D-serine is an endogenous N-methyl-D-aspartate (NMDA) receptor coagonist. It is synthesized from L-serine by serine racemase (SRR), but many aspects of its metabolism remain unclear, especially in the forebrain, which lacks active D-amino acid oxidase (DAO), the major D-serine degradative enzyme. Candidate mechanisms include SRR operating in α,β-eliminase mode (converting D-serine to pyruvate) and regulation by serine transport, in which the alanine-serine-cysteine transporter ASCT2 is implicated. Here we report studies in C6 glioma cells, which “simulate” the forebrain, in that the cells express SRR and ASCT2 but lack DAO activity. We measured D-serine, ASCT2, SRR, and DAO expression and DAO activity in two situations: after incubation of cells for 48 hr with serine isomers and after increased or decreased SRR expression by transfection and RNA interference, respectively. Incubation with serine enantiomers decreased [3H]D-serine uptake and ASCT2 mRNA and increased SRR immunoreactivity but did not alter DAO immunoreactivity, and DAO activity remained undetectable. SRR overexpression increased D-serine and pyruvate and decreased [3H]D-serine uptake and ASCT2 mRNA but did not affect DAO. SRR knockdown did not alter any of the parameters. Our data suggest that D-serine transport mediated by ASCT2 contributes prominently to D-serine homeostasis when DAO activity is absent. The factors regulating D-serine are important for understanding normal NMDA receptor function and because D-serine, along with DAO and SRR, is implicated in the pathogenesis and treatment of schizophrenia. © 2010 Wiley-Liss, Inc.
D-serine; eliminase; racemase; uptake; transporter; glia; DAAO
Roles for excitotoxicity and inflammation in Alzheimer's disease have been hypothesized. Proinflammatory stimuli, including amyloid β-peptide (Aβ), elicit a release of glutamate from microglia. We tested the possibility that a coagonist at the NMDA class of glutamate receptors, D-serine, could respond similarly.
Cultured microglial cells were exposed to Aβ. The culture medium was assayed for levels of D-serine by HPLC and for effects on calcium and survival on primary cultures of rat hippocampal neurons. Microglial cell lysates were examined for the levels of mRNA and protein for serine racemase, the enzyme that forms D-serine from L-serine. The racemase mRNA was also assayed in Alzheimer hippocampus and age-matched controls. A microglial cell line was transfected with a luciferase reporter construct driven by the putative regulatory region of human serine racemase.
Conditioned medium from Aβ-treated microglia contained elevated levels of D-serine. Bioassays of hippocampal neurons with the microglia-conditioned medium indicated that Aβ elevated a NMDA receptor agonist that was sensitive to an antagonist of the D-serine/glycine site (5,7-dicholorokynurenic acid; DCKA) and to enzymatic degradation of D-amino acids by D-amino acid oxidase (DAAOx). In the microglia, Aβ elevated steady-state levels of dimeric serine racemase, the apparent active form of the enzyme. Promoter-reporter and mRNA analyses suggest that serine racemase is transcriptionally induced by Aβ. Finally, the levels of serine racemase mRNA were elevated in Alzheimer's disease hippocampus, relative to age-matched controls.
These data suggest that Aβ could contribute to neurodegeneration through stimulating microglia to release cooperative excitatory amino acids, including D-serine.
In mammalian systems, D-serine is perhaps the most biologically active D-amino acid described to date. D-serine is a coagonist at the NMDA-receptor, and receptor activation is dependent on D-serine binding. Because D-serine binding dramatically increases receptor affinity for glutamate, it can produce excitotoxicity without any change in glutamate per se. D-serine is twofold higher in the spinal cords of mSOD1 (G93A) ALS mice, and the deletion of serine racemase (SR), the enzyme that produces D-serine, results in an earlier onset of symptoms, but with a much slower rate of disease progression. Localization studies within the brain suggest that mSOD1 and subsequent glial activation could contribute to the alterations in SR and D-serine seen in ALS. By also degrading both D-serine and L-serine, SR appears to be a prime bidirectional regulator of free serine levels in vivo. Therefore, accurate and reproducible measurements of D-serine are critical to understanding its regulation by SR. Several methods for measuring D-serine have been employed, and significant issues related to validation and standardization remain unresolved. Further insights into the intracellular transport and tissue-specific compartmentalization of D-serine within the CNS will aid in the understanding of the role of D-serine in the pathogenesis of ALS.
