Levels Of D-alanine Research Articles

Effects of d-alanine Intake on Amino Acid Metabolism and Kidney Function in Healthy Adults: A Multicenter, Randomized Pilot Study

Backgroundd-alanine administration prevented kidney damage in a murine acute kidney injury model. Further data are needed on the influence of d-alanine on kidney function in humans. ObjectiveThis study investigated the effects of d-alanine intake on amino acid metabolism and kidney function in healthy volunteers. MethodsThis multicenter pilot study randomly assigned individuals from the general Japanese population to receive 3 g or 6 g of d-alanine intake per day for 7 d in a 1:1 ratio. The primary endpoint was the mean change in plasma and urine d-alanine levels from baseline to 7 d after intake. The secondary endpoints were mean changes in kidney function and other clinical factors. Safety was assessed by evaluating adverse events and clinical parameters. ResultsWe randomly assigned 24 participants to the 3-g (n = 12) and 6-g d-alanine (n = 12) groups. The mean baseline estimated glomerular filtration rate (eGFR) was 73 mL/min/1.73 m2. The mean plasma d-alanine concentration increased from baseline by 77.5 ± 34.3 and 192.1 ± 80.9 nmol/mL in the 3-g and 6-g d-alanine groups (both p < 0.0001), respectively, in a dose-dependent manner (between-group difference: 114.6 nmol/mL; 95% CI: 62.1–167.2; P = 0.0002). A similar increase was observed for the urine d-alanine to creatinine ratio. The mean eGFR was elevated by 5.7 ± 8.8 mL/min/1.73 m2 in the 6-g d-alanine group (P = 0.045) but did not significantly change in the 3-g d-alanine group. Nonserious adverse events were reported in 11 participants. Conclusionsd-alanine intake increased plasma and urine d-alanine levels and was well tolerated in participants with normal kidney function. These results will be useful in future trials investigating the effects of d-alanine intake on kidney disease progression in patients with chronic kidney disease.This trial was registered at the UMIN Clinical Trials Registry as UMIN000051466.

D-alanine Inhibits Murine Intestinal Inflammation by Suppressing IL-12 and IL-23 Production in Macrophages.

Free D-amino acids, which have different functions from L-amino acids, have recently been discovered in various tissues. However, studies on the potential interactions between intestinal inflammation and D-amino acids are limited. We examined the inhibitory effects of D-alanine on the pathogenesis of intestinal inflammation. We investigated serum D-amino acid levels in 40 patients with ulcerative colitis and 34 healthy volunteers. For 7 days [d], acute colitis was induced using dextran sulphate sodium in C57BL/6J mice. Plasma D-amino acid levels were quantified in mice with dextran sulphate sodium-induced colitis, and these animals were administered D-alanine via intraperitoneal injection. IFN-γ, IL-12p35, IL-17A, and IL-23p19 mRNA expression in the colonic mucosa was measured using real-time polymerase chain reaction [PCR]. In vitro proliferation assays were performed to assess naïve CD4+ T cell activation under Th-skewing conditions. Bone marrow cells were stimulated with mouse macrophage-colony stimulating factor to generate mouse bone marrow-derived macrophages. Serum D-alanine levels were significantly lower in patients with ulcerative colitis than in healthy volunteers. Dextran sulphate sodium-treated mice had significantly lower plasma D-alanine levels than control mice. D-alanine-treated mice had significantly lower disease activity index than control mice. IFN-γ, IL-12p35, IL-17A, and IL-23p19 mRNA expression levels were significantly lower in D-alanine-administered mice than in control mice. D-alanine suppressed naïve T cell differentiation into Th1 cells in vitro, and inhibited the production of IL-12p35 and IL-23p19 in bone marrow-derived macrophages. Our results suggest that D-alanine prevents dextran sulphate sodium-induced colitis in mice and suppresses IL-12p35 and IL-23p19 production in macrophages.

