of aging. The target sequence and guide RNA of DAO were designed using the CCTop database, PPI analysis using the Search Tool for the Retrieval of Interacting Genes/Proteins (STRING) database, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses, Reactome biological pathway, protein docking using GalaxyTongDock database, and structure analysis. The translated target sequence of DAO lies between amino acids 43 to 50. The 10 proteins that were predicted to interact with DAO are involved in peroxisome pathways such as acyl-coenzyme A oxidase 1 (ACOX1), alanine-glyoxylate and serine-pyruvate aminotransferase (AGXT), catalase (CAT), carnitine O-acetyltransferase (CRAT), glyceronephosphate O-acyltransferase (GNPAT), hydroxyacid oxidase 1 (HAO1), hydroxyacid oxidase 2 (HAO2), trans-L-3-hydroxyproline dehydratase (L3HYPDH), polyamine oxidase (PAOX), and pipecolic acid and sarcosine oxidase (PIPOX). In summary, DAO mutation would most likely reduce activity with its interacting proteins that generate H2O2. However, DAO mutation may result in peroxisomal disorders, and thus, alternative techniques should be considered for an in vivo approach.

Pre-optimization and one-step preparation of cascade enzymes system with broad substrates by model guidance: Application of chiral L-norvaline and L-phenylglycine biosynthesis

Cascade biocatalyst systems with catalytic promiscuity can be used for synthesis of a class of chiral chemicals but the optimization of these systems by model guidance is poorly explored. In this study, a cascade system with broad substrate spectrum was characterized and simulated by kinetic model with substrates of DL-Norvaline (DL-Nor) and DL-Phenylglycine (DL-Phg) as examples. To evaluate the optimal cascade system, maximum accumulation of intermediate products and conversion rate in the process were investigated by simultaneous solution of the rate equations for varying enzyme quantities. According to the simulation results, the cascade system was optimized by regulating the expression of D-amino acid oxidase and formate dehydrogenase and was prepared by one-step. The conversion efficiency of DL-Nor and DL-Phg have been significantly improved compared with that of before optimization. Moreover, the total of L-Nor and L-Phg were reached 498.2 mM and 79.5 mM through a gradient fed-batch conversion strategy, respectively.

Oxygen-consumption based quantification of chemogenetic H2O2 production in live human cells.

Reactive Oxygen Species (ROS) in the form of H2O2 can act both as physiological signaling molecules as well as damaging agents, depending on their concentration and localization. The downstream biological effects of H2O2 were often studied making use of exogenously added H2O2, generally as a bolus and at supraphysiological levels. But this does not mimic the continuous, low levels of intracellular H2O2 production by for instance mitochondrial respiration. The enzyme d-Amino Acid Oxidase (DAAO) catalyzes H2O2 formation using d-amino acids, which are absent from culture media, as a substrate. Ectopic expression of DAAO has recently been used in several studies to produce inducible and titratable intracellular H2O2. However, a method to directly quantify the amount of H2O2 produced by DAAO has been lacking, making it difficult to assess whether observed phenotypes are the result of physiological or artificially high levels of H2O2. Here we describe a simple assay to directly quantify DAAO activity by measuring the oxygen consumed during H2O2 production. The oxygen consumption rate (OCR) of DAAO can directly be compared to the basal mitochondrial respiration in the same assay, allowing to estimate whether the ensuing level of H2O2 production is within the range of physiological mitochondrial ROS production. In the tested monoclonal RPE1-hTert cells, addition of 5 mM D-Ala to the culture media amounts to a DAAO-dependent OCR that surpasses ∼5% of the OCR that stems from basal mitochondrial respiration and hence produces supra-physiological levels of H2O2. We show that the assay can also be used to select clones that express differentially localized DAAO with the same absolute level of H2O2 production to be able to discriminate the effects of H2O2 production at different subcellular locations from differences in total oxidative burden. This method therefore greatly improves the interpretation and applicability of DAAO-based models, thereby moving the redox biology field forward.