Recently, d-serine has been identified as an important NMDA-receptor co-agonist, which might play a role in central nervous system development. We investigated this by studying rat P19 cells, an established model for neuronal and glial differentiation. Our results show that (1) the d-serine synthesizing enzyme serine racemase was expressed upon differentiation, (2) extracellular d-serine concentrations increased upon differentiation, which was inhibited by serine racemase antagonism, and (3) inhibition of d-serine synthesis or prevention of d-serine binding to the NMDA-receptor increased synaptophysin expression and intercellular connections, supporting a role for NMDA-receptor activation by d-serine, synthesized by serine racemase, in shaping synaptogenesis and neuronal circuitry during central nervous system development. In conjunction with recent evidence from literature, we therefore suggest that d-serine deficiency might be responsible for the severe neurological phenotype seen in patients with serine deficiency disorders. In addition, this may provide a pathophysiological mechanism for a role of d-serine deficiency in psychiatric disorders.
Abundant recent evidence favors a neurotransmitter/neuromodulator role for D-serine. D-serine is synthesized from L-serine by serine racemase in astrocytic glia that ensheath synapses, especially in regions of the brain that are enriched in NMDA-glutamate receptors. D-serine is more potent than glycine at activating the ‘glycine’ site of these receptors. Moreover, selective degradation of D-serine but not glycine by D-amino acid oxidase markedly reduces NMDA neurotransmission. D-serine appears to be released physiologically in response to activation by glutamate of AMPA-glutamate receptors on D-serine-containing glia. This causes glutamate-receptor-interacting protein, which binds serine racemase, to stimulate enzyme activity and D-serine release. Thus, glutamate triggers the release of D-serine so that the two amino acids can act together on postsynaptic NMDA receptors. D-serine also plays a role in neural development, being released from Bergmann glia to chemokinetically enhance the migration of granule cell cerebellar neurons from the external to the internal granular layer.
α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate receptor; D-serine; N-methyl-D-aspartate receptor; serine racemase
Pyrobaculum islandicum is an anaerobic hyperthermophilic archaeon that is most active at 100°C. A pyridoxal 5′-phosphate-dependent serine racemase called Srr was purified from the organism. The corresponding srr gene was cloned, and recombinant Srr was purified from Escherichia coli. It showed the highest racemase activity toward l-serine, followed by l-threonine, d-serine, and d-threonine. Like rodent and plant serine racemases, Srr is bifunctional, showing high l-serine/l-threonine dehydratase activity. The sequence of Srr is 87% similar to that of Pyrobaculum aerophilum IlvA (a putative threonine dehydratase) but less than 32% similar to any other serine racemases and threonine dehydratases. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration analyses revealed that Srr is a homotrimer of a 44,000-molecular-weight subunit. Both racemase and dehydratase activities were highest at 95°C, while racemization and dehydration were maximum at pH 8.2 and 7.8, respectively. Unlike other, related Ilv enzymes, Srr showed no allosteric properties: neither of these enzymatic activities was affected by either l-amino acids (isoleucine and valine) or most of the metal ions. Only Fe2+ and Cu2+ caused 20 to 30% inhibition and 30 to 40% stimulation of both enzyme activities, respectively. ATP inhibited racemase activity by 10 to 20%. The Km and Vmax values of the racemase activity of Srr for l-serine were 185 mM and 20.1 μmol/min/mg, respectively, while the corresponding values of the dehydratase activity of l-serine were 2.2 mM and 80.4 μmol/min/mg, respectively.