D -Alanine Affects the Circadian Clock to Regulate Glucose Metabolism in the Kidney.

Key Points d-Alanine affects the circadian clock to regulate gluconeogenesis in the kidney. d-Alanine itself has a clear intrinsic circadian rhythm, which is regulated by urinary excretion, and acts on the circadian rhythm. d-Alanine is a signal activator for circadian rhythm and gluconeogenesis through circadian transcriptional network. Background The aberrant glucose circadian rhythm is associated with the pathogenesis of diabetes. Similar to glucose metabolism in the kidney and liver, d-alanine, a rare enantiomer of alanine, shows circadian alteration, although the effect of d-alanine on glucose metabolism has not been explored. Here, we show that d-alanine acts on the circadian clock and affects glucose metabolism in the kidney. Methods The blood and urinary levels of d-alanine in mice were measured using two-dimensional high-performance liquid chromatography system. Metabolic effects of d-alanine were analyzed in mice and in primary culture of kidney proximal tubular cells from mice. Behavioral and gene expression analyses of circadian rhythm were performed using mice bred under constant darkness. Results d-Alanine levels in blood exhibited a clear intrinsic circadian rhythm. Since this rhythm was regulated by the kidney through urinary excretion, we examined the effect of d-alanine on the kidney. In the kidney, d-alanine induced the expressions of genes involved in gluconeogenesis and circadian rhythm. Treatment of d-alanine mediated glucose production in mice. Ex vivo glucose production assay demonstrated that the treatment of d-alanine induced glucose production in primary culture of kidney proximal tubular cells, where d-amino acids are known to be reabsorbed, but not in that of liver cells. Gluconeogenetic effect of d-alanine has an intraday variation, and this effect was in part mediated through circadian transcriptional network. Under constant darkness, treatment of d-alanine normalized the circadian cycle of behavior and kidney gene expressions. Conclusions d-Alanine induces gluconeogenesis in the kidney and adjusts the period of the circadian clock. Normalization of circadian cycle by d-alanine may provide the therapeutic options for life style–related diseases and shift workers.

L-Alanine Exporter AlaE Functions as One of the d-Alanine Exporters in Escherichia coli.

d-amino acids have recently been found to be present in the extracellular milieu at millimolar levels and are therefore assumed to play a physiological function. However, the pathway (or potential pathways) by which these d-amino acids are secreted remains unknown. Recently, Escherichia coli has been found to possess one or more energy-dependent d-alanine export systems. To gain insight into these systems, we developed a novel screening system in which cells expressing a putative d-alanine exporter could support the growth of d-alanine auxotrophs in the presence of l-alanyl-l-alanine. In the initial screening, five d-alanine exporter candidates, AlaE, YmcD, YciC, YraM, and YidH, were identified. Transport assays of radiolabeled d-alanine in cells expressing these candidates indicated that YciC and AlaE resulted in lower intracellular levels of d-alanine. Further detailed transport assays of AlaE in intact cells showed that it exports d-alanine in an expression-dependent manner. In addition, the growth constraints on cells in the presence of 90 mM d-alanine were mitigated by the overexpression of AlaE, implying that AlaE could export free d-alanine in addition to l-alanine under conditions in which intracellular d/l-alanine levels are raised. This study also shows, for the first time, that YciC could function as a d-alanine exporter in intact cells.

Determination of D-serine and D-alanine Tissue Levels in the Prefrontal Cortex and Hippocampus of Rats After a Single Dose of Sodium Benzoate, a D-Amino Acid Oxidase Inhibitor, with Potential Antipsychotic and Antidepressant Properties.

