The primary structure of D-amino acid oxidase from pig kidney. II. Isolation and sequence of overlap peptides and the complete sequence

Structural determinants for substrate specificity of flavoenzymes oxidizing d-amino acids

The oxidation of d-amino acids is relevant to neurodegenerative diseases, detoxification, and nutrition in microorganisms and mammals. It is also important for the resolution of racemic amino acid mixtures and the preparation of chiral building blocks for the pharmaceutical and food industry. Considerable biochemical and structural knowledge has been accrued in recent years on the enzymes that carry out the oxidation of the C_α-N bond of d-amino acids. These enzymes contain FAD as a required coenzyme, share similar overall three-dimensional folds and highly conserved active sites, but differ in their specificity for substrates with neutral, anionic, or cationic side-chains. Here, we summarize the current biochemical and structural knowledge regarding substrate specificity on d-amino acid oxidase, d-aspartate oxidase, and d-arginine dehydrogenase for which a wealth of biochemical and structural studies is available.

D-Amino acid dehydrogenase from Helicobacter pylori NCTC 11637

Helicobacter pylori is a microaerophilic bacterium, associated with gastric inflammation and peptic ulcers. D-Amino acid dehydrogenase is a flavoenzyme that digests free neutral D-amino acids yielding corresponding 2-oxo acids and hydrogen. We sequenced the H. pylori NCTC 11637 D-amino acid dehydrogenase gene, dadA. The primary structure deduced from the gene showed low similarity with other bacterial D-amino acid dehydrogenases. We purified the enzyme to homogeneity from recombinant Escherichia coli cells by cloning dadA. The recombinant protein, DadA, with 44 kDa molecular mass, possessed FAD as cofactor, and showed the highest activity to D-proline. The enzyme mediated electron transport from D-proline to coenzyme Q(1), thus distinguishing it from D-amino acid oxidase. The apparent K(m) and V(max) values were 40.2 mM and 25.0 micromol min(-1) mg(-1), respectively, for dehydrogenation of D-proline, and were 8.2 microM and 12.3 micromol min(-1) mg(-1), respectively, for reduction of Q(1). The respective pH and temperature optima were 8.0 and 37 degrees C. Enzyme activity was inhibited markedly by benzoate, and moderately by SH reagents. DadA showed more similarity with mammalian D-amino acid oxidase than other bacterial D-amino acid dehydrogenases in some enzymatic characteristics. Electron transport from D-proline to a c-type cytochrome was suggested spectrophotometrically.

Implication of a mutation in the flavin binding site on the specific activity and substrate specificity of glycine oxidase from Bacillus subtilis produced by directed evolution

Directed evolution was used to expand the substrate specificity and functionality of glycine oxidase by using a high-throughput screening assay based on the 4-aminoantipyrine peroxidase system, with a coefficient of variance below 4%. After screening the library, one mutant with the desired changes was found. The mutant was purified and characterized, showing important changes compared to the wild-type, especially towards cyclic d-amino acids. Amino acid substitution of Ile15 for Val, where the consensus sequence for flavin binding site is placed, seems to be responsible for these changes in specific activity and substrate specificity. The effect of this mutation was explained by using a computer-based three-dimensional model.

Characterization and structural modeling of a novel thermostable glycine oxidase from Geobacillus kaustophilus HTA426

Glycine oxidase from Geobacillus kaustophilus HTA426 (GOXK) is a 43 kDa monomer flavoenzyme containing noncovalently bound FAD. The induction of the enzyme resulted in the expression of a fully soluble protein with higher specific activity than those previously reported for GOX from B. subtilis (GOXB). A study of the kinetic properties of this novel GOXK revealed the lowest KM values for most of the substrates analyzed, with the exception of D-proline which kept a similar value and had the highest Vmax value reported. The Vmax/KM ratio maintained a substrate preference of GOXK for amines of small size, like glycine, sarcosine, N-ethyl-glycine, and glycine-ethyl-ester. GOXK presented good stability at 60-70 degrees C and in alkaline media (pH 6-9.5). The putative tridimensional structure was modeled by sequence alignment and by comparing the changes between GOXK and GOXB, and the residues that could be responsible for the substrate specificity as well as those essential for the catalytic activity were found. The comparison between the possible topology of GOXK with that of GOXB showed changes at the putative interactions between monomers for the building of the tetrameric oligomerization.

