Presence and origin of large amounts of d-proline in the urine of mutant mice lacking d-amino acid oxidase activity

Two-dimensional LC-MS/MS and three-dimensional LC analysis of chiral amino acids and related compounds in real-world matrices

Amino acids normally have a chiral carbon and d/l-enantiomers are present. Due to the homochirality features on the present Earth, l-enantiomers are predominant in the living beings and the d-enantiomers are rare. Along with the progress and development of cutting edge analytical methods, several d-amino acids were found even in the higher animals including humans, and their biological functions and diagnostic values have also been reported. However, the amounts of these d-amino acids are much lower than the l-forms, and development/utilization of highly sensitive and selective methods are practically essential to avoid the disturbance from uncountable intrinsic substances. In the present review, multi-dimensional HPLC methods for the determination of chiral amino acids, especially two-dimensional LC-MS/MS and three-dimensional LC methods, and their applications to a variety of real-world matrices are summarized.

Chiral Amino Acid Profiling in Serum Reveals Potential Biomarkers for Alzheimer's Disease

BACKGROUND

Alzheimer's disease (AD) is a complex neurodegenerative disease, and increasing evidence has linked dysregulation of amino acids to AD pathogenesis. However, the existing studies often ignore the chirality of amino acids, and some results are inconsistent and controversial. The changes of amino acid profiles in AD from the perspective of enantiomers remain elusive.

OBJECTIVE

The purpose of this study is to investigate whether the levels of amino acids, especially D-amino acids, are deregulated in the peripheral serum of AD patients, with the ultimate goal of discovering novel biomarkers for AD.

METHODS

The chiral amino acid profiles were determined by HPLC-MS/MS with a pre-column derivatization method. Experimental data obtained from 37 AD patients and 34 healthy controls (HC) were statistically analyzed.

RESULTS

Among the 35 amino acids detected, D-proline, D/total-proline ratio, D-aspartate, and D/total-aspartate ratio were decreased, while D-phenylalanine was elevated in AD compared to HC. Significant age-dependent increases in D-proline, D/total-proline ratio, and D-phenylalanine were observed in HC, but not in AD. Receiver operator characteristic analyses of the combination of D-proline, D-aspartate, D-phenylalanine, and age for discriminating AD from HC provided satisfactory area under the curve (0.87), specificity (97.0%), and sensitivity (83.8%). Furthermore, the D-aspartate level was significantly decreased with the progression of AD, as assessed by the Clinical Dementia Rating Scale and Mini-Mental State Examination.

CONCLUSION

The panels of D-proline, D-phenylalanine, and D-aspartate in peripheral serum may serve as novel biomarker candidates for AD. The latter parameter is further associated with the severity of AD.

Simultaneous Determination of D-amino Acids in Rat Urine by Highperformance Liquid Chromatography-tandem Mass Spectrometry Method: Application to Investigate the Clinical Value of D-amino Acids in the Early Diagnosis of Alzheimer’s Disease

Background:

D-amino acids are closely related to the development and progression of Alzheimer's disease (AD) and are expected as the novel biomarkers for AD diagnosis.

Objective:

The aim was to investigate the potential clinical value of D-amino acids for Alzheimer's disease.

Methods:

A simple and sensitive HPLC/MS-MS method was developed for the simultaneous determination of D-alanine, D-glutamine, D-proline and D-serine in rat urine. The samples were firstly pretreated by methanol, then derivatized by 7-chloro-4-nitrobenzoxadiazole with Fudosteine as internal standard, enantioseparated on Sumichiral OA-2500S column, using a mobile phase composed of acetonitrile- methanol (50:50, v/v) containing 0.5% formic acid, and detected with 4000 Qtrap MS/MS in electrospray-ionization source by negative ion mode.

Results:

The established method was successfully applied to determine the D-amino acid levels in rat urine from 20 Alzheimer's disease rats and 20 age-matched normal controls. The mean levels of Damino acids in the urine of Alzheimer's disease rats were all significantly lower than those in normal controls. Based on the contents of D-amino acids, the distinction model between Alzheimer's disease rats and normal controls was established by the Bayesian discriminant analysis.

