Fluorescence determination of D- and L-tryptophan concentrations in rat plasma following administration of tryptophan enantiomers using HPLC with pre-column derivatization

Enantioselective metabolomics by liquid chromatography-mass spectrometry

Metabolomics strives to capture the entirety of the metabolites in a biological system by comprehensive analysis, often by liquid chromatography hyphenated to mass spectrometry. A particular challenge thereby is the differentiation of structural isomers. Common achiral targeted and untargeted assays do not distinguish between enantiomers. This may lead to information loss. An increasing number of publications demonstrate that the enantiomeric ratio of certain metabolites can be meaningful biomarkers of certain diseases emphasizing the importance of introducing enantioselective analytical procedures in metabolomics. In this work, the state-of-the-art in the field of LC-MS based metabolomics is summarized with focus on developments in the recent decade. Methodologies, tagging strategies, workflows and general concepts are outlined. Selected biological applications in which enantioselective metabolomics has documented its usefulness are briefly discussed. In general, targeted enantioselective metabolomics assays are often based on a direct approach using chiral stationary phases (CSP) with polysaccharide derivatives, macrocyclic antibiotics, chiral crown ethers, chiral ion exchangers, donor-acceptor phases as chiral selectors. Rarely, these targeted assays focus on more than 20 analytes and usually are restricted to a certain metabolite class. In a variety of cases, pre-column derivatization of metabolites has been performed, especially for amino acids, to improve separation and detection sensitivity. Triple quadrupole instruments are the detection methods of first choice in targeted assays. Here, issues like matrix effect, absence of blank matrix impair accuracy of results. In selected applications, multiple heart cutting 2D-LC (RP followed by chiral separation) has been pursued to overcome this problem and alleviate bias due to interferences. Non-targeted assays, on the other hand, are based on indirect approach involving tagging with a chiral derivatizing agent (CDA). Besides classical CDAs numerous innovative reagents and workflows have been proposed and are discussed. Thereby, a critical issue for the accuracy is often neglected, viz. the validation of the enantiomeric impurity in the CDA. The majority of applications focus on amino acids, hydroxy acids, oxidized fatty acids and oxylipins. Some potential clinical applications are highlighted.

A review of chromatographic methods for bioactive tryptophan metabolites, kynurenine, kynurenic acid, quinolinic acid, and others, in biological fluids

sKynurenine (KYN) is synthesized from an essential amino acid, tryptophan by tryptophan 2,3-dioxygenase or indoleamine 2,3-dioxygenase via N-formyl- KYN in vivo. Subsequently, KYN acts as a precursor of some neuroactive metabolites such as kynurenic acid, quinolinic acid, and an important enzyme co-factor, nicotine adenine dinucleotide. These metabolites of tryptophan are a part of the "kynurenine pathway." In addition, KYN functions as an endogenous ligand for the aryl hydrocarbon receptor, which acts as a transcription factor. The levels of tryptophan metabolites are important for the assessment of the stage of neurological disorders, and hence, have garnered significant interest for clinical diagnosis. In this review, the detection of kynurenine, kynurenic acid, quinolinic acid, and other tryptophan metabolites performed via chromatographic methods such as HPLC using UV absorbance, fluorescence, and chromatographic-mass spectrometric detection is summarized.

Modulating D-amino acid oxidase (DAAO) substrate specificity through facilitated solvent access

D-amino acid oxidase (DAAO) degrades D-amino acids to produce α-ketoacids, hydrogen peroxide and ammonia. DAAO has often been investigated and engineered for industrial and clinical applications. We combined information from literature with a detailed analysis of the structure to engineer mammalian DAAOs. The structural analysis was complemented with molecular dynamics simulations to characterize solvent accessibility and product release mechanisms. We identified non-obvious residues located on the loops on the border between the active site and the secondary binding pocket essential for pig and human DAAO substrate specificity and activity. We engineered DAAOs by mutating such critical residues and characterised the biochemical activity of the resulting variants. The results highlight the importance of the selected residues in modulating substrate specificity, product egress and enzyme activity, suggesting further steps of DAAO re-engineering towards desired clinical and industrial applications.