The role of cofactor binding in tryptophan accessibility and conformational stability of His-tagged d-amino acid oxidase from Trigonopsis variabilis

d-amino acid oxidase from Trigonopsis variabilis (TvDAAO) is a flavoenzyme with high biotechnological and industrial interest. The overexpression and purification of the apoprotein form of a recombinant His-tagged TvDAAO allowed us to go deep into the structural differences between apoenzyme and holoenzyme, and on the cofactor binding and its contribution to enzyme stability. A significant decrease in intrinsic fluorescence emission took place upon FAD binding, associated to cofactor induced conformational transitions or subunit dimerization that could affect the local environment of protein tryptophan residues. Furthermore, acrylamide-quenching experiments indicated that one of the five tryptophan residues of TvDAAO became less accessible upon FAD binding. A K(d)=1.5+/-0.1x10(-7) M for the dissociation of FAD from TvDAAO was calculated from binding experiments based on both quenching of FAD fluorescence and activity titration curves. Secondary structure prediction indicated that TvDAAO is a mixed alpha/beta protein with 8 alpha-helices and 14 beta-sheets connected by loops. Prediction results were in good agreement with the estimates obtained by circular dichroism which indicated that both the apoenzyme and the holoenzyme had the same structural component ratios: 34% alpha-helix content, 20% beta-structure content (14% antiparallel and 6% parallel beta-sheet), 15% beta-turns and 31% of random structure. Circular dichroism thermal-transition curves suggested single-step denaturation processes with apparent midpoint transition temperatures (T(m)) of 37.9 degrees C and 41.4 degrees C for the apoenzyme and the holoenzyme, respectively. A three-dimensional model of TvDAAO built by homology modelling and consistent with the spectroscopic studies is shown. Comparing our results with those reported for pig kidney (pkDAAO) and Rhodotorula gracilis (RgDAAO) d-amino acid oxidases, a "head-to-head" interaction between subunits in the TvDAAO dimer might be expected.

Mutual regulation between serine and nitric oxide metabolism in human glioblastoma cells

D-Serine indirectly caused dose- and time-dependent inhibition of neuronal nitric oxide synthase (nNOS) without affecting endothelial nitric oxide synthase (eNOS) in human glioblastoma cell line U87. Activity of D-amino acid oxidase (DAAO), catalyzing the oxidative deamination of d-amino acid, was enhanced by NO in a dose-dependent manner. Recently, we have reported that serine racemase (SR) is inhibited by NO and activated by D-serine through nitrosylation and denitrosylation, respectively [K. Shoji, S. Mariotto, A.R. Ciampa, H. Suzuki, Regulation of serine racemase activity by D-serine and nitric oxide in human glioblastoma cells, Neurosci. Lett., in press]. Thus, the metabolism of both d-serine and NO in U87 cells is functionally correlated in a complex manner. Suppression of NO production by d-serine in U87 cells contrasts its known action in enhancing nNOS in neurons.

Guinea pig D-amino-acid oxidase cDNA and phylogenetic position

The nucleotide sequence of cDNA that encodes guinea pig D-amino-acid oxidase (DAO) was determined. The cDNA consisted of 1,399 nucleotides and a poly(A) tail. The cDNA encodes 347 amino acid residues. In contrast to the hamster, rat, and mouse DAOs, guinea pig DAO had the 25th amino acid residue. The homology in amino acid sequences between the guinea pig DAO and the rodent DAOs was not high in comparison to the homology in amino acid sequences between the guinea pig DAO and DAOs of humans, pigs and rabbits. The phylogenetic position of the guinea pig varied depending on the source of sequences (amino acids or nucleotides) and the methods of phylogenetic tree construction. These results suggest that the guinea pig is not a simple rodent.

