The Antiviral Drug Acyclovir Is a Slow-Binding Inhibitor of d-Amino Acid Oxidase

Identification of an l-serine/l-threonine dehydratase with glutamate racemase activity in mammals

Recent investigations have shown that multiple d-amino acids are present in mammals and these compounds have distinctive physiological functions. Free d-glutamate is present in various mammalian tissues and cells and in particular, it is presumably correlated with cardiac function, and much interest is growing in its unique metabolic pathways. Recently, we first identified d-glutamate cyclase as its degradative enzyme in mammals, whereas its biosynthetic pathway in mammals is unclear. Glutamate racemase is a most probable candidate, which catalyzes interconversion between d-glutamate and l-glutamate. Here, we identified the cDNA encoding l-serine dehydratase-like (SDHL) as the first mammalian clone with glutamate racemase activity. This rat SDHL had been deposited in mammalian databases as a protein of unknown function and its amino acid sequence shares ∼60% identity with that of l-serine dehydratase. Rat SDHL was expressed in Escherichia coli, and the enzymatic properties of the recombinant were characterized. The results indicated that rat SDHL is a multifunctional enzyme with glutamate racemase activity in addition to l-serine/l-threonine dehydratase activity. This clone is hence abbreviated as STDHgr. Further experiments using cultured mammalian cells confirmed that d-glutamate was synthesized and l-serine and l-threonine were decomposed. It was also found that SDHL (STDHgr) contributes to the homeostasis of several other amino acids.

Comparative studies on picosecond-resolved fluorescence of d-amino acid oxidases from human with one from porcine kidney. Photoinduced electron transfer from aromatic amino acids to the excited flavin

Fluorescence dynamics of human d-amino acid oxidase (hDAAO) and its five inhibitors have been studied in the picoseconds time domain, and compared with one in d-amino acid oxidase from porcine kidney (pkDAAO) reported. The fluorescence lifetimes were identified as 47 ps in the dimer, 235 ps in the monomer, which are compared with those of pkDAAO (45 ps-185 ps). The fluorescence lifetimes of the hDAAO did not change upon the inhibitor bindings despite of modifications in the absorption spectra. This indicates that the lifetimes of the complexes are too short to detect with the picosecond lifetime instrument. Numbers of the aromatic amino acids are similar between the both DAAOs. The fluorescence lifetimes of hDAAO were analysed with an ET theory using the crystal structure. The difference in the lifetimes of the dimer and monomer was well described in terms of difference in the electron affinity of the excited isoalloxazine (Iso*) between the two forms of the protein, though it is not known whether the structure of the monomer is different from the dimer. Three fastest ET donors were Tyr314, Trp52 and Tyr224 in the dimer, while Tyr314, Tyr224 and Tyr55 in the monomer, which are compared to those in pkDAAO, Tyr314, Tyr224 and Tyr228 in the dimer, and Tyr224, Tyr314 and Tyr228 in the monomer. The ET rate from Trp55 in hDAAO dimer was much faster compared to the rate in pkDAAO dimer. A rise component with negative pre-exponential factor was not observed in hDAAO, which are found in pkDAAO.

Structural and enzymatic properties of mammalian d-glutamate cyclase

d-Glutamate cyclase (DGLUCY) is a unique enzyme that reversibly converts free d-glutamate to 5-oxo-d-proline and H₂O. Mammalian DGLUCY is highly expressed in the mitochondrial matrix in the heart, and its downregulation disrupts d-glutamate and/or 5-oxo-d-proline levels, contributing to the onset and/or exacerbation of heart failure. However, detailed characterisation of DGLUCY has not yet been performed. Herein, the structural and enzymatic properties of purified recombinant mouse DGLUCY were examined. The results revealed a dimeric oligomerisation state, and both d-glutamate-to-5-oxo-d-proline and 5-oxo-d-proline-to-d-glutamate reactions were catalysed in a stereospecific manner. Catalytic activity is modulated by divalent cations and nucleotides including ATP and ADP. Interestingly, the presence of Mn²⁺ completely abolished the 5-oxo-d-proline-to-d-glutamate reaction but stimulated the d-glutamate-to-5-oxo-d-proline reaction. The optimum pH is ∼8.0, similar to that in the mitochondrial matrix, and the catalytic efficiency for d-glutamate is markedly higher than that for 5-oxo-d-proline. These findings suggest that DGLUCY functions as a metalloenzyme that degrades d-glutamate in the mitochondrial matrix in mammalian cells. The results also provide insight into the correlation between DGLUCY enzyme activity and the physiological and pathological roles of d-glutamate and 5-oxo-d-proline in cardiac function, which is of relevance to the risk of onset of heart failure.

Rat d-aspartate oxidase is more similar to the human enzyme than the mouse enzyme

d-Aspartate oxidase (DDO) is a degradative enzyme that is stereospecific for the acidic amino acid d-aspartate, an endogenous agonist of the N-methyl-d-aspartate (NMDA) receptor. Dysregulation of NMDA receptor-mediated neurotransmission has been implicated in the onset of various neuropsychiatric disorders including schizophrenia, as well as chronic pain. Thus, appropriate regulation of d-aspartate is believed to be important for maintaining proper neural activity in the nervous system. Accordingly, much attention has been paid to the role(s) of DDO in the metabolism of d-aspartate in vivo, and the physiological functions of DDO have been actively investigated using experimental rats and mice. However, detailed characterisation of rat DDO has not yet been performed, and little is known about species-specific differences in the properties of mammalian DDOs. In this study, the structural and enzymatic properties of purified recombinant rat, mouse and human DDOs were examined and compared. The results showed that rat DDO is more similar to human DDO than to mouse DDO. This work provides useful insight into the use of rats as an experimental model for investigating the biological significance of human DDO and/or d-aspartate. This article is part of a Special Issue entitled: d-Amino acids: biology in the mirror, edited by Dr. Loredano Pollegioni, Dr. Jean-Pierre Mothet and Dr. Molla Gianluca.

