Latest developments in tryptophan metabolism: Understanding its role in B cell immunity

One of the most essential and important building blocks of life is the tryptophan amino acid. As such, the pathways surrounding its metabolism are often crucial for the maintenance of proper cell activity and homeostasis. The ratios of tryptophan to kynurenine, mainly mediated by indoleamine 2,3-dioxygenase activity, is a key parameter in the inflammation as well as immunomodulation of both aseptic and septic diseases. As a result, several studies have been published to better understand the mechanisms by which the tryptophan pathways lead to such outcomes. Many have focused on gut health and cells associated with the given environment, the majority of which constitute regulatory T cells and T helper 17 cells. However, recent studies have highlighted the role of this molecular pathway on its capacity to modulate B cells functions and humoral immunity. Accordingly, the focus of this short review is to examine the key tryptophan pathways and their impact on B cells demonstrated by those studies. A better understanding of the role of tryptophan and its metabolites is crucial for its use in disease prevention and treatments.     

Metabonomic studies on potential plasma biomarkers in rats exposed to ionizing radiation and the protective effects of Hong Shan Capsule

An ultra performance liquid chromatography coupled to mass spectrometry-based metabonomic approach, combined with pattern recognition methods including PCA, PLS-DA, RF and heatmap, has been developed to characterize the global serum metabolic profile associated with ionizing radiation (IR). As the VIP-value threshold cutoff of the metabolites was set to 2, metabolites above this threshold were filtered out as potential target biomarkers. Nineteen distinct potential biomarkers in rat plasma were identified. To further elucidate the pathophysiology of IR, related metabolic pathways have been studied. It was found that IR was closely related to disturbed fatty acid metabolism, taurine and hypotaurine metabolism, sphingolipid metabolism, purine metabolism, pyrimidine metabolism, phospholipid catabolism, tryptophan metabolism, phenylalanine metabolism, and bile acid metabolism. With the presented metabonomic method, we systematically analyzed the protective effects of Traditional Chinese Medicine Hong Shan Capsule (HSC). The results demonstrated that HSC administration could provide satisfactory effects on IR through partially regulating the perturbed metabolic pathways.     

Model of Tryptophan Metabolism,Readily Scalable Using Tissue-specific Gene Expression Data

Tryptophan is utilized in various metabolic routes including protein synthesis, serotonin, and melatonin synthesis and the kynurenine pathway. Perturbations in these pathways have been associated with neurodegenerative diseases and cancer. Here we present a comprehensive kinetic model of the complex network of human tryptophan metabolism based upon existing kinetic data for all enzymatic conversions and transporters. By integrating tissue-specific expression data, modeling tryptophan metabolism in liver and brain returned intermediate metabolite concentrations in the physiological range. Sensitivity and metabolic control analyses identified expected key enzymes to govern fluxes in the branches of the network. Combining tissue-specific models revealed a considerable impact of the kynurenine pathway in liver on the concentrations of neuroactive derivatives in the brain. Moreover, using expression data from a cancer study predicted metabolite changes that resembled the experimental observations. We conclude that the combination of the kinetic model with expression data represents a powerful diagnostic tool to predict alterations in tryptophan metabolism. The model is readily scalable to include more tissues, thereby enabling assessment of organismal tryptophan metabolism in health and disease.     

Mechanism of indoleamine 2, 3-dioxygenase inhibiting cardiac allograft rejection in mice

Indoleamine 2, 3-dioxygenase (IDO)-mediated regulation of tryptophan metabolism plays an important role in immune tolerance in transplantation, but it has not been elucidated which mechanism specifically induces the occurrence of immune tolerance. Our study revealed that IDO exerts immunosuppressive effects through two pathways in mouse heart transplantation, ‘tryptophan depletion’ and ‘tryptophan metabolite accumulation’. The synergism between IDO⁺DC and TC (tryptophan catabolic products) has stronger inhibitory effects on T lymphocyte proliferation and mouse heart transplant rejection than the two intervention factors alone, and significantly prolong the survival time of donor-derived transplanted skin. This work demonstrates that the combination of IDO⁺DC and TC can induce immune tolerance to a greater extent, and reduce the rejection of transplanted organs.     

Hydrophilic interaction and reversed-phase ultraperformance liquid chromatography TOF-MS for serum metabonomic analysis of myocardial infarction in rats and its applications

An ultra performance liquid chromatography coupled to mass spectrometry-based metabonomic approach, which utilizes both reversed-performance (RP) chromatography and hydrophilic interaction chromatography (HILIC) separations, has been developed to characterize the global serum metabolic profile associated with myocardial infarction (MI). The HILIC was found necessary for a comprehensive serum metabonomic profiling, providing complementary information to RP chromatography. By combining with partial least squares discriminant analysis, 21 potential biomarkers in rat serum were identified. To further elucidate the pathophysiology of MI, related metabolic pathways have been studied. It was found that MI was closely related to disturbed sphingolipid metabolism, phospholipid catabolism, fatty acid transportation and metabolism, tryptophan metabolism, branched-chain amino acids metabolism, phenylalanine metabolism, and arginine and proline metabolism. With the presented metabonomic method, we systematically analyzed the therapeutic effects of Traditional Chinese Medicine Sini decoction (SND). The results demonstrated that SND administration could provide satisfactory effects on MI through partially regulating the perturbed metabolic pathways.     

