Measurement of kynurenine pathway metabolites by tandem mass spectrometry

The Biology and Biochemistry of Kynurenic Acid, a Potential Nutraceutical with Multiple Biological Effects

Kynurenic acid (KYNA) is an antioxidant degradation product of tryptophan that has been shown to have a variety of cytoprotective, neuroprotective and neuronal signalling properties. However, mammalian transporters and receptors display micromolar binding constants; these are consistent with its typically micromolar tissue concentrations but far above its serum/plasma concentration (normally tens of nanomolar), suggesting large gaps in our knowledge of its transport and mechanisms of action, in that the main influx transporters characterized to date are equilibrative, not concentrative. In addition, it is a substrate of a known anion efflux pump (ABCC4), whose in vivo activity is largely unknown. Exogeneous addition of L-tryptophan or L-kynurenine leads to the production of KYNA but also to that of many other co-metabolites (including some such as 3-hydroxy-L-kynurenine and quinolinic acid that may be toxic). With the exception of chestnut honey, KYNA exists at relatively low levels in natural foodstuffs. However, its bioavailability is reasonable, and as the terminal element of an irreversible reaction of most tryptophan degradation pathways, it might be added exogenously without disturbing upstream metabolism significantly. Many examples, which we review, show that it has valuable bioactivity. Given the above, we review its potential utility as a nutraceutical, finding it significantly worthy of further study and development.

Kynurenine Pathway Dysregulation and Pain Perception in Acute Pancreatitis: Has the Connection Unraveled?

AIM

Tryptophan (TRP), an essential amino acid, undergoes catabolism through various pathways. Notably, the kynurenine pathway (KP), constituting one of these pathways, exhibits a unidirectional impact on immune response and energy metabolism. Nonetheless, its influence on pain sensation is characterized by biphasic dynamics. This study aims to scrutinize the influence of the KP pathway on pain sensation, particularly within the context of pancreatic inflammation.

METHODS

Our prospective case-control study involved individuals diagnosed with acute pancreatitis and a control group matched for gender and age. The patient cohort was subsequently subdivided into severe and non-severe subgroups. To assess metabolites within KP, two blood samples were collected from the patient cohort, one at the time of diagnosis and another during the recovery phase. Furthermore, for pain quantification, daily pain scores utilizing the Visual Analog Scale (VAS) were extracted from the patients' medical records.

RESULTS

The study incorporated 30 patients along with an equivalent number of controls. A noticeable distinction was evident between the patient and control groups, characterized by an increase in kynurenine levels and a decrease in the tryptophan/kynurenine ratio. Throughout the process of disease recovery, a uniform decrease was observed in all KP metabolites, excluding 3-Hydroxykynurenine. Elevated levels of Kynurenic acid (KYNA) were correlated with increased pain scores. Critically, no apparent distinctions in KP metabolites were discerned concerning pain severity in patients with comorbidities characterized by neural involvement.

CONCLUSION

Based on our results, the kynurenine pathway (KP) is activated in instances of acute pancreatitis. Elevated levels of KYNA were found to be associated with heightened pain scores. The operative stages within the KP responsible for pain modulation are impaired in cases characterized by neuropathy-induced pain sensation.

Effects of Different Protein Sources on Amino Acid Absorption and Plasma Appearance of Tryptophan, Large Neutral Amino Acids, and Tryptophan Metabolites in Pigs

BACKGROUND

Absorption of tryptophan (TRP) across the gut epithelium is potentially modulated by competing large neutral amino acids (LNAAs), which could affect the appearance of TRP and its metabolites in the bloodstream.

OBJECTIVES

This study aimed to determine, in a growing pig model of an adult human, the absorption of TRP and other LNAAs from the gastrointestinal tract, and plasma appearance of TRP, LNAAs, and TRP metabolites, in response to dietary proteins varying in TRP content.