Glutamate racemase activity in Bacillus anthracis is of significant interest with respect to chemotherapeutic drug design, because l-glutamate stereoisomerization to d-glutamate is predicted to be closely associated with peptidoglycan and capsule biosynthesis, which are important for growth and virulence, respectively. In contrast to most bacteria, which harbor a single glutamate racemase gene, the genomic sequence of B. anthracis predicts two genes encoding glutamate racemases, racE1 and racE2. To evaluate whether racE1 and racE2 encode functional glutamate racemases, we cloned and expressed racE1 and racE2 in Escherichia coli. Size exclusion chromatography of the two purified recombinant proteins suggested differences in their quaternary structures, as RacE1 eluted primarily as a monomer, while RacE2 demonstrated characteristics of a higher-order species. Analysis of purified recombinant RacE1 and RacE2 revealed that the two proteins catalyze the reversible stereoisomerization of l-glutamate and d-glutamate with similar, but not identical, steady-state kinetic properties. Analysis of the pH dependence of l-glutamate stereoisomerization suggested that RacE1 and RacE2 both possess two titratable active site residues important for catalysis. Moreover, directed mutagenesis of predicted active site residues resulted in complete attenuation of the enzymatic activities of both RacE1 and RacE2. Homology modeling of RacE1 and RacE2 revealed potential differences within the active site pocket that might affect the design of inhibitory pharmacophores. These results suggest that racE1 and racE2 encode functional glutamate racemases with similar, but not identical, active site features.
The antibiotic d-cycloserine is an effective inhibitor of alanine racemase. The lack of inhibition by l-cycloserine of alanine racemase from Staphylococcus aureus led Roze and Strominger to formulate the cycloserine hypothesis. This hypothesis states that d-cycloserine has the conformation required of the substrates on the enzyme surface and that l-cycloserine cannot have this conformation. Alanine racemase from Escherichia coli W has been examined to establish whether these observations are a general feature of all alanine racemases. The enzyme (molecular weight = 95,000) has Michaelis-Menten constants of 4.6 × 10−4m and 9.7 × 10−4m for d- and l-alanine, respectively. The ratio of Vmax in the d- to l-direction is 2.3. The equilibrium constant calculated from the Haldane relationship is 1.11 ± 0.15. Both d- and l-cycloserine are competitive inhibitors with constants (Ki) of 6.5 × 10−4m and 2.1 × 10−3m, respectively. The ratio of Kmd-alanine to Kid-cycloserine is 0.71, and the ratio of Kml-alanine to Kil-cycloserine is 0.46. Since l-cycloserine is an effective inhibitor, it is concluded that the cycloserine hypothesis does not apply to the enzyme from E. coli W.
d-serine is an endogenous coagonist of the N-methyl-d-aspartate subtype glutamate receptor. Genetic association studies have implicated genes coding for enzymes associated with d-serine metabolism in schizophrenia and bipolar disorder.
Protein expression of serine racemase (SR) and its binding partner, protein interacting with C-kinase (PICK1), were examined by Western blotting in brains from wildtype and PICK1 knockout mice. Levels of d-serine in wildtype and PICK1 mice were also examined by an established high-pressure liquid chromatography protocol.
Expression of SR and PICK1 proteins was developmentally regulated. Although no change was observed in the level of SR protein, levels of d-serine were selectively decreased in the forebrain of neonatal PICK1 knockout mice, compared with those in wildtype mice.
PICK1 may be involved in the regulation of brain d-serine levels and SR in a spatially and temporally specific manner.