Assessment of the Target Engagement and D-Serine Biomarker Profiles of the D-Amino Acid Oxidase Inhibitors Sodium Benzoate and PGM030756

Irregular N-methyl-D-aspartate receptor (NMDAR) function is one of the main hypotheses employed to facilitate understanding of the underlying disease state of schizophrenia. Although direct agonism of the NMDAR has not yielded promising therapeutics, advances have been made by modulating the NMDAR co-agonist site which is activated by glycine and D-serine. One approach to activate the co-agonist site is to increase synaptic D-serine levels through inhibition of D-amino acid oxidase (DAO), the major catabolic clearance pathway for this and other D-amino acids. A number of DAO inhibitors have been developed but most have not entered clinical trials. One exception to this is sodium benzoate which has demonstrated efficacy in small trials of schizophrenia and Alzheimer's disease. Herein we provide data on the effect of sodium benzoate and an optimised Takeda compound, PGM030756 on ex vivo DAO enzyme occupancy and cerebellar D-serine levels in mice. Both compounds achieve high levels of enzyme occupancy; although lower doses of PGM030756 (1, 3 and 10 mg/kg) were required to achieve this compared to sodium benzoate (300, 1000 mg/kg). Cerebellar D-serine levels were increased by both agents with a delay of approximately 6 h after dosing before the peak effect was achieved. Our data and methods may be useful in understanding the effects of sodium benzoate that have been seen in clinical trials of schizophrenia and Alzheimer's disease and to support the potential clinical assessment of other DAO inhibitors, such as PGM030756, which demonstrate good enzyme occupancy and D-serine increases following administration of low oral doses.

Structure-function relationships in human d-aspartate oxidase: characterisation of variants corresponding to known single nucleotide polymorphisms

d-Aspartate oxidase (DDO) is a degradative enzyme that is stereospecific for the acidic amino acid d-aspartate, an endogenous agonist of the N-methyl-d-aspartate (NMDA) receptor. Dysregulation of NMDA receptor-mediated neurotransmission has been implicated in the onset of various neuropsychiatric disorders including schizophrenia and in chronic pain. Thus, appropriate regulation of the amount of d-aspartate is believed to be important for maintaining proper neural activity in the nervous system. Herein, the effects of the non-synonymous single nucleotide polymorphisms (SNPs) R216Q and S308N on several properties of human DDO were examined. Analysis of the purified recombinant enzyme showed that the R216Q and S308N substitutions reduce enzyme activity towards acidic d-amino acids, decrease the binding affinity for the coenzyme flavin adenine dinucleotide and decrease the temperature stability. Consistent with these findings, further experiments using cultured mammalian cells revealed elevated d-aspartate in cultures of R216Q and S308N cells compared with cells expressing wild-type DDO. Furthermore, accumulation of several amino acids other than d-aspartate also differed between these cultures. Thus, expression of DDO genes carrying the R216Q or S308N SNP substitutions may increase the d-aspartate content in humans and alter homeostasis of several other amino acids. This work may aid in understanding the correlation between DDO activity and the risk of onset of NMDA receptor-related diseases.

Identification and characterization of natural microbial products that alter the free d-aspartate content of mammalian cells

Mammalian cells possess the molecular apparatus necessary to take up, degrade, synthesize, and release free d-aspartate, which plays an important role in physiological functions within the body. Here, biologically active microbial compounds and pre-existing drugs were screened for their ability to alter the intracellular d-aspartate level in mammalian cells, and several candidate compounds were identified. Detailed analytical studies suggested that two of these compounds, mithramycin A and geldanamycin, suppress the biosynthesis of d-aspartate in cells. Further studies suggested that these compounds act at distinct sites within the cell. These compounds may advance our current understanding of biosynthesis of d-aspartate in mammals, a whole picture of which remains to be disclosed.

Identification of Novel D-Aspartate Oxidase Inhibitors by in Silico Screening and Their Functional and Structural Characterization in Vitro

D-Aspartate oxidase (DDO) is a degradative enzyme that is stereospecific for acidic D-amino acids, including D-aspartate, a potential agonist of the N-methyl-D-aspartate (NMDA) receptor. Dysfunction of NMDA receptor-mediated neurotransmission has been implicated in the onset of various mental disorders, such as schizophrenia. Hence, a DDO inhibitor that increases the brain levels of D-aspartate and thereby activates NMDA receptor function is expected to be a useful compound. To search for potent DDO inhibitor(s), a large number of compounds were screened in silico, and several compounds were identified as candidates. They were then characterized and evaluated as novel DDO inhibitors in vitro (e.g., the inhibitor constant value of 5-aminonicotinic acid for human DDO was 3.80 μM). The present results indicate that some of these compounds may serve as lead compounds for the development of a clinically useful DDO inhibitor and as active site probes to elucidate the structure-function relationships of DDO.

Diversity-oriented synthesis of acyclic nucleosides via ring-opening of vinyl cyclopropanes with purines

The diversity-oriented synthesis of acyclic nucleosides has been achieved via ring-opening of vinyl cyclopropanes with purines.