The effects of the N-methyl-D-aspartate receptor activators D-serine, D-alanine, and sarcosine against schizophrenia and depression are promising. Nevertheless, high doses of D-serine and sarcosine are associated with undesirable nephrotoxicity or worsened prostatic cancer. Thus, alternatives are needed. DAAO inhibition can increase D-serine as well as D-alanine and protect against D-serine-induced nephrotoxicity. Although several DAAO inhibitors improve the symptoms of schizophrenia and depression, they can increase the plasma levels but not brain levels of D-serine. The mechanism of action of DAAO inhibitors remains unclear. We investigated the effects of the DAAO inhibitor sodium benzoate on the prefrontal cortex and hippocampal level of D-alanine as known another substrate with antipsychotic and antidepressant properties and other NMDAR-related amino acids, such as, L-alanine, D-serine, L-serine, D-glutamate, L-glutamate, and glycine levels. Our results indicate that sodium benzoate exerts antipsychotic and antidepressant-like effects without changing the D-serine levels in the brain prefrontal cortex (PFC) and hippocampus. Moreover, D-alanine levels in the PFC and hippocampus did not change. Despite these negative findings regarding the effects of D-amino acids in the PFC and hippocampus, sodium benzoate exhibited antipsychotic and antidepressant-like effects. Thus, the therapeutic effects of sodium benzoate are independent of D-serine or D-alanine levels. In conclusion, sodium benzoate may be effective among patients with schizophrenia or depression; however, the mechanisms of actions remain to be elucidated.

Increased levels of oral Streptococcus-derived d-alanine in patients with chronic kidney disease and diabetes mellitus

The number of patients on hemodialysis is increasing globally; diabetes mellitus (DM) complications is the major cause of hemodialysis in patients with chronic kidney disease (CKD). The d-amino acid (AA) profile is altered in patients with CKD; however, it has not been studied in patients with CKD and DM. Furthermore, bacteria responsible for altering the D-AA profile are not well understood. Therefore, we examined the D-AA profiles and associated bacteria in patients with CKD, with and without DM. We enrolled 12 healthy controls and 54 patients with CKD, with and without DM, and determined their salivary, stool, plasma, and urine chiral AA levels using two-dimensional high-performance liquid chromatography. We performed 16S rRNA gene sequencing analysis of the oral and gut microbiota to determine the association between the abundance of bacterial species and D-AA levels. Plasma d-alanine and d-serine levels were higher in patients with CKD than in healthy adults (p < 0.01), and plasma d-alanine levels were higher in patients with CKD and DM than in those without DM. The abundance of salivary Streptococcus, which produced d-alanine, increased in patients with CKD and DM and was positively correlated with plasma d-alanine levels. Patients with CKD and DM had unique oral microbiota and d-alanine profiles. Plasma d-alanine is a potential biomarker for patients with CKD and DM.

D-Alanine as a biomarker and a therapeutic option for severe influenza virus infection and COVID-19

Since the outbreak of coronavirus disease 2019 (COVID-19), biomarkers for evaluating severity, as well as supportive care to improve clinical course, remain insufficient. We explored the potential of d-amino acids, rare enantiomers of amino acids, as biomarkers for assessing disease severity and as protective nutrients against severe viral infections. In mice infected with influenza A virus (IAV) and in patients with severe COVID-19 requiring artificial ventilation or extracorporeal membrane oxygenation, blood levels of d-amino acids, including d-alanine, were reduced significantly compared with those of uninfected mice or healthy controls. In mice models of IAV infection or COVID-19, supplementation with d-alanine alleviated severity of clinical course, and mice with sustained blood levels of d-alanine showed favorable prognoses. In severe viral infections, blood levels of d-amino acids, including d-alanine, decrease, and supplementation with d-alanine improves prognosis. d-Alanine has great potentials as a biomarker and a therapeutic option for severe viral infections.