Bipolar disorder; d-serine; knockout mice; PICK1; schizophrenia; serine racemase
The N-methyl D-aspartate (NMDA) type of glutamate receptor requires two distinct agonists to operate. Glycine is assumed to be the endogenous ligand for the NMDA receptor glycine site, but this notion has been challenged by the discovery of high levels of endogenous d-serine in the mammalian forebrain. I have outlined an evolutionary framework for the appearance of a glycine site in animals and the metabolic events leading to high levels of D-serine in brain. Sequence alignments of the glycine-binding regions, along with the scant experimental data available, suggest that the properties of invertebrate NMDA receptor glycine sites are probably different from those in vertebrates. The synthesis of D-serine in brain is due to a pyridoxal-5'-phosphate (B(6))-requiring serine racemase in glia. Although it remains unknown when serine racemase first evolved, data concerning the evolution of B(6) enzymes, along with the known occurrences of serine racemases in animals, point to D-serine synthesis arising around the divergence time of arthropods. D-Serine catabolism occurs via the ancient peroxisomal enzyme d-amino acid oxidase (DAO), whose ontogenetic expression in the hindbrain of mammals is delayed until the postnatal period and absent from the forebrain. The phylogeny of D-serine metabolism has relevance to our understanding of brain ontogeny, schizophrenia and neurotransmitter dynamics.
The NMDA receptor coagonist D-serine is important in a number of different processes in the central nervous system, ranging from synaptic plasticity to disease states, including schizophrenia. D-serine appears to be the major coagonist acting on retinal ganglion cell NMDA receptors, but the cell type from which it originates and whether its release can be modulated by activity are unknown. In this study, we utilized a mutant mouse line with elevated D-serine to investigate this question. Direct measurements of extracellular D-serine using capillary electrophoresis demonstrate that D-serine can be released from the intact mouse retina through an AMPA receptor dependent mechanism. AMPA-evoked D-serine release persisted in the presence of a cocktail of neural inhibitors but was abolished after administration of a glial toxin. These findings provide the first evidence that extracellular D-serine levels in the retina can be modulated, and that such modulation is contingent upon glial cell activity.
Retina; D-serine; NMDA receptor; AMPA receptor; capillary electrophoresis; glia
Experimental studies have demonstrated that not only dopaminergic signaling but also glutamatergic/NMDA receptor signaling play indispensable roles in the development of methamphetamine psychosis. Our recent genetic studies provided evidence that genetic variants of glutamate-related genes such as DTNBP1, GLYT1, and G72, which are involved in glutamate release and regulation of co-agonists for NMDA receptors, conferred susceptibility to methamphetamine psychosis. Serine racemase converts l-serine to d-serine, which is an endogenous co-agonist for NMDA receptors. Three single nucleotide polymorphisms (SNPs) in the promoter region of the serine racemase gene (SRR), rs224770, rs3760229, and rs408067, were proven to affect the transcription activity of SRR. Therefore, we examined these SNPs in 225 patients with methamphetamine psychosis and 291 age- and sex-matched controls. There was no significant association between methamphetamine psychosis and any SNP examined or between the disorder and haplotypes comprising the three SNPs. However, rs408067 was significantly associated with the prognosis for methamphetamine psychosis and multi-substance abuse status. The patients with C-positive genotypes (CC or CG) of rs408067 showed better prognosis of psychosis after therapy and less abuse of multiple substances than the patients with GG genotypes. Because the C allele of rs408067 reduces the expression of SRR, a lower d-serine level or reduced NMDA receptor activation may affect the prognosis of methamphetamine psychosis and multiple substance abuse. Our sample size is, however, not large enough to eliminate the possibility of a type I error, our findings must be confirmed by replicate studies with larger samples.
Methamphetamine psychosis; serine racemase; glutamate; NMDA receptors; SNP.
Schizosaccharomyces pombe has an open reading frame, which we named alr1+, encoding a putative protein similar to bacterial alanine racemase. We cloned the alr1+ gene in Escherichia coli and purified the gene product (Alr1p), with an Mr of 41,590, to homogeneity. Alr1p contains pyridoxal 5′-phosphate as a coenzyme and catalyzes the racemization of alanine with apparent Km and Vmax values as follows: for l-alanine, 5.0 mM and 670 μmol/min/mg, respectively, and for d-alanine, 2.4 mM and 350 μmol/min/mg, respectively. The enzyme is almost specific to alanine, but l-serine and l-2-aminobutyrate are racemized slowly at rates 3.7 and 0.37% of that of l-alanine, respectively. S. pombe uses d-alanine as a sole nitrogen source, but deletion of the alr1+ gene resulted in retarded growth on the same medium. This indicates that S. pombe has catabolic pathways for both enantiomers of alanine and that the pathway for l-alanine coupled with racemization plays a major role in the catabolism of d-alanine. Saccharomyces cerevisiae differs markedly from S. pombe: S. cerevisiae uses l-alanine but not d-alanine as a sole nitrogen source. Moreover, d-alanine is toxic to S. cerevisiae. However, heterologous expression of the alr1+ gene enabled S. cerevisiae to grow efficiently on d-alanine as a sole nitrogen source. The recombinant yeast was relieved from the toxicity of d-alanine.