Conclusion:

The relationship between Alzheimer's disease and D-amino acids revealed that D-amino acids would be potential biomarkers for Alzheimer’s disease.

Determination of phenylalanine enantiomers in the plasma and urine of mammals and ᴅ-amino acid oxidase deficient rodents using two-dimensional high-performance liquid chromatography

A two-dimensional (2D) HPLC system focusing on the determination of phenylalanine (Phe) enantiomers in mammalian physiological fluids has been developed. ᴅ-Phe is indicated to have potential values as a disease biomarker and therapeutic molecule in several neuronal and metabolic disorders, thus the regulation of ᴅ-Phe in mammals is a matter of interest. However, the precise determination of amino acid enantiomers is difficult in complex biological samples, and the development of an analytical method with practically acceptable sensitivity, selectivity and throughput is expected. In the present study, a 2D-HPLC system equipped with a reversed-phase column in the 1st dimension and an enantioselective column in the 2nd dimension has been designed, following the fluorescence derivatization of the target amino acid enantiomers with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The analytical method was validated using both plasma and urine samples, and successfully applied to human, rat and mouse fluids. Trace levels of ᴅ-Phe were determined in the plasma, and the %ᴅ values were around 0.1% for all species. In the urine, relatively large amounts of ᴅ-Phe were observed, and the %ᴅ values for humans, rats and mice were 3.99, 1.76 and 5.25%, respectively. The relationships between the enzymatic activity of ᴅ-amino acid oxidase (DAO) and the amounts of intrinsic ᴅ-Phe have also been clarified, and high ᴅ-Phe amounts were observed (around 0.3% in the plasma and around 50% in the urine) in the DAO deficient rats and mice.

Mouse d-Amino-Acid Oxidase: Distribution and Physiological Substrates

d-Amino-acid oxidase (DAO) catalyzes the oxidative deamination of d-amino acids. DAO is present in a wide variety of organisms and has important roles. Here, we review the distribution and physiological substrates of mouse DAO. Mouse DAO is present in the kidney, brain, and spinal cord, like DAOs in other mammals. However, in contrast to other animals, it is not present in the mouse liver. Recently, DAO has been detected in the neutrophils, retina, and small intestine in mice. To determine the physiological substrates of mouse DAO, mutant mice lacking DAO activity are helpful. As DAO has wide substrate specificity and degrades various d-amino acids, many d-amino acids accumulate in the tissues and body fluids of the mutant mice. These amino acids are d-methionine, d-alanine, d-serine, d-leucine, d-proline, d-phenylalanine, d-tyrosine, and d-citrulline. Even in wild-type mice, administration of DAO inhibitors elevates D-serine levels in the plasma and brain. Among the above d-amino acids, the main physiological substrates of mouse DAO are d-alanine and d-serine. These two d-amino acids are most abundant in the tissues and body fluids of mice. d-Alanine derives from bacteria and produces bactericidal reactive oxygen species by the action of DAO. d-Serine is synthesized by serine racemase and is present especially in the central nervous system, where it serves as a neuromodulator. DAO is responsible for the metabolism of d-serine. Since DAO has been implicated in the etiology of neuropsychiatric diseases, mouse DAO has been used as a representative model. Recent reports, however, suggest that mouse DAO is different from human DAO with respect to important properties.