LC-MS based serum metabonomic analysis for renal cell carcinoma diagnosis, staging, and biomarker discovery

A LC-MS based method, which utilizes both reversed-performance (RP) chromatography and hydrophilic interaction chromatography (HILIC) separations, has been carried out in conjunction with multivariate data analysis to discriminate the global serum profiles of renal cell carcinoma (RCC) patients and healthy controls. The HILIC was found necessary for a comprehensive serum metabonomic profiling as well as RP separation. The feasibility of using serum metabonomics for the diagnosis and staging of RCC has been evaluated. One-hundred percent sensitivity in detection has been achieved, and a satisfactory clustering between the early stage and advanced-stage patients is observed. The results suggest that the combination of LC-MS analysis with multivariate statistical analysis can be used for RCC diagnosis and has potential in the staging of RCC. The MS/MS experiments have been carried out to identify the biomarker patterns that made great contribution to the discrimination. As a result, 30 potential biomarkers for RCC are identified. It is possible that the current biomarker patterns are not unique to RCC but just the result of any malignancy disease. To further elucidate the pathophysiology of RCC, related metabolic pathways have been studied. RCC is found to be closely related to disturbed phospholipid catabolism, sphingolipid metabolism, phenylalanine metabolism, tryptophan metabolism, fatty acid beta-oxidation, cholesterol metabolism, and arachidonic acid metabolism.     

Deglutarylation of glutaryl-CoA dehydrogenase by deacylating enzyme SIRT5 promotes lysine oxidation in mice

A wide range of protein acyl modifications has been identified on enzymes across various metabolic processes; however, the impact of these modifications remains poorly understood. Protein glutarylation is a recently identified modification that can be nonenzymatically driven by glutaryl-CoA. In mammalian systems, this unique metabolite is only produced in the lysine and tryptophan oxidative pathways. To better understand the biology of protein glutarylation, we studied the relationship between enzymes within the lysine/tryptophan catabolic pathways, protein glutarylation, and regulation by the deglutarylating enzyme sirtuin 5 (SIRT5). Here, we identify glutarylation on the lysine oxidation pathway enzyme glutaryl-CoA dehydrogenase (GCDH) and show increased GCDH glutarylation when glutaryl-CoA production is stimulated by lysine catabolism. Our data reveal that glutarylation of GCDH impacts its function, ultimately decreasing lysine oxidation. We also demonstrate the ability of SIRT5 to deglutarylate GCDH, restoring its enzymatic activity. Finally, metabolomic and bioinformatic analyses indicate an expanded role for SIRT5 in regulating amino acid metabolism. Together, these data support a feedback loop model within the lysine/tryptophan oxidation pathway in which glutaryl-CoA is produced, in turn inhibiting GCDH function via glutaryl modification of GCDH lysine residues and can be relieved by SIRT5 deacylation activity.     

Tryptophan Metabolism in a Patient with Phenylketonuria and Scleroderma: A Proposed Explanation of the Indole Defect in Phenylketonuria

Phenylketonuria and severe focal scleroderma were observed in a white male child. This is the first instance in which the association of these two rare disorders has been reported. Studies carried out on this patient provide a possible explanation for the abnormalities of indole metabolism in phenylketonuria. On an unrestricted diet, when serum phenylalanine levels were elevated, excessive urinary excretion of indolic tryptophan metabolites was seen 18-24 hours after oral tryptophan loading, and tryptophan was demonstrable in the stool. This was not observed when the serum phenylalanine was within normal limits on a low phenylalanine diet. Impaired intestinal tryptophan absorption secondary to elevated serum phenylalanine, by providing tryptophan substrate for bacterial degradation to indolic compounds which are absorbed and excreted in the urine, may partially explain the abnormalities of indole metabolism in phenylketonuria.     

Aerobic methylobacteria are capable of synthesizing auxins

Obligately and facultatively methylotrophic bacteria with different pathways of C1 metabolism were found to be able to produce auxins, particularly indole-3-acetic acid (IAA), in amounts of 3-100 micrograms/ml. Indole-3-pyruvic acid and indole-3-acetamide were detected only in methylobacteria with the serine pathway of C1 metabolism, Methylobacterium mesophilicum and Aminobacter aminovorans. The production of auxins by methylobacteria was stimulated by the addition of tryptophan to the growth medium and was inhibited by ammonium ions. The methylobacteria under study lacked tryptophan decarboxylase and tryptophan side-chain oxidase. At the same time, they were found to contain several aminotransferases. IAA is presumably synthesized by methylobacteria through indole-3-pyruvic acid.     