The native l-serine deaminase (l-serine hydrolyase, deaminating, EC 184.108.40.206) of Escherichia coli K-12, which seems to be a very labile protein, is rather stable in concentrated solution. Dilution rapidly inactivates it, but in the presence of a saturating concentration of l-serine the molecule is protected from inactivation. It is a very specific enzyme; l-serine is the sole substrate with a Km value of 6.60 × 10−3m. d-Serine and l-cysteine are competitive inhibitors. Substrate saturation curves of the native enzyme show sigmoid shape, whereas the enzyme liberated from the bacteria in the presence of l-serine exhibits normal Michaelis-Menten kinetics.
D-serine is an endogenous neurotransmitter that binds to the NMDA receptor, thereby increasing the affinity for glutamate, and the potential for excitotoxicity. The primary source of D-serine in vivo is enzymatic racemization by serine racemase (SR). Regulation of D-serine in vivo is poorly understood, but is thought to involve a combination of controlled production, synaptic reuptake by transporters, and intracellular degradation by D-amino acid oxidase (DAO). However, SR itself possesses a well-characterized eliminase activity which effectively degrades D-serine as well. D-serine is increased two-fold in spinal cords of G93A SOD1 mice – the standard model of amyotrophic lateral sclerosis (ALS). ALS mice with SR disruption show earlier symptom onset, but survive longer (progression phase is slowed), in an SR-dependent manner. Paradoxically, administration of D-serine to ALS mice dramatically lowers cord levels of D-serine, leading to changes in onset and survival very similar to SR deletion. D-serine treatment also increases cord levels of the transporter Asc-1. Although the mechanism by which SOD1 mutations increases D-serine is not known, these results strongly suggest that SR and D-serine are fundamentally involved in both the presymptomatic and progression phases of disease, and offer a direct link between mutant SOD1 and a glial-derived toxic mediator.
ALS; D-serine; serine racemase; excitotoxicity; SOD1; DAO; ASC-1
An association between age-related memory impairments and changes in functional plasticity in the aging brain has been under intense study within the last decade. In this article, we show that an impaired activation of the strychnine-insensitive glycine site of N-methyl-d-aspartate receptors (NMDA-R) by its agonist d-serine contributes to deficits of synaptic plasticity in the hippocampus of memory-impaired aged rats. Supplementation with exogenous d-serine prevents the age-related deficits of isolated NMDA-R-dependent synaptic potentials as well as those of theta-burst-induced long-term potentiation and synaptic depotentiation. Endogenous levels of d-serine are reduced in the hippocampus with aging, that correlates with a weaker expression of serine racemase synthesizing the amino acid. On the contrary, the affinity of d-serine binding to NMDA-R is not affected by aging. These results point to a critical role for the d-serine-dependent pathway in the functional alterations of the brain underlying memory impairment and provide key information in the search for new therapeutic strategies for the treatment of memory deficits in the elderly.
memory; serine racemase; NMDA receptors; hippocampus; synaptic plasticity
D-serine, formed from L-serine by serine racemase (SR), is a physiologic co-agonist at NMDA receptors. Using mice with targeted deletion of SR, we demonstrate a role for D-serine in NMDA receptor mediated neurotoxicity and stroke. Brain cultures of SR deleted mice display markedly diminished nitric oxide (NO) formation and neurotoxicity. In intact SR knockout mice NO formation and nitrosylation of NO targets are substantially reduced. Infarct volume following middle cerebral artery occlusion is dramatically diminished in several regions of the brains of SR mutant mice despite evidence of increased NMDA receptor number and sensitivity.