The role of a conserved histidine residue, His324, in Trigonopsis variabilis D-amino acid oxidase

To investigate the functional role of an invariant histidine residue in Trigonopsis variabilis D-amino acid oxidase (DAAO), a set of mutant enzymes with replacement of the histidine residue at position 324 was constructed and their enzymatic properties were examined. Wild-type and mutant enzymes have been purified to homogeneity using the His-bound column and the molecular masses were determined to be 39.2 kDa. Western blot analysis revealed that the in vivo synthesized mutant enzymes are immuno-identical with that of the wild-type DAAO. The His324Asn and His324Gln mutants displayed comparable enzymatic activity to that of the wild-type enzyme, while the other mutant DAAOs showed markedly decreased or no detectable activity. The mutants, His324/Asn/Gln/Ala/Tyr/Glu, exhibited 38-181% increase in Km and a 2-10-fold reduction in kcat/Km. Based on the crystal structure of a homologous protein, pig kidney DAAO, it is suggested that His324 might play a structural role for proper catalytic function of T. variabilis DAAO.

Characterization of 2-oxo-3-pentynoate as an active-site-directed inactivator of flavoprotein oxidases: identification of active-site peptides in tryptophan 2-monooxygenase

2-oxo-3-pentynoate has been characterized as an active-site-directed inhibitor of selected flavoprotein oxidases. Tryptophan 2-monooxygenase is irreversibly inactivated in an active-site-directed fashion. The addition of FAD affords no protection from inactivation, whereas the competitive inhibitor indole-3-acetamide fully protects the enzyme from inactivation. The inactivation follows first-order kinetics for at least five half-lives. The rate of inactivation shows saturation kinetics, consistent with the formation of a reversible complex between the alkylating agent and the enzyme before inactivation occurs. Values of 0.017 +/- 0.0005 min-1 and 44 +/- 7 microM were determined for the limiting rate of inactivation and the apparent dissociation constant for 2-oxo-3-pentynoate, respectively. Tryptic maps of tryptophan 2-monooxygenase treated with 2-oxo-3-pentynoate show that two peptides are alkylated in the absence of indole-3-acetamide but not in its presence. The two peptides were identified by mass spectrometry as residues 333-349 and 503-536. Based upon sequence analysis, cysteine 511 and either cysteine 339 or histidine 338 are the likely sites of modification. In contrast, incubation of D-amino acid oxidase or nitroalkane oxidase with 2-oxo-3-pentynoate results in a loss of 55% or 100%, respectively, of the initial activity. In neither case does a competitive inhibitor affect the rate of inactivation, suggesting that the effect is not due to modification of active-site residues.

Rat D-amino-acid oxidase cDNA: rat D-amino-acid oxidase as an intermediate form between mouse and other mammalian D-amino-acid oxidases

Nucleotide sequence of cDNA encoding rat D-amino-acid oxidase (DAO) was determined. Two species of DAO mRNA were present in rat kidney, liver, and brain. They were probably produced by alternative splicing. Rat DAO cDNA encodes 346 amino acid residues, indicating that rat DAO is an intermediate form between mouse DAO (345 amino acids) and DAOs (347 amino acids) of human, rabbit, and pig. Deduced amino acid sequence indicates 93% identity between rat and mouse DAO. Northern hybridization and western blotting supported the sequence data.

Expression of thiamin biosynthetic genes (thiCOGE) and production of symbiotic terminal oxidase cbb3 in Rhizobium etli

In this paper we report the cloning and sequence analysis of four genes, located on plasmid pb, which are involved in the synthesis of thiamin in Rhizobium etli (thiC, thiO, thiG, and thiE). Two precursors, 4-methyl-5-(beta-hydroxyethyl)thiazole monophosphate and 4-amino-5-hydroxymethylpyrimidine pyrophosphate, are coupled to form thiamin monophosphate, which is then phosphorylated to make thiamin pyrophosphate. The first open reading frame (ORF) product, of 610 residues, has significant homology (69% identity) with the product of thiC from Escherichia coli, which is involved in the synthesis of hydroxymethylpyrimidine. The second ORF product, of 327 residues, is the product of a novel gene denoted thiO. A protein motif involved in flavin adenine dinucleotide binding was found in the amino-terminal part of ThiO; also, residues involved in the catalytic site of D-amino acid oxidases are conserved in ThiO, suggesting that it catalyzes the oxidative deamination of some intermediate of thiamin biosynthesis. The third ORF product, of 323 residues, has significant homology (38% identity) with ThiG from E. coli, which is involved in the synthesis of the thiazole. The fourth ORF product, of 204 residues, has significant homology (47% identity) with the product of thiE from E. coli, which is involved in the condensation of hydroxymethylpyrimidine and thiazole. Strain CFN037 is an R. etli mutant induced by a single Tn5mob insertion in the promoter region of the thiCOGE gene cluster. The Tn5mob insertion in CFN037 occurred within a 39-bp region which is highly conserved in all of the thiC promoters analyzed and promotes constitutive expression of thiC. Primer extension analysis showed that thiC transcription in strain CFN037 originates within the Tn5 element. Analysis of c-type protein content and expression of the fixNOQP operon, which codes for the symbiotic terminal oxidase cbb3, revealed that CFN037 produces the cbb3 terminal oxidase. These data show a direct relationship between expression of thiC and production of the cbb3 terminal oxidase. This is consistent with the proposition that a purine-related metabolite, 5-aminoimidazole-4-carboxamide ribonucleotide, is a negative effector of the production of the symbiotic terminal oxidase cbb3 in R. etli.