METHODS

Pigs were adapted for 7 d to each of 4 diets that differed in their protein source and TRP content: 1) alpha-lactalbumin (AL; 9.95 mg TRP/g diet DM), 2) whey protein (6.59 mg TRP/g), 3) casein (3.73 mg TRP/g), or 4) zein (0.14 mg TRP/g). On day 8, pigs were euthanised after a 12-h fast (baseline), or 1, 2, 3, 4, or 6 h after they received a test meal consisting of 45 g protein, or a protein-free meal (n = 6 pigs at each time in each meal group). Tryptophan and LNAA absorption from the small intestine, and appearance of TRP, LNAAs, and TRP metabolites (melatonin, serotonin, kynurenine pathway metabolites), in the portal vein and systemic circulation, were determined.

RESULTS

AL intake resulted in sustained elevated plasma TRP concentrations after an overnight fast. The amount of TRP absorbed was dose-dependently related to protein TRP content (P = 0.028), with fastest rates for pigs fed AL (371 mg/h). Portal and systemic plasma TRP, TRP/LNAA, and the TRP metabolites were highest (P ≤ 0.05) after AL intake, and remained above baseline levels for ∼4 h postprandially. Absorption rates of TRP correlated with postprandial plasma TRP and TRP metabolites (P ≤ 0.05).

CONCLUSIONS

In adult humans, postprandial plasma TRP and TRP metabolite concentrations can likely be modulated by the TRP content of the meal.

Impedimetric detection of gut-derived metabolites using 2D Germanene-based materials

Apart from the extensively researched graphene under the Group 14 2D materials, monolayered germanene and its derivatives have been gaining interest lately as alternative class of 2D materials owing to their facile synthesis, and attractive electronic and optical properties. Herein, three different functionalized germanene-based nanomaterials, namely Ge-H, Ge-CH3 and Ge-C3-CN were investigated on their novel incorporation in impedimetric immunosensors for the detection of gut-derived metabolites associated with neurological diseases, such as kynurenic acid (KA) and quinolinic acid (QA). The designed germanene-based immunosensor relies on an indirect competitive mechanism using disposable electrode printed chips. The competition for a fixed binding site of a primary antibody occurs between the bovine serum albumin-conjugated antigens on the electrode surface and the free antigens in the solution. Among the three materials, Ge-H displayed superior bioanalytical performance in KA and QA detection. Lower limits of detection of 5.07-11.38 ng/mL (26.79-68.11 nM) were attained for KA and QA with a faster reaction time than previously reported methods. Also, minimal cross-reactivity with interfering compounds, good reproducibility in impedimetric responses (RSD = 2.43-7.51 %) and long-term stability up to a month at 4 °C were the other attributes that the proposed Ge-H competitive impedimetric immunosensor has accomplished. The application of the developed Ge-H immunosensor to serum samples allowed an accurate KA and QA quantification at physiologically relevant levels. This work serves as a stepping-stone in the development of germanene-based nanomaterials for their implementation into cost-effective, miniaturized, portable and rapid impedimetric immunosensors, which are highly desirable for point-of-care testing in clinical settings.

Altered kynurenine pathway metabolism and association with disease activity in patients with systemic lupus

Systemic lupus erythematosus (SLE) is an autoimmune disease accompanied by increased release of proinflammatory cytokines that are known to activate the indoleamine 2,3-dioxygenase (IDO-1) enzyme, which catalyzes the rate-limiting step of the kynurenine pathway (KP). This study aimed to measure KP metabolite levels in patients with SLE and investigate the relationship between disease activity, clinical findings, and KP. The study included 100 patients with SLE and 100 healthy controls. Serum tryptophan (TRP), kynurenine (KYN), kynurenic acid (KYNA), 3-hydroxyanthranilic acid (3HAA), 3-hydroxykynurenine (3HK), quinolinic acid (QA) concentrations were measured with tandem mass spectrometry. Serum KYN, KYNA, 3HAA, 3HK, and QA levels of the patients with SLE were significantly higher than the control group. Serum QA levels were elevated in patients with neurological involvement (four patients with peripheral neuropathy and two patients with mononeuropathy), serum KYN levels and KYN/TRP ratio increased in patients with joint involvement, and serum KYN, 3HK, and 3HAA levels and the KYN/TRP ratio were increased in patients with renal involvement. Moreover, KYN and KYN/TRP ratios were positively correlated with the disease activity score. These findings indicated that imbalances in KP metabolites may be associated with the pathogenesis, activation, and clinical manifestations of SLE.