D-serine; Serine Racemase; Stroke; NMDA receptor; Nitric Oxide; Excitotoxicity
Phenytoin elimination is a saturable process obeying Michaelis-Menten kinetics. Plasma phenytoin levels are not related linearly to dose, and small changes in enzyme activity produced by concurrent drug therapy could alter plasma levels. Two cases of phenytoin intoxication associated with simultaneous administration of diazepam are reported. Intravenous phenytoin infusions were given and the apparent Km and Vmax computed from the resulting plasma phenytoin levels. In one case `Km' and `Vmax' were 0.8 μmol/1 and 1.3 μmol/1/hour respectively during concurrent diazepam administration, and 50.3 μmol/1 and 4.4 μmol/1/hour after discontinuation of diazepam. In the second case phenytoin infusion with diazepam gave `Km' and `Vmax' values of 0.012 μmol/1 and 0.95 μmol/1/hour. Without diazepam these were 28.8 μmol/1 and 0.92 μmol/1/hour respectively.
N-methyl--aspartate (NMDA) receptors are glutamate-gated cation channels that mediate excitatory neurotransmission in the central nervous system. In addition to glutamate, NMDA receptors are also activated by coagonist binding of the gliotransmitter, -serine. Neuronal NMDA receptors mediate activity-dependent blood flow regulation in the brain. Our objective was to determine whether NMDA receptors expressed by brain endothelial cells can induce vasodilation of isolated brain arteries. Adult mouse middle cerebral arteries (MCAs) were isolated, pressurized, and preconstricted with norepinephrine. N-methyl--aspartate receptor agonists, glutamate and NMDA, significantly dilated MCAs in a concentration-dependent manner in the presence of -serine but not alone. Dilation was significantly inhibited by NMDA receptor antagonists, -2-amino-5-phosphonopentanoate and 5,7-dichlorokynurenic acid, indicating a response dependent on NMDA receptor glutamate and -serine binding sites, respectively. Vasodilation was inhibited by denuding the endothelium and by selective inhibition or genetic knockout of endothelial nitric oxide synthase (eNOS). We also found evidence for expression of the pan-NMDA receptor subunit, NR1, in mouse primary brain endothelial cells, and for the NMDA receptor subunit NR2C in cortical arteries in situ. Overall, we conclude that NMDA receptor coactivation by glutamate and -serine increases lumen diameter in pressurized MCA in an endothelial and eNOS-dependent mechanism.
-serine; eNOS; glutamate; middle cerebral artery; NMDA receptor; NR2C
The N-methyl-d-aspartate receptor co-agonist d-serine is synthesized by serine racemase and degraded by d-amino acid oxidase. Both d-serine and its metabolizing enzymes are implicated in N-methyl-d-aspartate receptor hypofunction thought to occur in schizophrenia. We studied d-amino acid oxidase and serine racemase immunohistochemically in several brain regions and compared their immunoreactivity and their mRNA levels in the cerebellum and dorsolateral prefrontal cortex in schizophrenia. d-Amino acid oxidase immunoreactivity was abundant in glia, especially Bergmann glia, of the cerebellum, whereas in prefrontal cortex, hippocampus and substantia nigra, it was predominantly neuronal. Serine racemase was principally glial in all regions examined and demonstrated prominent white matter staining. In schizophrenia, d-amino acid oxidase mRNA was increased in the cerebellum, and as a trend for protein. Serine racemase was increased in schizophrenia in the dorsolateral prefrontal cortex but not in cerebellum, while serine racemase mRNA was unchanged in both regions. Administration of haloperidol to rats did not significantly affect serine racemase or d-amino acid oxidase levels. These findings establish the major cell types wherein serine racemase and d-amino acid oxidase are expressed in human brain and provide some support for aberrant d-serine metabolism in schizophrenia. However, they raise further questions as to the roles of d-amino acid oxidase and serine racemase in both physiological and pathophysiological processes in the brain.
