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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.

With oligonucleotides based on the amino-terminal and internal amino-acid sequences of a xylanase, two xylanase genes, cgxA and cgxB, were isolated and sequenced from Chaetomium gracile wild and mutant strains. Each gene isolated from both strains was essentially the same as far as nucleotide sequences were compared. The mature CgXA and CgXB xylanases comprise 189 and 211 amino acids, respectively, and share 68.5% homology. The CgXA was found to be the major enzyme in the mutant strain. Comparison of these amino-acid sequences with xylanase sequences from other origins showed that they have a high degree of identity to the family G xylanases. The cgxA and cgxB genes were introduced into Aspergillus nidulans and found to be expressed with their own promoters.