P-159 Impact of the symbiotic metabolites D-amino acids on preimplantation development

Abstract Study question Are the metabolites derived from the symbiotic microbiota of reproductive tract involved in preimplantation development? Summary answer The metabolites derived from the symbiotic microbiota of reproductive tract, D-amino acid（D-AA）, contributed to preimplantation development. What is known already It has been known that Lactobacillus exist in the vagina as symbiotic bacteria. Though it was thought that Lactobacillus keep the pH environment acidic and prevent infection by pathogenic bacteria, thereby keeping the uterus and fallopian tubes sterile in mammal, we have identified Staphylococcus aureus in oviductal lavage fluid from female mice and confirmed that the antibiotics disrupt the implantation pattern. In human, the vaginal microbiota is reported to be dominated by Lactobacillus, which produces D-amino acids (D-AA). These findings suggest the involvement of bacterial metabolites D-AA in the establishment of human pregnancy. Study design, size, duration 23 women undergoing in vitro fertilization (IVF) treatment at Keio University Hospital from 2019 to 2021 were assessed for eligibility. 3 were excluded due to spontaneous pregnancy or infectious disease. Twenty subjects were allocated to the successful pregnancy group (S-group (8); mean ± standard deviation 35.5 ± 2.8 years) defined as gestational sac detected on ultrasound at 5 weeks' gestation; and to the unsuccessful pregnancy group (U- group (12); 35.5 ± 2.7 years)). Participants/materials, setting, methods Genomic DNA (gDNA) extracted from vaginal and intrauterine samples at each menstrual cycle was used for metagenome analysis at the genus and the species levels by next-generation sequencing, and by quantitative real-time PCR. Amino acids were analyzed using a two-dimensional HPLC. Immunocytochemical staining by a polyclonal antibody to D-AA oxidase (DAO), which catabolizes D-AA, was performed using B6D2F1 mice and mice with a natural point-mutation G181R in DAO gene, which abolishes DAO activity. Main results and the role of chance Both vaginal/uterine bacterial counts were higher in the luteal phase than in the follicular phase. Vaginal bacterial counts in the S-group were lower than those in the U- group. The composition of microbiota of the vaginal samples (N = 23) shifted from the follicular to the luteal phase, with Lactobacillus spp. predominating. However, there was no significant difference in the ratio of Lactobacillus spp. between the S- and U-groups. The intrauterine samples of the S-group contained a high abundance of L. crispatus, L. gasseri, and L. ultunensis, while the U-group contained a high abundance of L. jensenii. Next, we analyzed the Lactobacillus metabolite, D-AA, and detected D-Ser, D-Ala, D-Asn, and D-Pro in the vaginal, uterine, and follicular fluid of the S-group. Follicular fluid that developed into blastocysts contained more D-AA than that of arrested embryos (N = 2). To investigate the significance of D-AA in the preimplantation development, mouse embryos were cultured in amino acid-depleted medium. All of them arrested (0.0%: 0/78), whereas 41% (25/61) of the embryos with D-AA added to the amino acid-depleted medium developed into blastocysts. We confirmed the expression of DAO in oocytes and blastocysts. DAO-null mice had fewer births than wild type. Limitations, reasons for caution The main limitation is the differences of symbiotic microbiota in reproductive tract and embryonic development between mice and humans; it may be difficult to apply the results of this analysis to humans. Wider implications of the findings Our results display a correlation between species-level microbiome profiling and IVF outcomes. In addition, microbiota-derived metabolites D-AA are suggested to be crucial mediators of embryo development. These findings could give rise to the development of new diagnostic techniques and drug. Trial registration number Not applicable

Involvement of DAAO Overexpression in Delayed Hippocampal Neuronal Death.

