D-amino acid oxidase (DAO) is a degradative enzyme that is stereospecific for D-amino acids, including D-serine and D-alanine, which are potential coagonists 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 DAO inhibitor that augments the brain levels of D-serine and/or D-alanine and thereby activates NMDA receptor function is expected to be an antipsychotic drug, for instance, in the treatment of schizophrenia. In the search for potent DAO inhibitor(s), a large number of compounds were screened in silico, and several compounds were estimated as candidates. These compounds were then characterized and evaluated as novel DAO inhibitors in vitro. The results reported in this study indicate that some of these compounds are possible lead compounds for the development of a clinically useful DAO inhibitor and have the potential to serve as active site probes to elucidate the structure-function relationships of DAO.

Human DDO is considered an attractive therapeutic target, and DDO inhibitors may be potential lead compounds for the development of new drugs against the aforementioned diseases. However, human DDO has not been characterized in detail and, although preclinical studies using experimental rodents are prerequisites for evaluating the in vivo effects of potential inhibitors, the existence of species-specific differences in the properties of human and rodent DDOs is still unclear. Here, the enzymatic activity and characteristics of purified recombinant human DDO were analyzed in detail. The kinetic and inhibitor-binding properties of this enzyme were also compared with those of purified recombinant rat and mouse DDOs. In addition, structural models of human, rat, and mouse DDOs were generated and compared. It was found that the differences among these DDO proteins occur in regions that appear involved in migration of the substrate/product in and out of the active site. In summary, detailed characterization of human DDO was performed and provides useful insights into the use of rats and mice as experimental models for evaluating the in vivo effects of DDO inhibitors. Alzheimer's disease, 9,10) and amyotrophic lateral sclerosis. Key words 11,12)Unlike the tissue-specific expression of D-Ser, substantial amounts of free D-Asp are present in a wide variety of mammalian tissues and cells, particularly those of the central nervous, neuroendocrine, and endocrine systems. Several lines of evidence suggest that D-Asp plays an important role in regulating developmental processes, hormone secretion, and steroidogenesis. [13][14][15] The amounts of D-Asp in human seminal plasma and spermatozoa are significantly lower in oligoasthenoteratospermic and azoospermic donors than normospermic donors. 16) Furthermore, in female patients undergoing in vitro fertilization, the D-Asp content of pre-ovulatory follicular fluid is lower in older patients than in younger patients; this decrease in D-Asp content appears to reflect a reduction in oocyte quality and fertilization competence. 17) Overall, current evidence suggests that decreases in D-Asp levels may be involved in the pathophysiology of infertility. Furthermore, D-Asp stimulates the NMDA receptor by acting as an agonist that binds to the L-Glu-binding site of the receptor. 18,19) Recent studies have suggested that, similar to D-Ser, D-Asp acts as a signaling molecule in nervous and neuroendocrine systems, at least in part, by binding to the NMDA receptor, and plays an important role in the regulation of brain functions. 14,15,20) In support of this proposal, it was reported recently that D-Asp levels in the prefrontal cortex and striatum of post-mortem brains of schizophrenic patients are significantly lower than those of non-psychiatrically ill individuals. 21) In mammalian tissues, two types of degradative enzymes that are stereospecific for D-amino acids have been identified, namely, D-amino acid oxidase (DAO, also abbreviated as DAAO; EC 1.4.3.3) and D-Asp oxidase (DDO, also ab...

D-Aspartate (D-Asp) has important physiological functions, and recent studies have shown that substantial amounts of free D-Asp are present in a wide variety of mammalian tissues and cells. Biosynthesis of D-Asp has been observed in several cultured rat cell lines, and a murine gene (glutamate-oxaloacetate transaminase 1-like 1, Got1l1) that encodes Asp racemase, a synthetic enzyme that produces D-Asp from L-Asp, was proposed recently. The product of this gene is homologous to mammalian glutamate-oxaloacetate transaminase (GOT). Here, we tested the hypothesis that rat and human homologs of mouse GOT1L1 are involved in Asp synthesis. The following two approaches were applied, since the numbers of attempts were unsuccessful to prepare soluble GOT1L1 recombinant proteins. First, the relationship between the D-Asp content and the expression levels of the mRNAs encoding GOT1L1 and D-Asp oxidase, a primary degradative enzyme of D-Asp, was examined in several rat and human cell lines. Second, the effect of knockdown of the Got1l1 gene on D-Asp biosynthesis during culture of the cells was determined. The results presented here suggest that the rat and human homologs of mouse GOT1L1 are not involved in D-Asp biosynthesis. Therefore, D-Asp biosynthetic pathway in mammals is still an urgent issue to be resolved.

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.

d-Amino acid oxidase (DAO) is a degradative enzyme that is stereospecific for d-amino acids, including d-serine and d-alanine, which are believed to be coagonists of the N-methyl-d-aspartate (NMDA) receptor. To identify a new class of DAO inhibitor(s) that can be used to elucidate the molecular details of the active site environment of DAO, manifold biologically active compounds of microbial origin and pre-existing drugs were screened for their ability to inhibit DAO activity, and several compounds were identified as candidates. One of these compounds, acyclovir (ACV), a well-known antiviral drug used for the treatment of herpesvirus infections, was characterized and evaluated as a novel DAO inhibitor in vitro. Analysis showed that ACV acts on DAO as a reversible slow-binding inhibitor, and interestingly, the time required to achieve equilibrium between DAO, ACV, and the DAO/ACV complex was highly dependent on temperature. The binding mechanism of ACV to DAO was investigated in detail by several approaches, including kinetic analysis, structural modeling of DAO complexed with ACV, and site-specific mutagenesis of an active site residue postulated to be involved in the binding of ACV. The results confirm that ACV is a novel, active site-directed inhibitor of DAO that can be a valuable tool for investigating the structure-function relationships of DAO, including the molecular details of the active site environment of DAO. In particular, it appears that ACV can serve as an active site probe to study the structural basis of temperature-induced conformational changes of DAO.

Multiple D-amino acids are present in mammalian cells, and these compounds have distinctive physiological functions. Among the free D-amino acids identified in mammals, D-aspartate plays critical roles in the neuroendocrine and endocrine systems, as well as in the central nervous system. Mammalian cells have the molecular apparatus necessary to take up, degrade, synthesize, and release D-aspartate. In particular, D-aspartate is degraded by D-aspartate oxidase (DDO), a peroxisome-localized enzyme that catalyzes the oxidative deamination of D-aspartate to generate oxaloacetate, hydrogen peroxide, and ammonia. However, little is known about the molecular mechanisms underlying D-aspartate homeostasis in cells. In this study, we established a cell line that overexpresses cytoplasm-localized DDO; this cell line cannot survive in the presence of high concentrations of D-aspartate, presumably because high levels of toxic hydrogen peroxide are produced by metabolism of abundant D-aspartate by DDO in the cytoplasm, where hydrogen peroxide cannot be removed due to the absence of catalase. Next, we transfected these cells with a complementary DNA library derived from the human brain and screened for clones that affected D-aspartate metabolism and improved cell survival, even when the cells were challenged with high concentrations of D-aspartate. The screen identified a clone of glyoxylate reductase/hydroxypyruvate reductase (GRHPR). Moreover, the GRHPR metabolites glyoxylate and hydroxypyruvate inhibited the enzymatic activity of DDO. Furthermore, we evaluated the effects of GRHPR and peroxisome-localized DDO on D-and L-aspartate levels in cultured mammalian cells. Our findings show that GRHPR contributes to the homeostasis of these amino acids in mammalian cells.

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