Mouse d-Amino-Acid Oxidase: Distribution and Physiological Substrates.

d-Amino-acid oxidase (DAO) catalyzes the oxidative deamination of d-amino acids. DAO is present in a wide variety of organisms and has important roles. Here, we review the distribution and physiological substrates of mouse DAO. Mouse DAO is present in the kidney, brain, and spinal cord, like DAOs in other mammals. However, in contrast to other animals, it is not present in the mouse liver. Recently, DAO has been detected in the neutrophils, retina, and small intestine in mice. To determine the physiological substrates of mouse DAO, mutant mice lacking DAO activity are helpful. As DAO has wide substrate specificity and degrades various d-amino acids, many d-amino acids accumulate in the tissues and body fluids of the mutant mice. These amino acids are d-methionine, d-alanine, d-serine, d-leucine, d-proline, d-phenylalanine, d-tyrosine, and d-citrulline. Even in wild-type mice, administration of DAO inhibitors elevates D-serine levels in the plasma and brain. Among the above d-amino acids, the main physiological substrates of mouse DAO are d-alanine and d-serine. These two d-amino acids are most abundant in the tissues and body fluids of mice. d-Alanine derives from bacteria and produces bactericidal reactive oxygen species by the action of DAO. d-Serine is synthesized by serine racemase and is present especially in the central nervous system, where it serves as a neuromodulator. DAO is responsible for the metabolism of d-serine. Since DAO has been implicated in the etiology of neuropsychiatric diseases, mouse DAO has been used as a representative model. Recent reports, however, suggest that mouse DAO is different from human DAO with respect to important properties.

D-Glutamate is metabolized in the heart mitochondria.

D-Amino acids are enantiomers of L-amino acids and have recently been recognized as biomarkers and bioactive substances in mammals, including humans. In the present study, we investigated functions of the novel mammalian mitochondrial protein 9030617O03Rik and showed decreased expression under conditions of heart failure. Genomic sequence analyses showed partial homology with a bacterial aspartate/glutamate/hydantoin racemase. Subsequent determinations of all free amino acid concentrations in 9030617O03Rik-deficient mice showed high accumulations of D-glutamate in heart tissues. This is the first time that a significant amount of D-glutamate was detected in mammalian tissue. Further analysis of D-glutamate metabolism indicated that 9030617O03Rik is a D-glutamate cyclase that converts D-glutamate to 5-oxo-D-proline. Hence, this protein is the first identified enzyme responsible for mammalian D-glutamate metabolism, as confirmed in cloning analyses. These findings suggest that D-glutamate and 5-oxo-D-proline have bioactivities in mammals through the metabolism by D-glutamate cyclase.

Interplay between microbial d-amino acids and host d-amino acid oxidase modifies murine mucosal defence and gut microbiota.

L-Amino acids are the building blocks for proteins synthesized in ribosomes in all kingdoms of life, but d-amino acids (d-aa) have important non-ribosome-based functions(1). Mammals synthesize d-Ser and d-Asp, primarily in the central nervous system, where d-Ser is critical for neurotransmission(2). Bacteria synthesize a largely distinct set of d-aa, which become integral components of the cell wall and are also released as free d-aa(3,4). However, the impact of free microbial d-aa on host physiology at the host-microbial interface has not been explored. Here, we show that the mouse intestine is rich in free d-aa that are derived from the microbiota. Furthermore, the microbiota induces production of d-amino acid oxidase (DAO) by intestinal epithelial cells, including goblet cells, which secrete the enzyme into the lumen. Oxidative deamination of intestinal d-aa by DAO, which yields the antimicrobial product H2O2, protects the mucosal surface in the small intestine from the cholera pathogen. DAO also modifies the composition of the microbiota and is associated with microbial induction of intestinal sIgA. Collectively, these results identify d-aa and DAO as previously unrecognized mediators of microbe-host interplay and homeostasis on the epithelial surface of the small intestine.

Enantioselective determination of citrulline and ornithine in the urine of d-amino acid oxidase deficient mice using a two-dimensional high-performance liquid chromatographic system.