Glycerol-assisted restorative adjustment of flavoenzyme conformation perturbed by site-directed mutagenesis

The replacement of histidine 307 with leucine in pig kidney D-amino acid oxidase perturbs its active site conformation accompanied by dramatic losses in protein-flavin interactions and enzymatic activity. However, the negative effect of this mutation on the holoenzyme structure is essentially eliminated in the presence of glycerol, resulting in up to 50% activity recovery and greater than 16-fold increase in the flavin affinity. Further analysis revealed that glycerol assists in the rearrangement of the protein toward its holoenzyme-like conformation together with reduction in the solvent-accessible protein hydrophobic area as demonstrated by limited proteolysis and use of affinity and hydrophobic probes. A substantial decrease in the protein-flavin interactions was demonstrated at a low temperature, but this reversible process was completely blocked in the presence of 40% glycerol. We suggest that the perturbation of the D-amino acid oxidase active site is due to the nonpolar nature of the mutation whose negative impact on the holoenzyme structure can be overcome by glycerol-induced strengthening of protein internal hydrophobic interactions.

Free D-aspartate and D-serine in the mammalian brain and periphery

It has long been assumed that L-forms of amino acids exclusively constitute free amino acid pools in mammals. However, a variety of studies in the last decade has demonstrated that free D-aspartate and D-serine occur in mammals and may have important physiological function in mammals. Free D-serine is confined predominantly to the forebrain structure, and the distribution and development of D-serine correspond well with those of the N-methyl-D-aspartate (NMDA)-type excitatory amino acid receptor. As D-serine acts as a potent and selective agonist for the strychnine-insensitive glycine site of the NMDA receptor, it is proposed that D-serine is a potential candidate for an NMDA receptor-related glycine site agonist in mammalian brain. In contrast, widespread and transient emergence of a high concentration of free D-aspartate is observed in the brain and periphery. Since the periods of maximal emergence of D-aspartate in the brain and periphery occur during critical periods of morphological and functional maturation of the organs, D-aspartate could participate in the regulation of these regulation of these developmental processes of the organs. This review deals with the recent advances in the studies of presence of free D-aspartate and D-serine and their metabolic systems in mammals. Since D-aspartate and D-serine have been shown to potentiate NMDA receptor-mediated transmission through the glutamate binding site and the strychnine-insensitive glycine binding site, respectively, and have been utilized extensively as potent and selective tools to study the excitatory amino acid system in the brain, we shall discuss also the NMDA receptor and uptake system of D-amino acids.

D-amino-acid oxidase is not present in the mouse liver

Since there are conflict reports on the presence of D-amino-acid oxidase in the mouse liver, this problem was examined. D-Amino-acid oxidase activity was not detected in the homogenates of the mouse liver, lung, or heart, whereas it was detected in the homogenates of the mouse kidney and brain. Western blotting showed that a protein which reacted with the antiserum against pig D-amino-acid oxidase was present in the homogenates of the mouse kidney and brain but not in those of the liver or heart. Northern hybridization using a D-amino-acid oxidase cDNA probe detected a hybridizing signal in poly(A)+ RNAs extracted from the mouse kidney and brain but not in those from the liver, heart, or lung. Reverse transcription-polymerase chain reaction using three primer pairs always amplified D-amino-acid oxidase cDNA fragments of expected sizes in the mouse kidney and brain but very rarely did so in the liver, heart, or lung. The results indicate that D-amino-acid oxidase is not present in the mouse liver in a measurable amount.