Chiral amino acid metabolomics for novel biomarker screening in the prognosis of chronic kidney disease

D-Amino acids, the enantiomers of L-amino acids, are increasingly recognized as novel biomarkers. Although the amounts of D-amino acids are usually very trace in human, some of them have sporadically been detected in blood from patients with kidney diseases. This study examined whether multiple chiral amino acids would be associated with kidney functions, comorbidities, and prognosis of chronic kidney disease (CKD) by enantioselective analyses of all chiral amino acids with a micro-two-dimensional high-performance liquid chromatograph (2D-HPLC)-based analytical platform. 16 out of 21 D-amino acids were detected in plasma from 108 CKD patients in a longitudinal cohort. The levels of D-Ser, D-Pro, and D-Asn were strongly associated with kidney function (estimated glomerular filtration ratio), the levels of D-Ala and D-Pro were associated with age, and the level of D-Asp and D-Pro were associated with the presence of diabetes mellitus. D-Ser and D-Asn were significantly associated with the progression of CKD in mutually-adjusted Cox regression analyses; the risk of composite end point (developing to ESKD or death before ESKD) was elevated from 2.7- to 3.8-fold in those with higher levels of plasma D-Ser and D-Asn. These findings identified chiral amino acids as potential biomarkers in kidney diseases.

A novel chiral electrochemiluminescence sensor that can discriminate proline enantiomers

A chiral stereoselective electrochemiluminescence (ECL) sensor was constructed by using Ru–AuNPs and β-CD–rGO composites to discriminate proline enantiomers.

Chiral amino acid analysis of Japanese traditional Kurozu and the developmental changes during earthenware jar fermentation processes

Enantioselective amino acid metabolome analysis of the Japanese traditional black vinegars (amber rice vinegar, Kurozu) was performed using two-dimensional high-performance liquid chromatography combining a microbore-monolithic ODS column and narrowbore-enantioselective columns. d-Amino acids, the enantiomers of widely observed l-amino acids, are currently paid attention as novel physiologically active substances, and the foodstuffs and beverages containing high amounts of d-amino acids are the subjects of interest. In the present study, the amino acid enantiomers were determined by two-dimensional HPLC techniques after pre-column fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole. In the first dimension, the amino acid enantiomers are separated as their d plus l mixtures by the reversed-phase mode, then the d-amino acids and their l-counterparts are separately determined in the second dimension by the enantioselective columns. As a result, large amounts of d-Ala (800-4000nmol/mL), d-Asp (200-400nmol/mL) and d-Glu (150-500nmol/mL) were observed in some of the traditionally produced Kurozu vinegars. Relatively large or small amounts of d-Ser (50-100nmol/mL), d-Leu (10-50nmol/mL) and d-allo-Ile (less than 20nmol/mL) were also present in these samples. Developmental changes in the d-amino acid amounts during the fermentation and aging processes have also been investigated.

Analysis of small amounts of D-amino acids and the study of their physiological functions in mammals

D-amino acids, the enantiomers of L-amino acids, are candidates to be novel physiologically active substances and the biomarkers in mammals. However, the amounts of D-amino acids in the tissues and physiological fluids are extremely small in most cases, and sensitive and selective analytical methods are needed for their determination. In the present manuscript, we review the analytical technologies including our recent advances for the determinations of small amounts of D-amino acids in mammals and the applications to clarify their physiological meanings.

Simultaneous determination of hydrophilic amino acid enantiomers in mammalian tissues and physiological fluids applying a fully automated micro-two-dimensional high-performance liquid chromatographic concept

A validated two-dimensional HPLC system combining a microbore-monolithic ODS column and a narrowbore-enantioselective column has been established for a sensitive and simultaneous analysis of hydrophilic amino acid enantiomers (His, Asn, Ser, Gln, Arg, Asp, allo-Thr, Glu and Thr) and the non-chiral amino acid, Gly, in biological samples. To accomplish this goal, the amino acids were first tagged with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F) to the respective fluorescent NBD derivatives which were separated in the first dimension by a micro-reversed-phase column. The automatically collected fractions of the target peaks were then transferred to the second dimension consisting of a Pirkle type enantioselective column generating separation factors higher than 1.13 for all the enantiomeric target analytes. The system was validated using standard amino acids and a rat plasma sample, and analytically satisfactory calibration and precision results were obtained. The present 2D-HPLC system enables the fully automated determination of hydrophilic amino acid enantiomers in mammalian samples. The d-isomers of all the investigated 9 amino acids were found in rat urine but at various enantiomeric ratios.