D-amino acid oxidase (DAAO) is a flavoenzyme that specifically catalyzes the deamination of many neutral and basic D-amino acids. This study aims to explore the pathological increment of hippocampal DAAO and its potential relationship with delayed hippocampal neuronal death. Ischemia-reperfusion was induced in mice through middle cerebral artery occlusion (MCAO). Neurological deficit scores and hippocampal neuronal death were assessed in MCAO mice. Immunofluorescent staining was applied to identify activated astrocytes and evaluate DAAO expression. TUNEL and Nissl staining were utilized to identify cell apoptosis of hippocampal neurons. Hippocampal astrocytic DAAO was strikingly increased following ischemic stroke, with the greatest increase on day 5 after surgery, followed by the manifestation of neurobehavioral deficits. Astrocytic DAAO was found to be mainly expressed in the hippocampal CA2 region and linked with subsequent specific neural apoptosis. Thus, it is supposed that the activation of astrocytic DAAO in ischemic stroke might contribute to neuronal death. An intravenous, twice-daily administration of 4H-furo[3,2-b]pyrrole-5-carboxylic acid (SUN, 10 mg/kg) markedly relieved behavioral status and delayed hippocampal neuronal death by 38.0% and 41.5%, respectively, compared to the model group treated with saline. In transfected primary astrocytes, DAAO overexpression inhibits cell activity, induces cytotoxicity, and promotes hippocampal neuronal death at least partly by enhancing H2O2 levels with subsequent activation of TRP calcium channels in neurons. Our findings suggest that increased hippocampal DAAO is causally associated with the development of delayed neuronal death after MCAO onset via astrocyte-neuron interactions. Hence, targeting DAAO is a promising therapeutic strategy for the management of neurological disorders.

Targeting D-Amino Acid Oxidase (DAAO) for the Treatment of Schizophrenia: Rationale and Current Status of Research.

In the brain, D-amino acid oxidase (DAAO) is a peroxisomal flavoenzyme. Through oxidative deamination by DAAO, D-serine, the main coagonist of synaptic N-methyl-D-aspartate receptors (NMDARs), is degraded into α-keto acids and ammonia; flavin adenine dinucleotide (FAD) is simultaneously reduced to dihydroflavine-adenine dinucleotide (FADH2), which is subsequently reoxidized to FAD, with hydrogen peroxide produced as a byproduct. NMDAR hypofunction is implicated in the pathogenesis of schizophrenia. In previous studies, compared with control subjects, patients with schizophrenia had lower D-serine levels in peripheral blood and cerebrospinal fluid but higher DAAO expression and activity in the brain. Inhibiting DAAO activity and slowing D-serine degradation by using DAAO inhibitors to enhance NMDAR function may be a new strategy for use in the treatment of schizophrenia. The aim of this leading article is to review the current research in DAAO inhibitors.

Astrocytic d-amino acid oxidase degrades d-serine in the hindbrain.

d-Serine modulates excitatory neurotransmission by binding to N-methyl-d-aspartate glutamate receptors. d-Amino acid oxidase (DAO) degrades d-amino acids, such as d-serine, in the central nervous system, and is associated with neurological and psychiatric disorders. However, cell types that express brain DAO remain controversial, and whether brain DAO influences systemic d-amino acids in addition to brain d-serine remains unclear. Here, we created astrocyte-specific DAO-conditional knockout mice. Knockout in glial fibrillary acidic protein-positive cells eliminated DAO expression in the hindbrain and increased d-serine levels significantly in the cerebellum. Brain DAO did not influence levels of d-amino acids in the forebrain or periphery. These results show that astrocytic DAO regulates d-serine specifically in the hindbrain.

Free d-Amino Acids in Salivary Gland in Rat.