Two-dimensional high-performance liquid chromatographic (2D-HPLC) and 2D-HPLC-mass spectrometric (2D-HPLC-MS) systems have been designed and developed for the determination of the citrulline (Cit) and ornithine (Orn) enantiomers. Several d-amino acids have already been identified as novel physiologically active molecules and biomarkers, and the enantioselective evaluation of the amounts, distributions and metabolisms of non-proteinogenic amino acids gain as well increasing interest. In the present study, highly selective analytical methods were developed using a capillary monolithic ODS column (0.53mm i.d. x 1000mm) for the reversed-phase separation of the target analytes from the matrix compounds in the first dimension, and a narrowbore-Pirkle type enantioselective column, KSAACSP-105S (1.5mm i.d. x 250mm), was used for the enantiomer separation in the second dimension. The amino acids were analyzed after pre-column derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) and detected by the fluorescence detector and MS. The systems were applied to the urine of d-amino acid oxidase (DAO) deficient B6DAO- mice and control C57BL mice to evaluate the presence and metabolism of the Cit and Orn enantiomers in mammals. As a result, all of the 4 target enantiomers (d-Cit, l-Cit, d-Orn, l-Orn) were found in the urine of both strains. The %D value of Cit (d-Cit/Cit×100) increased about 3-fold in the urine of the DAO deficient mice and that of Orn also tended to increase with the DAO deficiency. These results were definitely confirmed by a 2D-HPLC-MS detection system. Further investigations about the biological significance of these d-isomers are currently ongoing.

Chiral amino acid metabolomics for novel biomarker screening in the prognosis of chronic kidney disease.

D-Amino acids, the enantiomers of L-amino acids, are increasingly recognized as novel biomarkers. Although the amounts of D-amino acids are usually very trace in human, some of them have sporadically been detected in blood from patients with kidney diseases. This study examined whether multiple chiral amino acids would be associated with kidney functions, comorbidities, and prognosis of chronic kidney disease (CKD) by enantioselective analyses of all chiral amino acids with a micro-two-dimensional high-performance liquid chromatograph (2D-HPLC)-based analytical platform. 16 out of 21 D-amino acids were detected in plasma from 108 CKD patients in a longitudinal cohort. The levels of D-Ser, D-Pro, and D-Asn were strongly associated with kidney function (estimated glomerular filtration ratio), the levels of D-Ala and D-Pro were associated with age, and the level of D-Asp and D-Pro were associated with the presence of diabetes mellitus. D-Ser and D-Asn were significantly associated with the progression of CKD in mutually-adjusted Cox regression analyses; the risk of composite end point (developing to ESKD or death before ESKD) was elevated from 2.7- to 3.8-fold in those with higher levels of plasma D-Ser and D-Asn. These findings identified chiral amino acids as potential biomarkers in kidney diseases.

Establishment of a two-dimensional chiral HPLC system for the simultaneous detection of lactate and 3-hydroxybutyrate enantiomers in human clinical samples.

A two-dimensional chiral high-performance liquid chromatography system was established for simultaneous detection of lactate (LA) and 3-hydroxybutyrate (3HB) enantiomers in human clinical samples. d-LA is increased upon kidney damage but 3HB protected against kidney injury. Therefore, determining the concentrations of D,L-LA and D,L-3HB simultaneously would be useful for evaluating pathological conditions. LA and 3HB were pre-column-derivatized with the fluorescent reagent 4-(N-chloroformylmethyl-N-methylamino)-7-nitro-2,1,3-benzoxadiazole (NBD-COCl) at 60 °C for 15 min and separated in the first dimension with a capillary monolithic octadecylsilane column. The mobile phase consisted of 13% acetonitrile and 0.05% tirfluoroacetic acid in water. Chiralpak QD-AX and KSAACSP-001S enantioselective columns were used in the second dimension to separate LA and 3HB enantiomers, respectively. Mobile phases were mixed solutions of methanol and acetonitrile containing formic acid. The separation factors were 1.14 and 1.08, respectively. The detection limit of LA and 3HB enantiomers was 10 fmol/injection. This method was applied to human clinical samples; intra- and inter-day relative standard deviations of LA and 3HB enantiomers were, respectively, 1.04-3.25% and 1.61-5.12% in plasma, 9.19-11.2% and 4.60-5.89% in urine, and 7.12-8.90% and 2.86-6.97% in saliva. This novel analytical method is a powerful tool for investigating variations in LA and 3HB enantiomers under disease conditions.

Design and synthesis of a novel pre-column derivatization reagent with a 6-methoxy-4-quinolone moiety for fluorescence and tandem mass spectrometric detection and its application to chiral amino acid analysis.