D-glutamate, D-serine, and D-alanine differ in their roles in cognitive decline in patients with Alzheimer's disease or mild cognitive impairment

D-amino acids have been recognized as bioactive substances in humans. d-Serine and D-alanine are co-agonists of N-methyl-d-aspartate receptors. Glutamate has been suggested to be involved in the pathophysiology of Alzheimer's disease (AD). This study aimed to explore the roles of amino acids, particularly D-amino acids, in cognitive decline among patients with AD or mild cognitive impairment (MCI). We enrolled 144 patients: 20 amnestic MCI, 85 mild AD, 25 moderate AD, and 14 severe AD. Serum levels of amino acids were measured by high performance liquid chromatography and confirmed by D-amino acid oxidase assay. The cognitive function was mainly evaluated by Alzheimer's Disease Assessment Scale - Cognitive Subscale (ADAS-cog). ADAS-cog total scores were positively correlated with d-serine (r = 0.186, p = 0.026) and D-/Total- serine ratio (r = 0.191, p = 0.022). ADAS-cog behavior scores were negatively correlated with D-glutamate (r = -0.177, p = 0.034) and L-glutamate (r = -0.250, p = 0.003), but positively correlated with D-alanine (r = 0.236, p = 0.005) and D-/Total- alanine ratio (r = 0.252, p = 0.002). Among the 11 tasks of ADAS-cog, D-glutamate and d-serine were correlated with different items respectively, noteworthily in the opposite direction. This is the first study suggesting that D-amino acids in blood may be correlated with ADAS-cog in different items and in the opposite direction. Lower D-glutamate and higher D-alanine levels may predict more behavioral symptoms. In summary, D-glutamate, d-serine and D-alanine play different and characteristic roles in AD. Further longitudinal studies are warranted to elucidate the function and interaction of D-amino acids in specific cognitive domains as well as various phases of dementia.

Who's Your DadA? d-Alanine Levels Regulate Bacterial Stiffness.

A central question in mechanobiology is how cellular-scale structures are established and regulated. In bacteria, the cell envelope is essential for mechanical integrity, protecting against environmental stresses and bearing the load from high turgor pressures. Trivedi et al. (mBio 9:e01340-18, 2018, https://doi.org/10.1128/mBio.01340-18) screened a Pseudomonas aeruginosa transposon library and identified genes that influence cell stiffness by measuring cell growth while cells were embedded in an agarose gel. Their findings provide a broad knowledge base for how biochemical pathways regulate cellular mechanical properties in this pathogen. Dozens of genes across diverse functional categories were implicated, suggesting that cellular mechanics is a systems-level emergent property. Furthermore, changes in d-alanine levels in a dadA (d-alanine dehydrogenase) mutant resulted in decreases in the expression of cell wall enzymes, cross-linking density, and cell stiffness. These insights into the biochemical and mechanical roles of dadA highlight the importance of systems-level investigations into the physical properties of cells.

Lack of Effect of Sodium Benzoate at Reported Clinical Therapeutic Concentration on d-Alanine Metabolism in Dogs.

Cognitive decline and psychosis have been hypothesized to be mediated by N-methyl-d-aspartate receptor (NMDAR) hypofunction. Consistent with this hypothesis, chronic treatment with d-alanine, a coagonist at the glycine site of the NMDAR, leads to an improvement of positive and cognitive symptoms in schizophrenic patients. d-alanine is oxidized by d-amino acid oxidase (DAAO); thus, an inhibitor of DAAO would be expected to enhance d-alanine levels and likewise lead to desirable clinical outcomes. Sodium benzoate, on the basis of d-amino acid inhibition, was observed to display beneficial clinical effects in schizophrenic and Alzheimer's patients. However, in the clinical pilot studies using sodium benzoate, d-amino acids were not quantified to verify that sodium benzoate's efficacy was mediated through DAAO inhibition. In this study, d-alanine content was monitored in cerebral spinal fluid (CSF) of dogs treated with daily injections of d-alanine (30 mg/kg) alone and in combination with sodium benzoate (30 mg/kg) for seven consecutive days. We reasoned that the cerebral spinal fluid d-alanine quantity is reflective of the brain d-alanine levels and it would increase as a consequence of DAAO inhibition with sodium benzoate. We found that d-alanine treatment lead to maximal concentration of 7.51 μM CSF d-alanine level; however, coadministration of sodium benzoate and d-alanine did not change CSF d-alanine level beyond that of d-alanine treatment alone. As a consequence, we conclude that clinical efficacy associated with chronic administration of sodium benzoate in schizophrenic and Alzheimer's patients is likely not mediated through inhibition of DAAO.