Synthesis, characterization and preliminary crystallographic data of N6-(6-carbamoylhexyl)-FAD-D-amino-acid oxidase from pig kidney, a semi-synthetic oxidase

The FAD analogue, N6-(6-carboxyhexyl)-FAD, carrying a hexanoic acid residue at the N6 position of the adenine moiety was synthesized. A new semi-synthetic oxidase, N6-(6-carbamoylhexyl)-FAD-D-amino acid oxidase, was prepared by reacting the succinimido ester of N6-(6-carboxyhexyl)-FAD with apo-D-amino-acid oxidase from pig kidney in the presence of benzoate. Reaction conditions and methods have been developed for preparing pure semi-synthetic and fully active N6-(6-carbamoylhexyl)-FAD-D-amino acid oxidase that contains 1 covalently bound FAD analogue/subunit, as verified by redialysis, ultraviolet spectrophotometry, electrospray ionization (ESI)-MS and peptide mapping. Presumably, the N6-(6-carbamoylhexyl)-FAD moiety of this semi-synthetic D-amino-acid oxidase (DAAO), selectively bound to Lys163, has a structurally similar position to that of the non-covalently bound FAD of the native holoenzyme, since both DAAO forms show very similar kinetic properties (semi-synthetic DAAO, Vmax(app) = 17.7 mumol min-1 mg-1; KM(app) = 4.5 mM; native holo-DAAO, Vmax = 12.2 mumol min-1 mg-1; KM = 1.8 mM). Compared with the native holo-D-amino acid oxidase. this new semi-synthetic N6-(6-carbamoylhexyl)-FAD-D-amino acid oxidase is a considerably more stable enzyme that shows meso-thermostability and withstands inactivation on dilution. Probably, the lack of dissociation of FAD and, consequently, the absence of the instable apoenzyme are responsible for these phenomena. Preliminary investigations resulted in finding convenient and reproducible crystallization conditions for N6-(6-carbamoylhexyl)-FAD-D-amino acid oxidase. The single crystals, obtained by the sitting-drop method using ammonium sulfate as precipitant, belong to the tetragonal space group I422 with cell dimensions a = 16.3 nm, c = 13.6 nm. The crystals diffract to 0.3-nm resolution, with two molecules being present in the asymmetric unit, demonstrating the two-subunit quarternary structure of this semi-synthetic D-amino-acid oxidase.

Purification and properties of D-aspartate oxidase from Cryptococcus humicolus UJ1

D-Aspartate oxidase (EC 1.4.3.1), which is highly specific to D-aspartate, was inducibly produced by a yeast strain which was isolated from soil and identified as Cryptococcus humicolus UJ1. The enzyme was purified to homogeneity as indicated on SDS-polyacrylamide gel electrophoresis. The molecular mass of the monomer subunit was determined to be 40 kDa. The native enzyme was suggested to be a homotetramer by its behavior on gel filtration. The enzyme was shown to be a flavoprotein by its absorption spectral properties, and the flavin was found to be tightly, but not covalently, bound FAD. The purified preparation had a specific activity of 76.1 mumol/min per mg protein with D-aspartate as substrate. Optimum pH was 7.5 and optimum temperature was around 35 degrees C. D-Glutamate was a very poor substrate for the enzyme. N-Methyl-D-aspartate was better than D-glutamate as substrate but markedly poorer than D-aspartate. Malonate was the most effective competitive inhibitor of the compounds tested. The N-terminal amino-acid sequence of the enzyme showed a significant homology with those of D-aspartate oxidases from beef kidney and Octopus vulgaris and those of D-amino-acid oxidases from various sources.