Simple SummaryParotid, submandibular, and sublingual glands in rat were found to contain high concentrations of d-aspartic acid and low ones of d-serine and d-alanine. In addition to d-amino acid oxidase and d-aspartate oxidase, serine racemase was also detected in all three of these major salivary glands, as were N-methyl-d-aspartic acid receptor subunits NR1 and NR2D, but not NR2A, NR2B, or NR2C.Free d-amino acids, which are enantiomers of l-amino acids, are found in mammals, including humans, and play an important role in a range of physiological functions in the central nervous system and peripheral tissues. Several d-amino acids have been observed in saliva, but their origin and the enzymes involved in their metabolism and catabolism remain to be clarified. In the present study, large amounts of d-aspartic acid and small amounts of d-serine and d-alanine were detected in all three major salivary glands in rat. No other d-enantiomers were detected. Protein expression of d-amino acid oxidase and d-aspartate oxidase, the enzymes responsible for the oxidative deamination of neutral and dicarboxylic d-amino acids, respectively, were detected in all three types of salivary gland. Furthermore, protein expression of the d-serine metabolic enzyme, serine racemase, in parotid glands amounted to approximately 40% of that observed in the cerebral cortex. The N-methyl-d-aspartic acid subunit proteins NR1 and NR2D were detected in all three major salivary glands. The results of the present study suggest that d-amino acids play a physiological role in a range of endocrine and exocrine function in salivary glands.

Development of a Combination Fermentation Strategy to Simultaneously Increase Biomass and Enzyme Activity of D-amino Acid Oxidase Expressed in Escherichia coli.

D-amino acid oxidase (DAAO) is widely used in the industrial preparation of L-amino acids, and cultivating Escherichia coli (E. coli) expressing DAAO for the biosynthesis of L-phosphinothricin (L-PPT) is very attractive. At present, the biomass production of DAAO by fermentation is still limited in large-scale industrial applications because the expression of DAAO during the fermentation process inhibits the growth of host cells, which limits higher cell density. In this study, the factors that inhibit the growth of bacterial cells during a 5-L fed-batch fermentation process were explored, and the fermentation process was optimized by co-expressing catalase (CAT), by balancing the biomass and the enzyme activity, and by adding exogenous D-alanine (D-Ala) to relieve the limitation of DAAO on the cells and optimize fermentation. Under optimal conditions, the DO-STAT feeding mode with DO controlled at 30% ± 5% and the addition of 27.5 g/L lactose mixed with 2 g/L D-Ala during induction at 28 °C resulted in the production of 26.03 g dry cell weight (DCW)/L biomass and 390.0 U/g DCW specific activity of DAAO; an increase of 78% and 84%, respectively, compared with the initial fermentation conditions. The fermentation strategy was successfully scale-up to a 5000-L fermenter.

Factors regulating serine racemase and d-amino acid oxidase expression in the mouse striatum

d-Serine plays an important role in modulating N-methyl-d-aspartate receptor (NMDAR) neurotransmission in the mammalian brain by binding to the receptor’s glycine modulatory site (GMS). The cytosolic enzyme serine racemase (SR) converts L-serine to d-serine, while the peroxisomal enzyme d-amino acid oxidase (DAAO) catalyzes the breakdown of d-serine. Although it is important to understand how the activities of SR and DAAO regulate d-serine levels, very little is known about the mechanisms that regulate the expression of SR and DAAO. In this study, we investigated whether the different centrally active drugs affect the expression of SR and DAAO in adult mouse brain. We found that the NMDAR antagonist, MK801, and cocaine, psychotropic drugs that both augment glutamate release, reduce the expression of SR and DAAO. This regulation is brain region selective, and in the case of cocaine, is reversed in part byα-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX). However, d-serine and antipsychotics do not regulate SR and DAAO protein levels. In a genetic model of SR disruption, we found that DAAO expression was unaltered in SR conditional knockout mice, in which tissue d-serine content remains fairly stable despite marked reduction in SR expression. This study reveals a new mechanism by which AMPAR activity could regulate NMDAR function via d-serine availability.

