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.
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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...