Studies on the inactivation of the flavoprotein D-amino acid oxidase from Trigonopsis variabilis

Inactivation of D-amino acid oxidase occurred by different mechanisms. The enzyme showed a rapid loss of activity in the presence of micromolar amounts of Cu2+ and Hg2+. It was also sensitive to oxidative inactivation by Fe2+ and H2O2 when both reagents were added in millimolar amounts. When oxidatively inactivated D-amino acid oxidase and a corresponding non-treated control were modified with the sulfhydryl-modifying, fluorescent reagent monobromobimane and subsequently digested with endoproteinase Glu-C, Cys-298 was identified to be a target for oxidative modification according to differences in the known peptide profile of fluorescence intensity. Another reason for the observed loss of enzyme activity in crude extracts was the specific proteolytic digestion of D-amino acid oxidase, which was dependent on the growth phase of the cells used. This cleavage was catalyzed by a serine-type proteinase and was the introductory step for the further complete degradation of the enzyme. In addition, a coenriched 50-kDa protein, identified as NADPH-specific glutamate dehydrogenase, significantly decreased the stability of the D-amino acid oxidase activity. Treatment of apo-D-amino acid oxidase from T. variabilis with monobromobimane resulted in a significantly increased fluorescence of two peptides, neither of which contained any cysteine residue. Thus, an involvement of cysteine residues in binding the FAD coenzyme should be excluded.

Reactivity of histidyl residues in D-amino acid oxidase from Rhodotorula gracilis

Incubation of D-amino acid oxidase from the yeast Rhodotorula gracilis with excess dansyl chloride at pH 6.6 and 18 degrees C caused an irreversible inactivation of D-amino acid oxidase. Benzoate, a competitive inhibitor of the enzyme, completely protected the enzyme from inactivation. The dansylated-enzyme, isolated by gel-filtration, was in part still active while the substrate specificity was altered substantially. It was completely reduced by D-alanine in anaerobiosic conditions and did stabilize the red anion semiquinone upon photochemical reduction with EDTA. The results provide evidence for the presence of essential histidyl residue(s) in the active center of the yeast enzyme.

Evidence for the functional importance of Cys298 in D-amino acid oxidase from Trigonopsis variabilis

D-Amino acid oxidase from Trigonopsis variabilis was purified to homogeneity by a combination of freeze/thawing, isoelectric precipitation and chromatography on Mono Q. This purification procedure required very little working effort. The homogeneous enzyme exhibited a ratio A280/A450 of about 6.5 and was obtained in high yield (63%) and a good stability. Using D-methionine as a substrate, a specific activity of 120 U/mg was determined colorimetrically at 26 degrees C, corresponding to 185 U/mg polarographically at 37 degrees C. Polyclonal antibodies were raised against the homogeneous protein and Western immunoblot analysis showed that the 39-kDa subunit can undergo defined cleavages at the carboxy terminus of amino acid positions 104, 106 and 108, leading to 27-kDa and 12-kDa fragments as revealed by SDS/PAGE, which are still enzymically active in their native form. The enzyme was inactivated by all sulfhydryl-modifying reagents tested. Inactivation by 5,5'-dithiobis(-2-nitrobenzoate) was correlated with a modification of up to 2 mol/mol protein of the six cysteine residues present in the monomer. Identification of the most reactive cysteine was achieved by inactivation of the enzyme with the fluorescent, sulfhydryl-modifying reagent monobromobimane. In the presence of a substrate amino acid, under anaerobic conditions, the protein could be protected from modification and, thus, inactivation by this reagent. Peptide mapping by reverse-phase chromatography of endoproteinase Glu-C-digested monobromobimane-labeled enzyme revealed one major fluorescence peak which was not obtained when the protein was modified in the presence of a substrate amino acid under anaerobic conditions. Isolation and sequencing of the labeled peptide led to the identification of Cys298 as the reactive cysteine residue.

A single-base-pair substitution abolishes D-amino-acid oxidase activity in the mouse

Mutant ddY/DAO- mice lacking D-amino-acid oxidase (DAO) activity were examined for the cause of their lack of enzyme activity. Total RNA was extracted from the kidney of the ddY/DAO- mice and cDNA was synthesized. After cDNA encoding DAO was amplified by the polymerase chain reaction it was cloned into a plasmid and sequenced. Comparison of the DAO cDNA sequence with that of normal BALB/c mice revealed the presence of a single-base substitution (G----A) which causes a Gly-181----Arg substitution in the middle of the enzyme molecule. The mutant DAO cDNA was inserted into an expression vector and was expressed in transfected COS-1 cells. The transfected cells synthesized the DAO mutant protein, but they did not show DAO activity. In contrast, when cells were transfected with an expression vector carrying wild-type DAO cDNA, where the substituted base-pair was replaced by a normal base-pair, they showed DAO activity. These results indicate that the single base-pair substitution is the cause of the loss of DAO activity in the ddY/DAO- mice.