Effects of high-salinity seawater acclimation on the levels of d-alanine in the muscle and hepatopancreas of kuruma prawn, Marsupenaeus japonicus

Changes in d- and l-alanine contents were determined in the muscle and hepatopancreas of kuruma prawn Marsupenaeus japonicus, during acclimation from seawater containing 100% salinity to artificial seawater containing 150% salinity. In the hepatopancreas, contents of both amino acids increased by approximately threefold. The activity of alanine racemase, which catalyzes the interconversion of d- and l-alanine, also increased in the high-salinity seawater. In addition, the expression of the gene encoding alanine racemase increased in the hepatopancreas with an increase in the alanine racemase activity. These data indicate that the biosynthesis of d- and l-alanine is controlled by the gene expression level of alanine racemase, and d-alanine in the hepatopancreas functions as a major osmolyte for isosmotic regulation. In contrast, the content of d-alanine and alanine racemase activity did not change in the muscle during hyper-osmotic acclimation. Therefore, we suggest that d-alanine, which exists in the several tissues of M. japonicus, is considered to be utilized in some different physiological phenomena in different tissues.

D-Alanine in the islets of Langerhans of rat pancreas

Relatively high levels of d-alanine (d-Ala), an endogenous d-amino acid, have been found in the endocrine systems of several animals, especially in the anterior pituitary; however, its functional importance remains largely unknown. We observed d-Ala in islets of Langerhans isolated from rat pancreas in significantly higher levels than in the anterior/intermediate pituitary; specifically, 180±60fmol d-Ala per islet (300±100nmol/gislet), and 10±2.5nmol/g of wet tissue in pituitary. Additionally, 12±5% of the free Ala in the islets was in the d form, almost an order of magnitude higher than the percentage of d-Ala found in the pituitary. Surprisingly, glucose stimulation of the islets resulted in d-Ala release of 0.6±0.5fmol per islet. As d-Ala is stored in islets and released in response to changes in extracellular glucose, d-Ala may have a hormonal role.

Metabolomics Analysis Identifies d-Alanine-d-Alanine Ligase as the Primary Lethal Target of d-Cycloserine in Mycobacteria

d-Cycloserine is an effective second line antibiotic used as a last resort to treat multi (MDR)- and extensively (XDR) drug resistant strains of Mycobacterium tuberculosis . d-Cycloserine interferes with the formation of peptidoglycan biosynthesis by competitive inhibition of alanine racemase (Alr) and d-alanine-d-alanine ligase (Ddl). Although the two enzymes are known to be inhibited, the in vivo lethal target is still unknown. Our NMR metabolomics work has revealed that Ddl is the primary target of DCS, as cell growth is inhibited when the production of d-alanyl-d-alanine is halted. It is shown that inhibition of Alr may contribute indirectly by lowering the levels of d-alanine, thus allowing DCS to outcompete d-alanine for Ddl binding. The NMR data also supports the possibility of a transamination reaction to produce d-alanine from pyruvate and glutamate, thereby bypassing Alr inhibition. Furthermore, the inhibition of peptidoglycan synthesis results in a cascading effect on cellular metabolism as there is a shift toward the catabolic routes to compensate for accumulation of peptidoglycan precursors.