A new pre-column derivatization reagent with a 6-methoxy-4-quinolone (6-MOQ) moiety for amino acid analysis, 2,5-dioxopyrrolidin-1-yl(2-(6-methoxy-4-oxoquinolin-1(4H)-yl)ethyl) carbonate (6-MOQ-EtOCOOSu), was designed and synthesized. 6-MOQ is a thermo/photostable fluorophore with a high proton-affinity site and sensitive determination could be carried out by a fluorescence detector and also by an electrospray ionization mass spectrometer. Derivatization of amino acids with 6-MOQ-EtOCOOSu was completed within 1 min under mild basic conditions at room temperature. The 6-MOQ derivatives of all chiral proteinogenic amino acids were separated using the combination of three enantioselective columns, Chiralpak QN-AX, Chiralpak ZXIX(+), and KSAACSP-001S, with separation factors of higher than 1.07. The present reagent enables the sensitive determination of amino acid enantiomers, and the values of LLOD using a chiral-HPLC-MS/MS system were 0.05-50 fmol/injection.

Changes in D-aspartic acid and D-glutamic acid levels in the tissues and physiological fluids of mice with various D-aspartate oxidase activities.

D-Aspartic acid (D-Asp) and D-glutamic acid (D-Glu) are currently paid attention as modulators of neuronal transmission and hormonal secretion. These two D-amino acids are metabolized only by D-aspartate oxidase (DDO) in mammals. Therefore, in order to design and develop new drugs controlling the D-Asp and D-Glu amounts via regulation of the DDO activities, changes in these acidic D-amino acid amounts in various tissues are expected to be clarified in model animals having various DDO activities. In the present study, the amounts of Asp and Glu enantiomers in 6 brain tissues, 11 peripheral tissues and 2 physiological fluids of DDO(+/+), DDO(+/-) and DDO(-/-) mice were determined using a sensitive and selective two-dimensional HPLC system. As a result, the amounts of D-Asp were drastically increased with the decrease in the DDO activity in all the tested tissues and physiological fluids. On the other hand, the amounts of D-Glu were almost the same among the 3 strains of mice. The present results are useful for designing new drug candidates, such as DDO inhibitors, and further studies are expected.

Glycolytic flux controls D-serine synthesis through glyceraldehyde-3-phosphate dehydrogenase in astrocytes.

D-Serine is an essential coagonist with glutamate for stimulation of N-methyl-D-aspartate (NMDA) glutamate receptors. Although astrocytic metabolic processes are known to regulate synaptic glutamate levels, mechanisms that control D-serine levels are not well defined. Here we show that d-serine production in astrocytes is modulated by the interaction between the D-serine synthetic enzyme serine racemase (SRR) and a glycolytic enzyme, glyceraldehyde 3-phosphate dehydrogenase (GAPDH). In primary cultured astrocytes, glycolysis activity was negatively correlated with D-serine level. We show that SRR interacts directly with GAPDH, and that activation of glycolysis augments this interaction. Biochemical assays using mutant forms of GAPDH with either reduced activity or reduced affinity to SRR revealed that GAPDH suppresses SRR activity by direct binding to GAPDH and through NADH, a product of GAPDH. NADH allosterically inhibits the activity of SRR by promoting the disassociation of ATP from SRR. Thus, astrocytic production of D-serine is modulated by glycolytic activity via interactions between GAPDH and SRR. We found that SRR is expressed in astrocytes in the subiculum of the human hippocampus, where neurons are known to be particularly vulnerable to loss of energy. Collectively, our findings suggest that astrocytic energy metabolism controls D-serine production, thereby influencing glutamatergic neurotransmission in the hippocampus.

Boronophenylalanine, a boron delivery agent for boron neutron capture therapy, is transported by ATB0,+, LAT1 and LAT2.

The efficacy of boron neutron capture therapy relies on the selective delivery of boron carriers to malignant cells. p-Boronophenylalanine (BPA), a boron delivery agent, has been proposed to be localized to cells through transporter-mediated mechanisms. In this study, we screened aromatic amino acid transporters to identify BPA transporters. Human aromatic amino acid transporters were functionally expressed in Xenopus oocytes and examined for BPA uptake and kinetic parameters. The roles of the transporters in BPA uptake were characterized in cancer cell lines. For the quantitative assessment of BPA uptake, HPLC was used throughout the study. Among aromatic amino acid transporters, ATB(0,+), LAT1 and LAT2 were found to transport BPA with Km values of 137.4 ± 11.7, 20.3 ± 0.8 and 88.3 ± 5.6 µM, respectively. Uptake experiments in cancer cell lines revealed that the LAT1 protein amount was the major determinant of BPA uptake at 100 µM, whereas the contribution of ATB(0,+) became significant at 1000 µM, accounting for 20-25% of the total BPA uptake in MCF-7 breast cancer cells. ATB(0,+), LAT1 and LAT2 transport BPA at affinities comparable with their endogenous substrates, suggesting that they could mediate effective BPA uptake in vivo. The high and low affinities of LAT1 and ATB(0,+), respectively, differentiate their roles in BPA uptake. ATB(0,+), as well as LAT1, could contribute significantly to the tumor accumulation of BPA at clinical dose.