Presence of D-amino-acid oxidase protein in mutant mice lacking D-amino-acid oxidase activity

1. Mutant mice lacking D-amino-acid oxidase activity were examined as to whether they possessed the enzyme protein. 2. Immunoblotting using an antibody against hog kidney D-amino-acid oxidase showed that kidney homogenates of the mutant mice as well as that of the normal mice had proteins reactive to the antibody. 3. Peroxisomal proteins of the kidney cells of the mutant mice were not different from those of the normal mice. 4. The peroxisomes of the mutant mice possessed a protein reactive to the antibody in the immunoblotting whose size was the same as the D-amino-acid oxidase protein present in the peroxisomes of the normal mice. 5. These results suggest that the mutant mice synthesize the D-amino-acid oxidase protein and integrate it into peroxisomes, though it is a nonfunctional enzyme.

Immunochemical relationship of D-amino acid oxidases in various tissues and animals

1. By means of an enzyme immunoassay, the contents of D-amino acid oxidase (DAO) were determined in kidney, liver, cerebellum and lung of hog, but the oxidase was not detectable in heart or cerebrum. 2. The oxidases in kidney, liver and cerebellum of hog were indistinguishable as regards immunoreactivity toward anti-hog kidney DAO antibody, specific activity and molecular weight. 3. The oxidases in rat and dog kidneys immunochemically cross-reacted with anti-hog DAO antibody. 4. The overall structure of the hog oxidase was more similar to that of the dog enzyme than that of the rat, while the structure around the catalytic site of the hog oxidase was more similar to that of the rat oxidase than that of the dog enzyme. 5. On immunoblot analysis, two forms of the oxidase were detected in extracts of hog, rat and dog kidneys.

Expression of normal and abnormal porcine kidney D-amino acid oxidase in Escherichia coli: purification and characterization of the enzymes

Expression plasmids for normal and abnormal porcine D-amino acid oxidases (E.C. 1.4.3.3, DAO) have been constructed from cloned cDNA that encodes the entire protein sequence of DAO, and the enzymes were expressed in Escherichia coli cells on a large scale. The expressed enzymes were purified to apparent homogeneity. The molecular weight of the normal DAO (38 kD) was identical with that of DAO purified from porcine kidney, whereas that of the abnormal DAO was 39 kD, which comprised the normal DAO with an additional decapeptide at its amino terminus. However, the specific activities of the two enzymes were comparable with that of natural DAO. The results indicate that the bulky decapeptide does not affect the structure necessary for the catalytic function of DAO in the amino-terminal region. The use of a GTG triplet in the 5'-untranslated region of DAO cDNA as the initiation codon for the synthesis of the abnormal DAO is suggested.

Cloning and sequencing of the peroxisomal amine oxidase gene from Hansenula polymorpha

We have cloned the AMO gene, encoding the microbody matrix enzyme amine oxidase (EC 1.4.3.6) from the yeast Hansenula polymorpha. The gene was isolated by differential screening of a cDNA library, immunoselection, and subsequent screening of a H. polymorpha genomic library. The nucleotide sequence of a 3.6 kilobase stretch of DNA containing the amine oxidase (AMO) gene was determined. The AMO gene contains an open reading frame of 692 amino acids, with a relative molecular mass of 77,435. The 5' and 3' ends of the gene were mapped and show that the transcribed region measures 2134 nucleotides. The derived amino-acid sequence was confirmed by sequencing an internal proteolytic fragment of the purified protein. Amine oxidase contains the tripeptide sequence Ser-Arg-Leu, located 9 residues from the carboxy terminus, which may represent the topogenic signal for protein import into microbodies.

Porcine D-amino acid oxidase: production of the biologically active enzyme in Escherichia coli

DNA molecules coding either for mature porcine D-amino acid oxidase or for truncated forms of the enzyme have been obtained by stepwise addition of synthetic oligonucleotides to a partial cDNA. Under the control of the lambda PL thermoregulatable promoter, these DNAs were respectively expressed in Escherichia coli as 36, 28 and 25 kilodalton polypeptides, specifically recognised by antibodies raised against the natural enzyme. None of the truncated proteins were biologically active whereas the mature recombinant species was able to hydrolyze D-alanine in vitro as efficiently as the natural product.