DAAO; DAO; d-serine; mRNA; SRR
D-serine and L-glutamate play crucial roles in excitotoxicity through N-methyl-D-aspartate receptor coactivation, but little is known about the temporal profile of efflux during cerebral ischemia. We utilized a newly designed brain slice microperfusion device coupled offline to capillary electrophoresis laser-induced fluorescence to monitor dynamic efflux of endogenous D-ser and L-glu in response to oxygen glucose deprivation (OGD) in single acute hippocampus slices. Efflux profiles with 2-min temporal resolution in response to 24-min OGD show that efflux of D-ser slightly precedes efflux of L-glu by one 2-min sampling interval. Thus both coagonists are available to activate NMDA receptors by the time when glu is released. The magnitude of D-ser efflux relative to baseline values is, however, less than that for L-glu. Peak efflux during OGD, expressed as pre-OGD baseline values, was as follows: D-ser 254% ± 24%, L-glu 1,675% ± 259%, L-asp 519% ± 128%, and L-thr 313% ± 33%. L-glutamine efflux was shown to decrease significantly in response to OGD. The microperfusion/CE-LIF approach shows several promising attributes for studying endogenous chemical efflux from single, acute brain slices.
D-serine; L-glutamate; excitotoxicity; microperfusion; stroke; ischemia; cardiac arrest; capillary electrophoresis
The uptake and degradation of nanomolar levels of [methyl-14C]choline in estuarine water samples and in seawater filtrate cultures composed mainly of natural free-living bacteria was studied. Uptake of [14C]choline exhibited Michaelis-Menten kinetics, with Kt + Sn values of 1.7 to 2.9 nM in filtrate cultures and 1.7 to 4.1 nM in estuarine-water samples. Vmax values ranged from 0.5 to 3.3 nM · h−1. The uptake system for choline in natural microbial assemblages therefore displays very high affinity and appears able to scavenge this compound at the concentrations expected in seawater. Uptake of choline was inhibited by some natural structural analogs and p-chloromercuribenzoate, indicating that the transporter may be multifunctional and may involve a thiol binding site. When 11 nM [14C]choline was added to water samples, a significant fraction (>50%) of the methyl carbon was respired to CO2 in incubations lasting 10 to 53 h. Cells taking up [14C]choline produced [14C]glycine betaine ([14C]GBT), and up to 80% of the radioactivity retained by cells was in the form of GBT, a well-known osmolyte. Alteration of the salinity in filtrate cultures affected the relative proportion of [14C]choline degraded or converted to [14C]GBT, without substantially affecting the total metabolism of choline. Increasing the salinity from 14 to 25 or 35 ppt caused more [14C]GBT to be produced from choline but less 14CO2 to be produced than in the controls. Lowering the salinity to 7 ppt decreased [14C]GBT production and increased 14CO2 production slightly. Intracellular accumulations of [14C]GBT in the salt-stressed cultures were osmotically significant (34 mM). Choline may be used as an energy substrate by estuarine bacteria and may also serve as a precursor of the osmoprotectant GBT, particularly as bacteria are mixed into higher-salinity waters.
The N-methyl-D-aspartate (NMDA) receptors play a role in behavioral abnormalities observed after administration of the psychostimulant, methamphetamine (METH). Serine racemase (SRR) is an enzyme which synthesizes D-serine, an endogenous co-agonist of NMDA receptors. Using Srr knock-out (KO) mice, we investigated the role of SRR on METH-induced behavioral abnormalities in mice.