Chiral amino acid analysis of Japanese traditional Kurozu and the developmental changes during earthenware jar fermentation processes.

Enantioselective amino acid metabolome analysis of the Japanese traditional black vinegars (amber rice vinegar, Kurozu) was performed using two-dimensional high-performance liquid chromatography combining a microbore-monolithic ODS column and narrowbore-enantioselective columns. d-Amino acids, the enantiomers of widely observed l-amino acids, are currently paid attention as novel physiologically active substances, and the foodstuffs and beverages containing high amounts of d-amino acids are the subjects of interest. In the present study, the amino acid enantiomers were determined by two-dimensional HPLC techniques after pre-column fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole. In the first dimension, the amino acid enantiomers are separated as their d plus l mixtures by the reversed-phase mode, then the d-amino acids and their l-counterparts are separately determined in the second dimension by the enantioselective columns. As a result, large amounts of d-Ala (800-4000nmol/mL), d-Asp (200-400nmol/mL) and d-Glu (150-500nmol/mL) were observed in some of the traditionally produced Kurozu vinegars. Relatively large or small amounts of d-Ser (50-100nmol/mL), d-Leu (10-50nmol/mL) and d-allo-Ile (less than 20nmol/mL) were also present in these samples. Developmental changes in the d-amino acid amounts during the fermentation and aging processes have also been investigated.

Ischemic acute kidney injury perturbs homeostasis of serine enantiomers in the body fluid in mice: early detection of renal dysfunction using the ratio of serine enantiomers.

The imbalance of blood and urine amino acids in renal failure has been studied mostly without chiral separation. Although a few reports have shown the presence of D-serine, an enantiomer of L-serine, in the serum of patients with severe renal failure, it has remained uncertain how serine enantiomers are deranged in the development of renal failure. In the present study, we have monitored serine enantiomers using a two-dimensional HPLC system in the serum and urine of mice after renal ischemia-reperfusion injury (IRI), known as a mouse model of acute kidney injury. In the serum, the level of D-serine gradually increased after renal IRI in parallel with that of creatinine, whereas the L-serine level decreased sharply in the early phase after IRI. The increase of D-serine was suppressed in part by genetic inactivation of a D-serine-degrading enzyme, D-amino acid oxidase (DAO), but not by disruption of its synthetic enzyme, serine racemase, in mice. Renal DAO activity was detected exclusively in proximal tubules, and IRI reduced the number of DAO-positive tubules. On the other hand, in the urine, D-serine was excreted at a rate nearly triple that of L-serine in mice with sham operations, indicating that little D-serine was reabsorbed while most L-serine was reabsorbed in physiological conditions. IRI significantly reduced the ratio of urinary D-/L-serine from 2.82 ± 0.18 to 1.10 ± 0.26 in the early phase and kept the ratio lower than 0.5 thereafter. The urinary D-/L-serine ratio can detect renal ischemia earlier than kidney injury molecule-1 (KIM-1) or neutrophil gelatinase-associated lipocalin (NGAL) in the urine, and more sensitively than creatinine, cystatin C, or the ratio of D-/L-serine in the serum. Our findings provide a novel understanding of the imbalance of amino acids in renal failure and offer a potential new biomarker for an early detection of acute kidney injury.

Two-dimensional high-performance liquid chromatographic determination of day-night variation of D-alanine in mammals and factors controlling the circadian changes.

D-Alanine (D-Ala) is one of the naturally occurring D-amino acids in mammals, and its amount is known to have characteristic circadian changes. It is a candidate for a novel physiologically active substance and/or a biomarker, and the regulation mechanisms of the intrinsic amounts of D-Ala are expected to be clarified. In the present study, the effects of the possible factors controlling the D-Ala amounts, e.g., diet, D-amino acid oxidase (DAO) and intestinal bacteria, on the day-night changes in the intrinsic D-Ala amounts have been investigated using a highly sensitive and selective two-dimensional high-performance liquid chromatographic system combining a reversed-phase column and an enantioselective column. The circadian rhythm was not changed under fasting conditions. In the mice lacking D-amino acid oxidase activity (ddY/DAO(-) mice), clear day-night changes were still observed, suggesting that the factors controlling the D-Ala rhythm were not their food and DAO activity. On the other hand, in the germ-free mice, quite low amounts of D-Ala were detected compared with those in the control mice, indicating that the main origin of D-Ala in the mice is intestinal bacteria. Because the D-Ala amounts in the digesta containing intestinal bacteria did not show the day-night changes, the controlling factor of the circadian changes of the D-Ala amount was suggested to be the intestinal absorption.