In vivo detection of d-amino acid oxidase with hyperpolarized d-[1-13 C]alanine.

d-amino acid oxidase (DAO) is a peroxisomal enzyme that catalyzes the oxidative deamination of several neutral and basic d-amino acids to their corresponding α-keto acids. In most mammalian species studied, high DAO activity is found in the kidney, liver, brain and polymorphonuclear leukocytes, and its main function is to maintain low circulating d-amino acid levels. DAO expression and activity have been associated with acute and chronic kidney diseases and with several pathologies related to N-methyl-d-aspartate (NMDA) receptor hypo/hyper-function; however, its precise role is not completely understood. In the present study we show that DAO activity can be detected in vivo in the rat kidney using hyperpolarized d-[1-13 C]alanine. Following a bolus of hyperpolarized d-alanine, accumulation of pyruvate, lactate and bicarbonate was observed only when DAO activity was not inhibited. The measured lactate-to-d-alanine ratio was comparable to the values measured when the l-enantiomer was injected. Metabolites downstream of DAO were not observed when scanning the liver and brain. The conversion of hyperpolarized d-[1-13 C]alanine to lactate and pyruvate was detected in blood ex vivo, and lactate and bicarbonate were detected on scanning the blood pool in the heart in vivo; however, the bicarbonate-to-d-alanine ratio was significantly lower compared with the kidney. These results demonstrate that the specific metabolism of the two enantiomers of hyperpolarized [1-13 C]alanine in the kidney and in the blood can be distinguished, underscoring the potential of d-[1-13 C]alanine as a probe of d-amino acid metabolism.

Evaluation of H2S-producing enzymes in cerebrospinal fluid and its relationship with interleukin-6 and neurologic deficits in subarachnoid hemorrhage

Background and purposeRecent studies have suggested that H2S may be involved in the pathophysiology of subarachnoid hemorrhage (SAH). Endogenous H2S is mainly formed by cystathionine β-synthase (CBS), d-amino-acid oxidase (DAO), and 3-mercaptopyruvate sulfotransferase (3-MST) from the substrate cysteine in the central nervous system. In this study, we assessed the expression of CBS, 3-MST, and DAO in cerebrospinal fluid (CSF) from patients with SAH and rats and the expression in the rat brain. MethodsCSF samples were collected within 48 h of aneurysm rupture in SAH patients. The CBS, DAO and 3-MST levels in CSF were measured using Western blot analyses, and correlations with the inflammatory parameter Interleukin-6 (IL-6) were assessed. Six months after SAH, the clinical outcomes were assessed. ResultsIn human CSF samples, the CBS and DAO protein levels were detected and increased after SAH. However, 3-MST was not detected in the control group CSF but increased after SAH. Strong correlations were observed between the increasing levels of CBS, DAO, and 3-MST and IL-6 2 days after SAH. Furthermore, high CBS, 3-MST and DAO levels in the CSF samples were correlated with poor outcomes at 6 months after SAH onset. We also found that the expression of CBS, DAO and 3-MST in the rat CSF and brain (parietal cortex and hippocampus) increased following SAH. We detected strong correlations between the increases in CBS, 3-MST and IL-6 in the rat CSF and brain samples. ConclusionsThese results indicate that the upregulated expression of CBS, DAO and 3-MST after SAH was closely associated with the inflammatory response and neurological deficits after SAH.

Effects of arsenic exposure on D-serine metabolism in the hippocampus of offspring mice at different developmental stages.