Decreased levels of free d-aspartic acid in the forebrain of serine racemase (Srr) knock-out mice

d-Serine, an endogenous co-agonist of the N-methyl-d-aspartate (NMDA) receptor is synthesized from l-serine by serine racemase (SRR). A previous study of Srr knockout (Srr-KO) mice showed that levels of d-serine in forebrain regions, such as frontal cortex, hippocampus, and striatum, but not cerebellum, of mutant mice are significantly lower than those of wild-type (WT) mice, suggesting that SRR is responsible for d-serine production in the forebrain. In this study, we attempted to determine whether SRR affects the level of other amino acids in brain tissue. We found that tissue levels of d-aspartic acid in the forebrains (frontal cortex, hippocampus and striatum) of Srr-KO mice were significantly lower than in WT mice, whereas levels of d-aspartic acid in the cerebellum were not altered. Levels of d-alanine, l-alanine, l-aspartic acid, taurine, asparagine, arginine, threonine, γ-amino butyric acid (GABA) and methionine, remained the same in frontal cortex, hippocampus, striatum and cerebellum of WT and mutant mice. Furthermore, no differences in d-aspartate oxidase (DDO) activity were detected in the forebrains of WT and Srr-KO mice. These results suggest that SRR and/or d-serine may be involved in the production of d-aspartic acid in mouse forebrains, although further detailed studies will be necessary to confirm this finding.

Effects of D-Amino Acid Oxidase Inhibitor on the Extracellular D-Alanine Levels and the Efficacy of D-Alanine on Dizocilpine-Induced Prepulse Inhibition Deficits in Mice

D-Alanine, one of D-amino acids present in the mammalian brain, is a selective and potent agonist at the N- methyl-D-aspartate (NMDA) receptors. Like D-serine, D-alanine is reported to be effective in the treatment of schizo- phrenia. However, orally given D-alanine is metabolized substantially by D-amino acid oxidase (DAAO), diminishing its oral bioavailability. In this study, we studied the effects of oral D-alanine administration with or without the novel DAAO inhibitor, 5-chloro-benzo(d)isoxazol-3-ol (CBIO), on the extracellular D-alanine levels in the brain and on the prepulse inhibition (PPI) deficits after administration of the NMDA receptor antagonist dizocilpine. Co-administration of CBIO (30 mg/kg) with D-alanine (100 mg/kg), but not D-alanine (100 mg/kg) alone, significantly attenuated dizocilpine (0.1 mg/kg)-induced PPI deficits in mice. The in vivo microdialysis study of the conscious and free moving mice revealed that co-administration of CBIO (30 mg/kg) significantly increased extracellular levels of D-alanine in the frontal cortex after oral administration of D-alanine (100 mg/kg). These findings suggest that co-administration of CBIO can increase the bioavailability of D-alanine after oral administration of D-alanine, and that co-administration of CBIO can enhance the ef- ficacy of D-alanine on dizocilpine-induced PPI deficits. Therefore, combination of D-alanine and a DAAO inhibitor such as CBIO offers new therapeutic potential for treatment of schizophrenia.

An assessment of the microbial contribution to aquatic dissolved organic nitrogen using amino acid enantiomeric ratios

There is increasing evidence that certain microbially-derived compounds may account for part of the aquatic dissolved organic nitrogen (DON) pool. Enantiomeric ratios of amino acids were used to assess the microbial input to the DON pool in the Florida Everglades, USA. Elevated levels of d-alanine, d-aspartic acid, d-glutamic acid and d-serine indicated the presence of peptidoglycan in the samples. The estimated peptidoglycan contribution to amino acid nitrogen ranged from 2.8 ± 0.1% to 6.4 ± 0.9%, increasing with salinity from freshwater to coastal waters. The distribution of individual d-amino acids in the samples suggests additional inputs to DON, possibly from archaea or from abiotic racemization of l-amino acids.