Half-life of Glycated Tryptophan in the Plasma of Chickens

Tryptophan, an essential amino acid, is enzymatically metabolized to two compounds, kynurenine and serotonin, and 95% of tryptophan is metabolized to kynurenine. As chickens have hyperglycemia and high temperature, tryptophan glycation occurs more easily in chickens than in mammals. Part of tryptophan is non-enzymatically converted to two types of glycated tryptophan, tryptophan-Amadori product and (1R, 3S)-1-(d-gluco-1, 2, 3, 4, 5-pentahydroxypentyl)-1,2,3,4-tetrahydro-β-carboline-3-carboxylic acid (PHP-THβC). Although these compounds are detected in the plasma of chickens, information on the half-life of PHP-THβC in the blood circulation is limited. Therefore, the present study aimed to measure the half-life of plasma PHP-THβC in chickens. PHP-THβC (114 nmol/0.2 mL/70 g body weight) was intravenously administered to chickens via the wing vein, and blood samples were collected at 0, 15, 30, 60, 180, 360, 720, and 1440 min after administration. Plasma concentrations of PHP-THβC were measured by liquid chromatography-mass spectrometry. Plasma PHP-THβC reached to a peak concentration of 16.1 βM at 30 min after administration, and then decreased rapidly to return to the physiological level (0 min) at 360 min after administration. The half-life of plasma PHP-THβC was calculated by non-linear regression analysis, and it was found to be 107 min. This study was the first to measure plasma half-life of glycated tryptophan.

Chiral recognition of tryptophan enantiomers using chitosan-capped silver nanoparticles: Scanometry and spectrophotometry approaches

A new, fast and inexpensive colorimetric sensor was developed for chiral recognition of tryptophan enantiomers using chitosan-capped silver nanoparticles. The function of the sensor was based on scanometry and spectrophotometry of the colored product of a reaction solution containing a mixture of chitosan-capped silver nanoparticles, phosphate buffer and tryptophan enantiomers. The image of the colored solution was taken using the scanometer and the corresponding color values were obtained using Photoshop software which subsequently were used for optimization of the experimental parameters as the analytical signal. Two types of color values system were investigated: RGB (red, green and blue values) and CMYK (cyan, magenta, yellow and black values). The color values indicated that L-tryptophan had better interaction than D-tryptophan with chitosan-capped silver nanoparticles. A linear relationship between the analytical signal and the concentration of L-tryptophan was obtained in the concentration range of 1.3 × 10^-5-4.6 × 10^-4molL^-1. Detection limits, were obtained to be 2.1 × 10^-6, 2.4 × 10^-6 and 3.8 × 10^-6molL^-1 for L-tryptophan based on R (red), G (green) and B (blue) values, respectively.

Chromatographic analysis of tryptophan metabolites

The kynurenine pathway generates multiple tryptophan metabolites called collectively kynurenines and leads to formation of the enzyme cofactor nicotinamide adenine dinucleotide. The first step in this pathway is tryptophan degradation, initiated by the rate-limiting enzymes indoleamine 2,3-dioxygenase, or tryptophan 2,3-dioxygenase, depending on the tissue. The balanced kynurenine metabolism, which has been a subject of multiple studies in last decades, plays an important role in several physiological and pathological conditions such as infections, autoimmunity, neurological disorders, cancer, cataracts, as well as pregnancy. Understanding the regulation of tryptophan depletion provide novel diagnostic and treatment opportunities, however it requires reliable methods for quantification of kynurenines in biological samples with complex composition (body fluids, tissues, or cells). Trace concentrations, interference of sample components, and instability of some tryptophan metabolites need to be addressed using analytical methods. The novel separation approaches and optimized extraction protocols help to overcome difficulties in analyzing kynurenines within the complex tissue material. Recent developments in chromatography coupled with mass spectrometry provide new opportunity for quantification of tryptophan and its degradation products in various biological samples. In this review, we present current accomplishments in the chromatographic methodologies proposed for detection of tryptophan metabolites and provide a guide for choosing the optimal approach.

Role of d-amino acid oxidase in the production of kynurenine pathway metabolites from d-tryptophan in mice

The kynurenine pathway (KP), the major catabolic route of the essential amino acid l-tryptophan (l-TRP), contains several neuroactive compounds, including kynurenic acid, 3-hydroxykynurenine (3-HK), and quinolinic acid (QUIN). The role of the d-enantiomer (d-TRP) in KP metabolism has received little attention so far. d-TRP can be converted to l-TRP by d-amino acid oxidase, and the same enzyme can produce d-kynurenine, a known bioprecursor of KYNA. To analyze these complex metabolic events systematically in vivo, we injected mice with d-TRP (300 mg/kg, i.p.) and examined KP metabolism in the absence or presence of the d-amino acid oxidase inhibitor 3-methylpyrazole-5-carboxylic acid (MPC; 100 mg/kg, i.p.,). After 90 min, newly formed l-TRP was recovered in plasma, liver, forebrain, and cerebellum, and MPC prevented its neosynthesis in all tissues. In the same animals, de novo production of d-kynurenine from d-TRP was also observed, but was much higher in the periphery than in the brain. d-TRP administration raised KYNA, 3-HK, and QUIN levels in all tissues examined, and KYNA production from d-TRP was significantly reduced after pre-treatment with MPC. These results indicate that catabolic routes other than those classically ascribed to l-TRP and l-kynurenine can account for the synthesis of KYNA, 3-HK and QUINin vivo. The essential amino acid l-tryptophan is catabolized via the kynurenine pathway (KP). We explored the role of the d-enantiomer in KP metabolism in mice in vivo. We report that d-tryptophan is metabolized in both brain and periphery and converted to KP metabolites, including d-kynurenine and l-kynurenine, kynurenic acid, 3-hydroxykynurenine, and quinolinic acid. Pharmacological experiments confirm the involvement of d-amino acid oxidase in these processes. Our results indicate that this enzyme participates in the synthesis of KP metabolites from d-tryptophan.

