In eukaryotes, hydrogen sulphide acts as a signalling molecule and cytoprotectant. Hydrogen sulphide is known to be produced from L-cysteine by cystathionine b-synthase, cystathionine g-lyase and 3-mercaptopyruvate sulfurtransferase coupled with cysteine aminotransferase. Here we report an additional biosynthetic pathway for the production of hydrogen sulphide from D-cysteine involving 3-mercaptopyruvate sulfurtransferase and D-amino acid oxidase. Unlike the L-cysteine pathway, this D-cysteine-dependent pathway operates predominantly in the cerebellum and the kidney. Our study reveals that administration of D-cysteine protects primary cultures of cerebellar neurons from oxidative stress induced by hydrogen peroxide and attenuates ischaemia-reperfusion injury in the kidney more than L-cysteine. This study presents a novel pathway of hydrogen sulphide production and provides a new therapeutic approach to deliver hydrogen sulphide to specific tissues.

D D-Amino acid oxidase (DAAO) has been proposed to be involved in the oxidation of D D-serine, an allosteric activator of the NMDA-type glutamate receptor in the brain, and to be associated with the onset of schizophrenia. The recombinant human DAAO was expressed in Escherichia coli and was isolated as an active homodimeric flavoenzyme. It shows the properties of the dehydrogenase-oxidase class of flavoproteins, possesses a low kinetic efficiency, and follows a ternary complex (sequential) kinetic mechanism. In contrast to the other known DAAOs, the human enzyme is a stable homodimer even in the apoprotein form and weakly binds the cofactor in the free form.

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In the brain, the extensively studied FAD-dependent enzyme D-amino acid oxidase (DAO) degrades the gliotransmitter D-serine, a potent activator of N-methyl-D-aspartate type glutamate receptors, and evidence suggests that DAO, together with its activator G72 protein, may play a key role in the pathophysiology of schizophrenia. Indeed, its potential clinical importance highlights the need for structural and functional analyses of human DAO. We recently succeeded in purifying human DAO, and found that it weakly binds FAD and shows a significant slower rate of flavin reduction compared with porcine DAO. However, the molecular basis for the different kinetic features remains unclear because the active site of human DAO was considered to be virtually identical to that of porcine DAO, as would be expected from the 85% sequence identity. To address this issue, we determined the crystal structure of human DAO in complex with a competitive inhibitor benzoate, at a resolution of 2.5 Å . The overall dimeric structure of human DAO is similar to porcine DAO, and the catalytic residues are fully conserved at the re-face of the flavin ring. However, at the si-face of the flavin ring, despite the strict sequence identity, a hydrophobic stretch (residues 47-51, VAAGL) exists in a significantly different conformation compared with both of the independently determined porcine DAO-benzoate structures. This suggests that a context-dependent conformational variability of the hydrophobic stretch accounts for the low affinity for FAD as well as the slower rate of flavin reduction, thus highlighting the unique features of the human enzyme.Keywords: D-amino acid oxidase; Homo sapiens; X-ray crystallography; structurally ambivalent peptides; conformational variability D-amino acid oxidase (DAO) (EC 1.4.3.3) was first identified by Hans Krebs in 1935 and was later recognized to be the first enzyme known to use FAD as a cofactor (Krebs 1935). DAO noncovalently binds FAD as a prosthetic group and catalyzes the oxidative deamination of D-amino acids to their corresponding imino acids with concomitant reduction of FAD. The reduced flavin is subsequently reoxidized by molecular oxygen generating H 2 O 2 , and the imino acid is released into the solvent where it nonenzymatically hydrolyzes, yielding the corresponding a-keto acid and ammonia. DAO exhibits optimal activity toward neutral amino acids and marginal activity toward basic ones; acidic D-amino acids are oxidized by another flavoprotein, D-aspartate oxidase.Reprint requests to: Kiyoshi Fukui, The Institute for Enzyme Research, The University of Tokushima, Japan; Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi

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