The main purpose of this study was to verify the hypothesis that cognitive dysfunctions induced by arsenic exposure were related to the changes of D-serine metabolism in the hippocampus of offspring mice. Mother mice and their offsprings were exposed to 0, 15, 30 or 60mg/L sodium arsenite (NaAsO2) through drinking water from the first day of gestation until the end of lactation. D-serine levels in the hippocampus of mice of postnatal day (PND) 10, 20 and 40 were examined by high-performance liquid chromatography. Expressions of serine racemase (SR), D-amino acid oxidase (DAAO), alanine-serine-cysteine transporter-1 (asc-1) and subunits of N-methyl-D-aspartate receptors (NMDARs) in the hippocampus of mice were measured by Western blot and Real-time RT-PCR. Results showed that arsenic exposure significantly decreased D-serine levels of mice exposed to 60mg/L NaAsO2. Exposure to 60mg/L NaAsO2 could inhibit both mRNA and protein expression of SR, whereas increase in the protein expression of DAAO, only enhances the mRNA levels of DAAO of PND 20 mice. In addition, arsenic exposure could upregulate protein expression of asc-1. The mRNA and protein levels of NR1, NR2A and NR2B in the hippocampus of mice were down-regulated by arsenic. Findings from this study suggested that SR might play an important role in the reduction of D-serine levels caused by arsenic exposure, which might further influence the levels of NMDAR subunits especially on PND20, and then might disturb the function of NMDARs and cause the deficits of learning and memory ability of offspring mice.

Efficient production of S-adenosyl-l-methionine from dl-methionine in metabolic engineered Saccharomyces cerevisiae.

S-Adenosyl-l-methionine (SAM) is an important small molecule compound widely used in treating various diseases. Although l-methionine is generally used, the low-cost dl-methionine is more suitable as the substrate for industrial production of SAM. However, d-methionine is inefficient for SAM formation due to the substrate-specificity of SAM synthetase. In order to increase the utilization efficiency of dl-methionine, intracellular conversion of d-methionine to l-methionine was investigated in the type strain Saccharomyces cerevisiae BY4741 and an industrial strain S. cerevisiae HDL. Firstly, via disruption of HPA3 encoding d-amino acid-N-acetyltransferase, d-methionine was accumulated in vivo and no N-acetyl-d-methionine production was observed. Further, codon-optimized d-amino acid oxidase (DAAO) gene from Trigonopsis variabilis (Genbank MK280686) and l-phenylalanine dehydrogenase gene (l-PheDH) from Rhodococcus jostii (Genbank MK280687) were introduced to convert d-methionine to l-methionine, SAM concentration and content was increased by 110% and 72.1% in BY4741 (plasmid borne) and increased by 38.2% and 34.1% in HDL (genome integrated), by feeding 0.5 g/L d-methionine. Using the recently developed CRISPR tools, the DAAO and l-PheDH expression cassettes were integrated into the HPA3 and SAH1 loci while SAM2 expression was integrated into the SPE2 and GLC3 loci of HDL, and the resultant strain HDL-R2 accumulated 289% and 192% more SAM concentration and content, respectively, by feeding 0.5 g/L dl-methionine. Further, in a 10 L fed-batch fermentation process, 10.3 g/L SAM were accumulated with the SAM content of 242 mg/g dry cell weight by feeding 16 g/L dl-methionine. The strategies used here provided a promising approach to enhance SAM production using low-cost dl-methionine.

Involvement of d-amino acid oxidase in cerebral ischaemia induced by transient occlusion of the middle cerebral artery in mice.

d-Amino acid oxidase (DAAO) is a flavine adenine dinucleotide-containing flavoenzyme and specifically catalyses oxidative deamination of d-amino acids. This study aimed to explore the association between increased cerebral DAAO expression or enzymic activity and the development of cerebral ischaemia. A mouse model of transient (90 min) middle cerebral artery occlusion (MCAO) was established, and western blotting, enzymic activity assay, and fluorescent immunostaining techniques were used. The expression and enzymic activity of DAAO increased over time in the cortical peri-infarct area of the mice subjected to transient MCAO. The DAAO was specifically expressed in astrocytes, and its double immunostaining with the astrocytic intracellular marker, glial fibrillary acidic protein, in the cortical peri-infarct area was up-regulated following ischaemic insult, with peak increase on Day 5 after MCAO. Single intravenous injection of the specific and potent DAAO inhibitor Compound SUN reduced the cerebral DAAO enzymic activity and attenuated neuronal infarction and neurobehavioural deficits with optimal improvement apparent immediately after the MCAO procedure. The neuroprotective effect was dose dependent, with ED50 values of 3.9-4.5mg·kg-1 . Intracerebroventricular injection of the DAAO gene silencer siRNA/DAAO significantly reduced cerebral DAAO expression and attenuated MCAO-induced neuronal infarction and behavioural deficits. Our results, for the first time, demonstrated that increased cerebral astrocytic DAAO expression and enzymic activity were causally associated with the development of neuronal destruction following ischaemic insults, suggesting that targeting cerebral DAAO could be a potential approach for treatment of neurological conditions following cerebral ischaemia.