Enantioselective two-dimensional high-performance liquid chromatographic determination of N-methyl-D-aspartic acid and its analogues in mammals and bivalves.

A validated and fully automated chiral 2D-HPLC system was developed for the simultaneous determination of N-methyl-d-aspartic acid (NMDA) analogues by combining a long microbore-monolithic ODS column (0.53 mm i.d.× 1,000 mm) and narrowbore-enantioselective columns (1.5mm i.d.×150 or 250 mm). The target analytes, enantiomers of N-methylaspartic acid (NMA) and N-methylglutamic acid (NMG), were precolumn-derivatized with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) and detected by their fluorescence. The values of the lower limit of quantification for these enantiomers were 2.5 fmol. In the tissues and plasma of rats, neither NMA nor NMG were detected. On the other hand, in the mantle and foot of Scapharca broughtonii, a large amount of NMDA was present (170.1 and 43.5 nmol/g), and the enantiomers of NMG were also observed. Meretrix lusoria contained NMDA (29.3 nmol/g) and NMLG (13.8 nmol/g), and Ruditapes philippinarum contained only NMLG (2.6 nmol/g). The obtained results were confirmed using three different enantioselective columns and also using a 2D-HPLC-MS/MS system. These results indicated that neuroactive d-amino acid, NMDA, and its analogues were present in animals, and their physiological significance is expected to be clarified.

Alteration of intrinsic amounts of D-serine in the mice lacking serine racemase and D-amino acid oxidase.

For elucidation of the regulation mechanisms of intrinsic amounts of D-serine (D-Ser) which modulates the neuro-transmission of N-methyl-D-aspartate receptors in the brain, mutant animals lacking serine racemase (SRR) and D-amino acid oxidase (DAO) were established, and the amounts of D-Ser in the tissues and physiological fluids were determined. D-Ser amounts in the frontal brain areas were drastically decreased followed by reduced SRR activity. On the other hand, a moderate but significant decrease in D-Ser amounts was observed in the cerebellum and spinal cord of SRR knock-out (SRR(-/-)) mice compared with those of control mice, although the amounts of D-Ser in these tissues were low. The amounts of D-Ser in the brain and serum were not altered with aging. To clarify the uptake of exogenous D-Ser into the brain tissues, we have determined the D-Ser of SRR(-/-) mice after oral administration of D-Ser for the first time, and a drastic increase in D-Ser amounts in all the tested tissues was observed. Because both DAO and SRR are present in some brain areas, we have established the double mutant mice lacking SRR and DAO for the first time, and the contribution of both enzymes to the intrinsic D-Ser amounts was investigated. In the frontal brain, most of the intrinsic D-Ser was biosynthesized by SRR. On the other hand, half of the D-Ser present in the hindbrain was derived from the biosynthesis by SRR. These results indicate that the regulation of intrinsic D-Ser amounts is different depending on the tissues and provide useful information for the development of treatments for neuronal diseases.

D-Amino acids in the brain and mutant rodents lacking D-amino-acid oxidase activity.

D-Amino acids are stereoisomers of L-amino acids. They are often called unnatural amino acids, but several D-amino acids have been found in mammalian brains. Among them, D-serine is abundant in the forebrain and functions as a co-agonist of NMDA receptors to enhance neurotransmission. D-Amino-acid oxidase (DAO), which degrades neutral and basic D-amino acids, is mainly present in the hindbrain. DAO catabolizes D-serine and, therefore, modulates neurotransmission. In the brains of mutant mice and rats lacking DAO activity, the amounts of D-serine and other D-amino acids are markedly increased. Mutant mice manifested behavioral changes characteristic of altered NMDA receptor activity, likely due to increased levels of D-serine. D-Serine and DAO have been demonstrated to play important roles in cerebellar development and synaptic plasticity. They have also implicated in amyotrophic lateral sclerosis and pain response. There have also been several lines of evidence correlating DAO with schizophrenia. Taken together, the experiments indicate that D-amino acids and DAO have pivotal functions in the central nervous system.