Tryptophan metabolism, from nutrition to potential therapeutic applications

Tryptophan is an indispensable amino acid that should to be supplied by dietary protein. Apart from its incorporation into body proteins, tryptophan is the precursor for serotonin, an important neuromediator, and for kynurenine, an intermediary metabolite of a complex metabolic pathway ending with niacin, CO₂, and kynurenic and xanthurenic acids. Tryptophan metabolism within different tissues is associated with numerous physiological functions. The liver regulates tryptophan homeostasis through degrading tryptophan in excess. Tryptophan degradation into kynurenine by immune cells plays a crucial role in the regulation of immune response during infections, inflammations and pregnancy. Serotonin is synthesized from tryptophan in the gut and also in the brain, where tryptophan availability is known to influence the sensitivity to mood disorders. In the present review, we discuss the major functions of tryptophan and its role in the regulation of growth, mood, behavior and immune responses with regard to the low availability of this amino acid and the competition between tissues and metabolic pathways for tryptophan utilization.     

Tryptophan Metabolism During the Menstrual Cycle

The metabolism of tryptophan and tryptophan metabolites was investigated during the follicular, luteal and premenstrual phases of the menstrual cycle in 33 healthy women across one cycle. The metabolites of all three pathways of tryptophan ie the serotonergic pathway, the pyrollase pathway and the indole acetic acid pathway, were assayed from urinary prebreakfast samples collected on a repeated measures basis. Urinary 3 hydroxy kynurenine excretion was significantly elevated in the luteal phase (p=0.030). The relative activity of the serotonergic pathway to the kynurenergic pathway (identified by the ratios 5HT+HIAA/KY+HK and 5HT/KY+HK) were significantly elevated in both the luteal and premenstrual phases compared to the follicular phase (p=0.009 and p=0.005 respectively); indicating that the kynurenergic pathway of tryptophan metabolism may modulate serotonergic metabolism (via HK) during the menstrual cycle; and that the relative and not actual levels of serotonin metabolism may be the important factor when investigating any cyclical effects of the neurotransmitter serotonin.     

The role of liver tryptophan pyrrolase in the opposite effects of chronic administration and subsequent withdrawal of drugs of dependence on rat brain tryptophan metabolism.

1. Chronic administration of morphine, nicotine or phenobarbitone has previously been shown to inhibit rat liver tryptophan pyrrolase activity by increasing hepatic [NADPH], whereas subsequent withdrawal enhances pyrrolase activity by a hormonal-type mechanism. 2. It is now shown that this enhancement is associated with an increase in the concentration of serum corticosterone. 3. Chronic administration of the above drugs enhances, whereas subsequent withdrawal inhibits, brain 5-hydroxytryptamine synthesis. Under both conditions, tryptophan availability to the brain is altered in the appropriate direction. 4. The chronic drug-induced enhancement of brain tryptophan metabolism is reversed by phenazine methosulphate, whereas the withdrawal-induced inhibition is prevented by nicotinamide. 5. The chronic morphine-induced changes in liver [NADPH], pyrrolase activity, tryptophan availability to the brain and brain 5-hydroxytryptamine synthesis are all reversed by the opiate antagonist naloxone. 6. It is suggested that the opposite effects on brain tryptophan metabolism of chronic administration and subsequent withdrawal of the above drugs of dependence are mediated by the changes in liver tryptophan pyrrolase activity. 6. Similar conclusions based on similar findings have previously been made in relation to chronic administration and subsequent withdrawal of ethanol. These findings with all four drugs are briefly discussed in relation to previous work and the mechanism(s) of drug dependence.     

Tryptophan Metabolism During the Menstrual Cycle

The metabolism of tryptophan and tryptophan metabolites was investigated during the follicular, luteal and premenstrual phases of the menstrual cycle in 33 healthy women across one cycle. The metabolites of all three pathways of tryptophan ie the serotonergic pathway, the pyrollase pathway and the indole acetic acid pathway, were assayed from urinary prebreakfast samples collected on a repeated measures basis. Urinary 3 hydroxy kynurenine excretion was significantly elevated in the luteal phase (p=0.030). The relative activity of the serotonergic pathway to the kynurenergic pathway (identified by the ratios 5HT+HIAA/KY+HK and 5HT/KY+HK) were significantly elevated in both the luteal and premenstrual phases compared to the follicular phase (p=0.009 and p=0.005 respectively); indicating that the kynurenergic pathway of tryptophan metabolism may modulate serotonergic metabolism (via HK) during the menstrual cycle; and that the relative and not actual levels of serotonin metabolism may be the important factor when investigating any cyclical effects of the neurotransmitter serotonin.     

Factors controlling the selection of tissue-specific metabolic pathways in early embryonic cells

Rana pipiens embryos from the mid-blastula to the early gastrula stage were dissociated into cell cultures, and incubated with ¹⁴C-labeled tryptophan. The uptake of the tryptophan by the cells, its incorporation into protein and its metabolism by enzymes of the serotonin and kynurenine pathways were measured as a function of time, tryptophan concentration, and embryonic stage. It was found that the intracellular concentration of tryptophan was a constant fraction of the extracellular level except for a brief period around stage , during which the cells accumulated the amino acid to a higher concentration than in the external medium. The dominant metabolic pathway of tryptophan was a function of the intracellular concentration; at the lowest levels reported here most of the tryptophan was metabolized via the kynurenine pathway; at the highest levels most was metabolized via the serotonin pathway.