No effect of oral L-tryptophan or alpha-lactalbumin on total tryptophan levels in breast milk

Postpartum depression (PPD) is the most common complication of childbearing with a 13% prevalence rate. Sleep disturbances are also common, particularly during early postpartum. In theory, l-tryptophan could improve sleep and reduce depressed mood in early postpartum; however, the first step in clinical development of tryptophan for use in postpartum is to measure the effect of oral l-tryptophan on its concentrations in breast milk, which is presently unknown. The aims were to investigate the effect of oral l-tryptophan and alpha-lactalbumin, a protein with high tryptophan concentration, on total and free tryptophan levels in breast milk and plasma, and to compare free tryptophan levels in breast milk with those in common infant formulas. Thirty healthy breastfeeding women were randomly allocated to receive 2g or 4g of l-tryptophan, or, 20g or 40g of alpha-lactalbumin or no supplement. Free tryptophan levels were also measured in 12 different infant formulas. Total tryptophan in breast milk was unaffected by oral administration of l-tryptophan or alpha-lactalbumin (repeated measures of ANOVA (rANOVA), group effect: p=0.93). Both l-tryptophan and alpha-lactalbumin were associated with greater free tryptophan levels in breast milk (rANOVA, group effect: p<0.001) (representing 2% of total tryptophan), but these concentrations were within the range of commonly used infant formulas. In contrast to most sleep inducing medications, l-tryptophan does not affect its total concentration in breast milk. These results support further investigation of dietary l-tryptophan and alpha-lactalbumin as part of a dietary supplementation approach to address sleep disturbances in postpartum and reduce risk of PPD.

Determination of l-tryptophan and l-kynurenine in Human Serum by using LC-MS after Derivatization with (R)-DBD-PyNCS

Concentrations of l-tryptophan (l-Trp) and its metabolite, l-kynurenine (l-KYN), in sera of 19 normal subjects (age: 23.6 ± 3.5 y, male: 8, female: 11) were determined by high-performance liquid chromatography with mass-spectrometric detection, following their derivatization with (R)-(−)-4-(N, N-dimethylaminosulfonyl)-7-(3-isothiocyanatopyrrolidin-1-yl)-2,1,3-benzoxadiazole (DBD-PyNCS). A significant positive correlation between l-Trp and l-KYN concentrations was observed (r = 0.532, P < 0.05). Serum l-Trp concentration in male subjects (95.65 ± 4.27 μM) was significantly higher than that in female subjects (79.20 ± 3.34 μM; P < 0.05), while no significant differences in l-KYN concentration or the l-KYN:l-Trp ratio were observed between male and female subjects.

Alteration in the plasma concentration of a DAAO inhibitor, 3-methylpyrazole-5-carboxylic acid, in the ketamine-treated rats and the influence on the pharmacokinetics of plasma D-tryptophan

A determination method for 3-methylpyrazole-5-carboxylic acid (MPC), an inhibitor of D-amino acid oxidase (DAAO), in rat plasma was developed by using high-performance liquid chromatography-mass spectrometry (LC-MS). The structural isomer of MPC, 3-methylpyrazole-4-carboxylic acid, was used as an internal standard, and the intra- and inter-day accuracies and precisions were satisfactory for the determination of plasma MPC.Next, the LC-MS method was applied to determine the plasma MPC concentration in ketamine (Ket)-treated rats after intraperitoneal administration of MPC (5.0 or 50 mg·kg(-1)). The C(max) value of plasma MPC concentration in the Ket-treated rats was significantly higher than that in the control group when a high dose of MPC (50 mg·kg(-1)) was administered. In addition, it was found that plasma D-tryptophan (D-Trp) concentration in Ket-treated rats administered D-Trp was not significantly increased by MPC, suggesting that the DAAO-inhibitory effect of MPC is attenuated in Ket-treated rats.