D-Alanine metabolism in the lucinid clam Lucinoma aequizonata

The chemoautotrophic symbiont-bearing clam Lucinoma aequizonata contains very high levels of free d-alanine in all tissues. The possible sources for this amino acid and its involvement in the clams' metabolism were investigated. Very low levels of d-alanine (generally below 1 μmol·l-1) were measured in the sediment porewaters from the habitat of the clams. Experiments with 14C-labeled tracers demonstrate an active metabolism of d-alanine in the clams rather than a role as inert waste product. d-alanine is metabolized at about 0.12 μmol·g fw-1·h-1. Label from aspartate, but not glucose and CO2, is incorporated into d-alanine. Incubation with labeled d-alanine did not result in formation of radioactive l-alanine. Tests for alanine racemase (EC 5.1.1.1) and d-amino acid oxidase (EC 1.4.3.3.) did not show activity in either gill, i.e. symbiont and host, or foot tissue. d-Alanine amino transferase (EC 2.6.1.b.) was demonstrated in gill and foot tissues. Two sources for d-alanine are proposed: a degradation of cell walls of symbiotic bacteria and production by the host using a d-specific alanine transaminase.

Amino acid levels in d-alanine-administered mutant mice lacking d-amino acid oxidase

d-Alanine was administered orally to mutant mice lacking d-amino acid oxidase (EC 1.4.3.3). The mice had free access to drinking water containing 0.5% d- or l-alanine or 0.1% d-alanine for 2 weeks. The mice were then killed, and levels of the d- and l-enantiomers of free alanine, serine, proline, glutamate, and aspartate were determined in serum, liver, kidney, cerebrum, and cerebellum tissues. d-Alanine content increased by 60-fold (liver) to 110-fold (serum, brain), although the l-alanine level did not change. The increase of serum and brain d-alanine concentrations in animals fed 0.5% d-alanine was approximately five times more than that in animals fed 0.1% d-alanine, ie, the increase was roughly d-alanine dose-dependent in these tissues. The increase due to 0.5% d-alanine administration was reduced by 50% 17 hours after administration of d-alanine was stopped. Administration-induced increases in d-alanine levels in the cerebrum and cerebellum were not less than those in the serum, suggesting that d-alanine passed the blood-brain barrier quite freely. In the liver but not in other tissues, there were slight increases in d-serine and d-proline levels after administration of d-alanine. Administration of d-alanine produced no alterations in free glutamate and aspartate levels. No d-enantiomers of alanine, serine, proline, glutamate, or aspartate were detected in the liver and kidney tissue proteins of any animals, even in the mutant mice that received 0.5% d-alanine.

The relation of d-alanine and alanine racemase activity in molluscs

1. 1. Eighteen molluscan species were examined for the presence of d-alanine and alanine racemase activity to probe the probable relation between them. 2. 2. Two bivalve species had high concentration of d-alanine and l-alanine (1:1) and showed high activities of alanine racemase. In these species, the occurrence of d-alanine could be explained by the action of alanine racemase. 3. 3. In other species, the levels of d-alanine and enzyme activity were low, and the occurrence of d-alanine did not correspond with the presence of alanine racemase activity. 4. 4. The mechanism of the occurrence of d-alanine in molluscan tissues seems to vary from species to species and seems not to be associated with the phylogenic situation or habitats of the respective species.

Presence of d-alanine in proteins of normal and Alzheimer human brain

This report constitutes the first demonstration of the presence of d-alanine in the proteins of the human nervous system. Proteins of the frontal lobe white and gray matter of human brains, both normal and Alzheimer subjects, contain d-alanine at concentrations between 0.50 and 1.28 μmol/g of wet tissue, 50–70-times lower than the concentration of d-alanine. Both white and gray matter of Alzheimer brains contain d-alanine 1.4-times higher than the respective regions of normal brains. The gray matter proteins of Alzheimer brains show a highly significant 8% decrease in total alanine content, when compared with normal brain gray matter proteins. Since Alzheimer's disease is exhibited by deterioration of the gray matter, the occurence of elevated d-alanine levels in the gray matter of Alzheimer brains is a significant discovery and raises the question whether this enantiomer causes the degeneration of the gray matter proteins in Alzheimer's disease, or whether it is an effect of the disease.