Age- and gender-dependent D-amino acid oxidase activity in mouse brain and peripheral tissues: implication for aging and neurodegeneration.

D-amino acid oxidase (DAO) is a flavoenzyme, catalysing oxidative deamination of D-amino acids to produce corresponding α-keto acids, ammonia and hydrogen peroxide. In our search for DAO activity among various tissues, we developed a sensitive assay based on hydrogen peroxide production involving enzyme-coupled colorimetric assay with peroxidase. We first optimized buffer components to extract DAO protein from mouse tissues. Here we show that DAO activity was detected in kidney, cerebellum, medulla oblongata, midbrain and spinal cord, but not in liver. In addition, we observed that DAO activity and expression were decreased in thoracic and lumbar regions of spinal cord in aged mice when compared with young mice, indicating that decreased DAO is involved in motoneuron degeneration during senescence. We also found gender difference in DAO activity in the kidney, suggesting that DAO activity is influenced by sexual dimorphism. We newly detected DAO activity in the epididymis, although undetected in testis. Furthermore, DAO activity was significantly higher in the caput region than corpus and cauda regions of epididymis, indicating that D-amino acids present in the testis are eliminated in epididymis. Taken together, age- and gender-dependent DAO activity in each organ may underlie the human pathophysiology regulated by D-amino acid metabolism.

D-amino acid oxidase promotes cellular senescence via the production of reactive oxygen species.

d-amino acid oxidase (DAO) is a flavin adenine dinucleotide (FAD)-dependent oxidase metabolizing neutral and polar d-amino acids. Unlike l-amino acids, the amounts of d-amino acids in mammalian tissues are extremely low, and therefore, little has been investigated regarding the physiological role of DAO. We have recently identified DAO to be up-regulated in cellular senescence, a permanent cell cycle arrest induced by various stresses, such as persistent DNA damage and oxidative stress. Because DAO produces reactive oxygen species (ROS) as byproducts of substrate oxidation and the accumulation of ROS mediates the senescence induction, we explored the relationship between DAO and senescence. We found that inhibition of DAO impaired senescence induced by DNA damage, and ectopic expression of wild-type DAO, but not enzymatically inactive mutant, enhanced it in an ROS-dependent manner. Furthermore, addition of d-amino acids and riboflavin, a metabolic precursor of FAD, to the medium potentiated the senescence-promoting effect of DAO. These results indicate that DAO promotes senescence through the enzymatic ROS generation, and its activity is regulated by the availability of its substrate and coenzyme.

Human D-Amino Acid Oxidase: Structure, Function, and Regulation.