Evaluations of behavior in acute hyperlocomotion, behavioral sensitization, and conditioned place preference (CPP) were performed. The role of SRR on the release of dopamine (DA) in the nucleus accumbens after administration of METH was examined using in vivo microdialysis technique. Additionally, phosphorylation levels of ERK1/2 proteins in the striatum, frontal cortex and hippocampus were examined using Western blot analysis. Acute hyperlocomotion after a single administration of METH (3 mg/kg) was comparable between wild-type (WT) and Srr-KO mice. However, repeated administration of METH (3 mg/kg/day, once daily for 5 days) resulted in behavioral sensitization in WT, but not Srr-KO mice. Pretreatment with D-serine (900 mg/kg, 30 min prior to each METH treatment) did not affect the development of behavioral sensitization after repeated METH administration. In the CPP paradigm, METH-induced rewarding effects were demonstrable in both WT and Srr-KO mice. In vivo microdialysis study showed that METH (1 mg/kg)-induced DA release in the nucleus accumbens of Srr-KO mice previously treated with METH was significantly lower than that of the WT mice previously treated with METH. Interestingly, a single administration of METH (3 mg/kg) significantly increased the phosphorylation status of ERK1/2 in the striatum of WT, but not Srr-KO mice.
These findings suggest first, that SRR plays a role in the development of behavioral sensitization in mice after repeated administration of METH, and second that phosphorylation of ERK1/2 by METH may contribute to the development of this sensitization as seen in WT but not Srr-KO mice.
The kinetic parameters Km, Vmax, Tt (turnover time), and v (natural velocity) were determined for H2 and acetate conversion to methane by Wintergreen Lake sediment, using short-term (a few hours) methods and incubation temperatures of 10 to 14°C. Estimates of the Michaelis-Menten constant, Km, for both the consumption of hydrogen and the conversion of hydrogen to methane by sediment microflora averaged about 0.024 μmol g−1 of dry sediment. The maximal velocity, Vmax, averaged 4.8 μmol of H2 g−1 h−1 for hydrogen consumption and 0.64 μmol of CH4 g−1 h−1 for the conversion of hydrogen to methane during the winter. Estimated natural rates of hydrogen consumption and hydrogen conversion to methane could be calculated from the Michaelis-Menten equation and estimates of Km, Vmax, and the in situ dissolved-hydrogen concentration. These results indicate that methane may not be the only fate of hydrogen in the sediment. Among several potential hydrogen donors tested, only formate stimulated the rate of sediment methanogenesis. Formate conversion to methane was so rapid that an accurate estimate of kinetic parameters was not possible. Kinetic experiments using [2-14C]acetate and sediments collected in the summer indicated that acetate was being converted to methane at or near the maximal rate. A minimum natural rate of acetate conversion to methane was estimated to be about 110 nmol of CH4 g−1 h−1, which was 66% of the Vmax (163 nmol of CH4 g−1 h−1). A 15-min preincubation of sediment with 5.0 × 10−3 atm of hydrogen had a pronounced effect on the kinetic parameters for the conversion of acetate to methane. The acetate pool size, expressed as the term Km + Sn (Sn is in situ substrate concentration), decreased by 37% and Tt decreased by 43%. The Vmax remained relatively constant. A preincubation with hydrogen also caused a 37% decrease in the amount of labeled carbon dioxide produced from the metabolism of [U-14C]valine by sediment heterotrophs.
We probed the substrate specificity of a recombinant noncovalent complex of the full-length hepatitis C virus (HCV) NS3 serine protease and NS4A cofactor, using a series of small synthetic peptides derived from the three trans-cleavage sites of the HCV nonstructural protein sequence. We observed a distinct cleavage site preference exhibited by the enzyme complex. The values of the turnover number (k(cat)) for the most efficient NS4A/4B, 4B/5A, and 5A/5B peptide substrates were 1.6, 11, and 8 min(-1), respectively, and the values for the corresponding Michaelis-Menten constants (Km) were 280, 160, and 16 microM, providing catalytic efficiency values (k(cat)/Km) of 92, 1,130, and 8,300 M(-1) s(-1). An alanine-scanning study for an NS5A/5B substrate (P6P4') revealed that P1 Cys and P3 Val were critical. Finally, substitutions at the scissile P1 Cys residue by homocysteine (Hcy), S-methylcysteine (Mcy), Ala, S-ethylcysteine (Ecy), Thr, Met, D-Cys, Ser, and penicillamine (Pen) produced progressively less efficient substrates, revealing a stringent stereochemical requirement for a Cys residue at this position.