D-amino acid oxidase: physiological role and applications

D-Amino acids play a key role in regulation of many processes in living cells. FAD-dependent D-amino acid oxidase (DAAO) is one of the most important enzymes responsible for maintenance proper level of D-amino acids. The most interesting and important data for regulation of the nervous system, hormone secretion, and other processes by D-amino acids as well as development of different diseases under changed DAAO activity are presented. The mechanism of regulation is complex and multi-parametric because the same enzyme simultaneously influences the level of different D-amino acids, which can result in opposing effects. Use of DAAO for diagnostic and therapeutic purposes is also considered.

Treatment with sodium benzoate leads to malformation of zebrafish larvae

Sodium benzoate (SB) is a commonly used food preservative and anti-microbial agent in many foods from soup to cereals. However, little is known about the SB-induced toxicity and teratogenicity during early embryonic development. Here, we used zebrafish as a model to test the toxicity and teratogenicity because of their transparent eggs; therefore, the organogenesis of zebrafish embryos is easy to observe. After low dosages of SB (1-1000 ppm) treatment, the zebrafish embryos exhibited a 100% survival rate. As the exposure dosages increased, the survival rates decreased. No embryos survived after treatment with 2000 ppm SB. The 50% lethal dose (LD(50)) of zebrafish is found to be in the range of 1400-1500 ppm. Gut abnormalities, malformation of pronephros, defective hatching gland and edema in pericardial sac were observed after treatment with SB. Compared to untreated littermates (vehicle-treated control), SB-treated embryos exhibited significantly reduced tactile sensitivity frequencies of touch-induced movement (vehicle-treated control: 27.60+/-1.98 v.s. 1000 ppm SB: 7.89+/-5.28; N=30). Subtle changes are easily observed by staining with specific monoclonal antibodies F59, Znp1 and alpha6F to detect morphology changes in muscle fibers, motor axons and pronephros, respectively. Our data showed that the treatment of SB led to misalignment of muscle fibers, motor neuron innervations, excess acetyl-choline receptor cluster and defective pronephric tubes. On the basis of these observations, we suggest that sodium benzoate is able to induce neurotoxicity and nephrotoxicity of zebrafish larvae.

Functional genomics and schizophrenia: endophenotypes and mutant models

This article summarizes the rationale, methods, and results of gene discovery programs in schizophrenia research and describes functional methods of investigating potential candidate genes. It focuses next on the most prominent current candidate genes and describes (1) evidence for their association with schizophrenia and research into the function of each gene; (2) investigation of the clinical phenotypes and endophenotypes associated with each gene, at the levels of psychopathologic, neurocognitive, electrophysiologic, neuroimaging, and neuropathologic findings; and (3) research into the ethologic, cognitive, social, and psychopharmacologic phenotype of mutants with targeted deletion of each gene. It examines gene-gene and gene-environment interactions. Finally, it looks at future directions for research.

Sensitive Two-Dimensional Determination of Small Amounts of D-Amino Acids in Mammals and the Study on Their Functions

D-amino acids are the candidates of novel physiologically active substances and the marker molecules of diseases in mammals. In the present study, the two-dimensional determination of small amounts of D-amino acids in mammals has been performed after sensitive pre-column fluorescence derivatization with 4-fluoro-7-nitro-2,1,3-benzoxadiazole (NBD-F). The two-dimensional HPLC system includes the isolation of the target amino acids as D+L mixtures using the micro-ODS column, and the determination of the enantiomers using the chiral column. This method enables the sensitive and selective determination of small amounts of D-amino acids in mammals without the interference of L-amino acids and peptides, and the presence and distribution of D-Leu, D-Ala, D-Pro, D-Thr and D-allo-Thr has been demonstrated in rats and mice. The regulation and the origins of these D-amino acids, and their relationships to the biological rhythms are also discussed.