D-Amino acid oxidase (DAAO) is an FAD-containing flavoenzyme that catalyzes with absolute stereoselectivity the oxidative deamination of all natural D-amino acids, the only exception being the acidic ones. This flavoenzyme plays different roles during evolution and in different tissues in humans. Its three-dimensional structure is well conserved during evolution: minute changes are responsible for the functional differences between enzymes from microorganism sources and those from humans. In recent years several investigations focused on human DAAO, mainly because of its role in degrading the neuromodulator D-serine in the central nervous system. D-Serine is the main coagonist of N-methyl D-aspartate receptors, i.e., excitatory amino acid receptors critically involved in main brain functions and pathologic conditions. Human DAAO possesses a weak interaction with the FAD cofactor; thus, in vivo it should be largely present in the inactive, apoprotein form. Binding of active-site ligands and the substrate stabilizes flavin binding, thus pushing the acquisition of catalytic competence. Interestingly, the kinetic efficiency of the enzyme on D-serine is very low. Human DAAO interacts with various proteins, in this way modulating its activity, targeting, and cell stability. The known properties of human DAAO suggest that its activity must be finely tuned to fulfill a main physiological function such as the control of D-serine levels in the brain. At present, studies are focusing on the epigenetic modulation of human DAAO expression and the role of post-translational modifications on its main biochemical properties at the cellular level.

Controversial Effects of D-Amino Acid Oxidase Activator (DAOA)/G72 on D-Amino Acid Oxidase (DAO) Activity in Human Neuronal, Astrocyte and Kidney Cell Lines: The N-methyl D-aspartate (NMDA) Receptor Hypofunction Point of View.

Dysfunction of D-amino acid oxidase (DAO) and DAO activator (DAOA)/G72 genes have been linked to neuropsychiatric disorders. The glutamate hypothesis of schizophrenia has proposed that increased DAO activity leads to decreased D-serine, which subsequently may lead to N-methyl-D-aspartate (NMDA) receptor hypofunction. It has been shown that DAOA binds to DAO and increases its activity. However, there are also studies showing DAOA decreases DAO activity. Thus, the effect of DAOA on DAO is controversial. We aimed to understand the effect of DAOA on DAO activity in neuron-like (SH-SY5Y), astrocyte-like (1321N1) and kidney-like (HEK293) human cell lines. DAO activity was measured based on the release of hydrogen peroxide and its interaction with Amplex Red reagent. We found that DAOA increases DAO activity only in HEK293 cells, but has no effect on DAO activity in SH-SY5Y and 1321N1 cells. This might be because of different signaling pathways, or due to lower DAO and DAOA expression in SH-SY5Y and 1321N1 cells compared to HEK293 cells, but also due to different compartmentalization of the proteins. The lower DAO and DAOA expression in neuron-like SH-SY5Y and astrocyte-like 1321N1 cells might be due to tightly regulated expression, as previously reported in the human post-mortem brain. Our simulation experiments to demonstrate the interaction between DAOA and human DAO (hDAO) showed that hDAO holoenzyme [hDAO with flavine adenine dinucleotide (FAD)] becomes more flexible and misfolded in the presence of DAOA, whereas DAOA had no effect on hDAO apoprotein (hDAO without FAD), which indicate that DAOA inactivates hDAO holoenzyme. Furthermore, patch-clamp analysis demonstrated no effect of DAOA on NMDA receptor activity in NR1/NR2A HEK293 cells. In summary, the interaction between DAO and DAOA seems to be cell type and its biochemical characteristics dependent which still needs to be elucidated.

Efficient production of d-amino acid oxidase in Escherichia coli by a trade-off between its expression and biomass using N-terminal modification

Native d-amino acid oxidase (DAAO) that is expressed mostly as inclusion body and its toxicity for E. coli hamper efficient heterologous expression. In this study, the soluble expression of DAAO from Rhodosporidium toruloides (RtDAAO) was improved in E. coli through N-terminal modification, but the cell biomass was decreased. Then a trade-off between DAAO expression and biomass was achieved to obtain the highest volumetric activity of DAAO through regulated the number of N-terminus histidine residues. When variant 2H3G was fused with three N-terminus histidine residues, the volumetric activity was increased by 3.1 times and the biomass was not significant change compared with the wild type. Finally, the N-terminus disordered region of RtDAAO (HSQK) was replaced with HHHG and the variant enzyme activity reached 80.7U/mL (with a 40 percent of inactive DAAO reduced) in a 7.5L fermenter in 24h.