HPLC analysis of naturally occurring free D-amino acids in mammals.

D-amino acids are currently recognized as naturally occurring physiologically active substances and biomarkers in mammals. The progress of analytical technologies, mostly high resolution chromatographic or electrodriven separation methods, has significantly contributed to the advances in D-amino acid research in real biological matrices. In this review, we would like to describe the D-amino acid research, from the discovery of appreciable amounts of free D-amino acids in mammals to the current metabolomics study focusing on amino acid enantiomers. The liquid phase enantioselective analytical methods utilized for the determination of D-amino acids in mammals including human beings will be discussed.

D-amino acid oxidase controls motoneuron degeneration through D-serine.

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder involving an extensive loss of motoneurons. Aberrant excitability of motoneurons has been implicated in the pathogenesis of selective motoneuronal death in ALS. D-serine, an endogenous coagonist of N-methyl-D-aspartate receptors, exacerbates motoneuronal death and is increased both in patients with sporadic/familial ALS and in a G93A-SOD1 mouse model of ALS (mSOD1 mouse). More recently, a unique mutation in the D-amino acid oxidase (DAO) gene, encoding a D-serine degrading enzyme, was reported to be associated with classical familial ALS. However, whether DAO affects the motoneuronal phenotype and D-serine increase in ALS remains uncertain. Here, we show that genetic inactivation of DAO in mice reduces the number and size of lower motoneurons with axonal degeneration, and that suppressed DAO activity in reactive astrocytes in the reticulospinal tract, one of the major inputs to the lower motoneurons, predominantly contributes to the D-serine increase in the mSOD1 mouse. The DAO inactivity resulted from expressional down-regulation, which was reversed by inhibitors of a glutamate receptor and MEK, but not by those of inflammatory stimuli. Our findings provide evidence that DAO has a pivotal role in motoneuron degeneration through D-serine regulation and that inactivity of DAO is a common feature between the mSOD1 ALS mouse model and the mutant DAO-associated familial ALS. The therapeutic benefit of reducing D-serine or controlling DAO activity in ALS should be tested in future studies.

Enantioselective micro-2D-HPLC determination of aspartic acid in the pineal glands of rodents with various melatonin contents.

Enantioselective determination of aspartic acid (Asp) in the pineal gland of rodents with various melatonin contents was performed using a highly sensitive and selective two-dimensional HPLC system. After derivatization of the amino group with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F), NBD-Asp was separated using a capillary monolithic ODS column in the first dimension. The fraction of NBD-Asp was automatically collected and transferred to the second dimension, and the D- and L-Asp were separated and determined using a narrowbore enantioselective column. Large amounts of D-Asp were observed in the pineal gland of the rats and specific strains of mice (C3H and CBA) possessing a high concentration of melatonin in their pineal gland. On the other hand, the amounts of D-Asp were small in the pineal gland of mice possessing a trace or no melatonin in their pineal gland (ddY, ICR, C57BL and BALB/c). In other tissues and physiological fluids, no significant strain-dependent changes of the D-Asp amounts were observed. These results indicate that large amounts of D-Asp are present only in the pineal gland containing large amounts of melatonin, and special care should be taken when selecting mouse strains for the investigation of D-Asp.

D-Amino acid metabolism in mammals: biosynthesis, degradation and analytical aspects of the metabolic study.

It was believed for long time that d-amino acids are not present in mammals. However, current technological advances and improvements in analytical instruments have enabled studies that now indicate that significant amounts of D-amino acids are present in mammals. The most abundant D-amino acids are D-serine and D-aspartate. D-Serine, which is synthesized by serine racemase and is degraded by D-amino-acid oxidase, is present in the brain and modulates neurotransmission. D-Aspartate, which is synthesized by aspartate racemase and degraded by D-aspartate oxidase, is present in the neuroendocrine and endocrine tissues and testis. It regulates the synthesis and secretion of hormones and spermatogenesis. D-Serine and D-aspartate bind to the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors and function as a coagonist and agonist, respectively. The enzymes that are involved in the synthesis and degradation of these D-amino acids are associated with neural diseases where the NMDA receptors are involved. Knockout mice for serine racemase and D-aspartate oxidase have been generated, and natural mutations in the d-amino-acid oxidase gene are present in mice and rats. These mutant animals display altered behaviors caused by enhanced or decreased NMDA receptor activity. In this article, we review currently available studies on D-amino acid metabolism in mammals and discuss analytical methods used to assay activity of amino acid racemases and D-amino-acid oxidases.