Properties of D-amino-acid oxidase from Rhodotorula gracilis

The flavoprotein D-amino-acid oxidase was purified to homogeneity from the yeast Rhodotorula gracilis by a highly reproducible procedure. The amino acid composition of the protein was determined; the protein monomer had a molecular mass of 39 kDa and contained one molecule of FAD. The ratio between A274/A455 was about 8.2. D-Amino-acid oxidase from yeast showed typical flavin spectral perturbations on binding of the competitive inhibitor benzoate and was reduced by D-alanine under anaerobiosis. The enzyme reacted readily with sulfite to form a covalent reversible adduct and stabilized the red anionic form of the flavin semiquinone on photoreduction in the presence of 5-deazariboflavin; the 3,4-dihydro-FAD form was not detectable after reduction with sodium borohydride. Thus D-amino-acid oxidase from yeast exhibited most of the general properties of the dehydrogenase/oxidase class of flavoproteins; at the same time, the enzyme showed some peculiar features with respect to the same protein from pig kidney.

Nucleotide sequence of the gene for cholesterol oxidase from a Streptomyces sp

The nucleotide sequence of a 2.1-kilobase-pair fragment containing the Streptomyces choA gene, which codes a secreted cholesterol oxidase, was determined. A single open reading frame encodes a mature cholesterol oxidase of 504 amino acids, with a calculated Mr of 54,913. The leader peptides extend over 42 amino acids and have the characteristics of a signal sequence, including basic amino acids near the amino terminus and a hydrophobic core near the signal cleavage site. Analyses of the total amino acid composition and amino acid sequencing of the first 21 amino acids from the N terminus of the purified extracellular enzyme agree with the values deduced from nucleotide sequencing data.

Lack of D-amino-acid oxidase activity causes a specific renal aminoaciduria in the mouse

Thin-layer chromatography and amino acid analysis showed that urine of mutant ddY/DAO- mice lacking D-amino-acid oxidase activity contained more serine, proline, alanine and methionine than that of normal ddY/DAO+ mice. Among these four, an increase in alanine was conspicuous. However, the urinary levels of 11 other amino acids and glucose were not different between the ddY/DAO- and ddY/DAO+ mice. Amino acid analysis showed that the plasma levels of serine, proline and methionine were not elevated in the ddY/DAO- mice, though a slight increase in alanine was observed. Genetic crosses showed that aminoaciduria and lack of D-amino-acid oxidase activity were concomitantly transmitted as a set through generations. These results indicated that the lack of enzyme activity caused a specific renal aminoaciduria. Whether this enzyme merely diminishes the D-amino acid load presented for reabsorption, or actually participates catalytically in the reabsorption process, remains undetermined.

Effect of site-specific mutagenesis of tyrosine-55, methionine-110 and histidine-217 in porcine kidney D-amino acid oxidase on its catalytic function

To assess the contributions of Tyr-55, Met-110 and His-217 in porcine kidney D-amino acid oxidase (EC 1.4.3.3, DAO) to its catalytic function, we constructed three mutant cDNAs coding for the enzymes possessing Phe-55, Leu-110 and Leu-217 by site-specific mutagenesis. The mutant and wild type cDNAs could be expressed in vitro with similar efficiency. The three mutant enzymes thus synthesized showed catalytic activities comparable to that of the wild type oxidase. It is concluded that Tyr-55, Met-110 and His-217 are not directly involved in the catalytic function.

Molecular cloning and sequence analysis of cDNA encoding human kidney D-amino acid oxidase

cDNA clones encoding D-amino acid oxidase were isolated from a human kidney cDNA library by hybridization with cDNA for the pig enzyme. The cDNA insert of 2.0 kilobase pairs long provided coding information for a protein consisting of 347 amino acids. The molecular mass of the enzyme was calculated to be 39,410 Da. The amino acid sequence similarity between the pig and human enzymes is 84.4%, and among the active site residues proposed from chemical modification studies, methionine-110 of the pig enzyme was replaced by threonine. Northern blot analysis confirmed the expression of an mRNA of 2.0 kilobases encoding the D-amino acid oxidase in human kidney.