Peripheral tryptophan-kynurenine pathway dysfunction in first-episode schizophrenia

Abstract

The tryptophan (TRP)-kynurenine (KYN) pathway is involved in the pathogenesis of schizophrenia. This study aimed to investigate the levels of TRP-KYN metabolites in serum and urine of patients with first-episode schizophrenia (FES) and their association with clinical manifestations. This study included 38 drug-naive patients with FES and 43 healthy controls (HCs). Clinical symptoms were evaluated using the Positive and Negative Syndrome Scale (PANSS). Levels of TRP-KYN metabolites in serum and urine were quantified. Patients with FES showed significantly higher serum quinolinic acid/kynurenic acid (QUIN/KYNA) ratio and urine KYN/TRP ratio compared to HCs, while neuroprotective metabolites, including serum KYNA, xanthurenic acid (XA), and urine picolinic acid (PIC) levels, were significantly reduced, along with a decreased urine PIC/QUIN ratio (p < 0.05). The urine KYNA/KYN ratio was negatively correlated with PANSS general psychopathology scores (r = -0.35, p = 0.04) and with PANSS total scores (r = -0.35, p = 0.046). Patients with FES exhibited dysregulation of the peripheral TRP-KYN pathway, characterized by an increased neurotoxic-to-neuroprotective QUIN/KYNA ratio and reduced levels of neuroprotective metabolites. This shift towards increased neurotoxic product generation suggests that the dysregulation of the TRP-KYN pathway could play a role in the pathophysiology of schizophrenia.

Supplementary Information

The online version contains supplementary material available at 10.1038/s41598-025-86390-4.

Introduction

Schizophrenia is a complex psychiatric disorder characterized by a spectrum of psychotic symptoms, including delusions, hallucinations, negative symptoms, and cognitive deficits¹. Despite substantial progress in understanding this multifaceted condition, the precise mechanisms underlying schizophrenia remain partially elucidated. The etiology of schizophrenia is understood to result from an complex interplay of genetic, environmental, neurochemical, and neuroanatomical factors². Recent studies have increasingly focused on the dysregulation of the tryptophan (TRP)-kynurenine (KYN) metabolic pathway as a critical component in the pathogenesis of schizophrenia^3–5.

Approximately 95% of TRP catabolism occurs via the KYN pathway^5,6. TRP, an essential amino acid predominantly sourced from diet, is metabolized into N-formylkynurenine by the enzymes indoleamine 2,3-dioxygenase (IDO) and tryptophan 2,3-dioxygenase, and then further converted to KYN⁷. KYN is subsequently processed into KYNA and 3-hydroxykynurenine (3-HK) by kynurenine aminotransferase (KAT) and kynurenine 3-monooxygenase (KMO), respectively⁸. 3-HK is further metabolized into quinolinic acid (QUIN) and xanthurenic acid (XA)⁷. QUIN, an N-methyl-D-aspartate receptors (NMDARs) agonist, is neurotoxic and can induce neuronal damage⁹, whereas KYNA and XA possess neuroprotective effects and may impact neuropsychiatric conditions^10–12. Additionally, picolinic acid (PIC), another downstream metabolite of KYN, also shows neuroprotective and immunomodulatory properties¹³.

The KYN pathway in the brain is intricately connected to the peripheral KYN pathway, with approximately 40% of KYN synthesized in the brain and 60% derived from peripheral sources that cross the blood-brain barrier (BBB)^3,14. While TRP, KYN, and 3-HK can cross the BBB and directly influence brain KYN metabolism, KYNA and QUIN are restricted from crossing the BBB due to their polar nature^3,14. In schizophrenia, dysfunction in the TRP-KYN pathway is significantly influenced by inflammatory mediators that activate this pathway and enhance the conversion of TRP to KYN^14,15. Specifically, kynurenic acid (KYNA), a metabolite of KYN, acts as an antagonist at NMDARs and alpha7 nicotinic acetylcholine receptors (alpha7nAChRs). Increased KYNA levels lead to increased inhibition of α7nAChRs, thereby disrupting the balance of glutamate, dopamine, and acetylcholine systems—neurotransmitter systems implicated in schizophrenia³.

Patients with schizophrenia exhibited significantly elevated levels of KYN and KYNA in cerebrospinal fluid^16,17 and, in post-mortem studies, in prefrontal cortex tissue^18,19. The plasma KYN/TRP ratio was inversely correlated with dorsolateral prefrontal cortex volume¹⁹ and frontal glutamate levels²⁰ in schizophrenia, indicating that peripheral KYN levels may serve as biomarkers for brain changes associated with disruptions in cortical neurotransmission. Additionally, stress-induced increases in salivary KYNA were negatively correlated with anterior cingulate cortex glutamate levels in schizophrenia²¹, suggesting a link between peripheral KYNA responses and central neurotransmitter alterations. These findings highlight the potential of KYN pathway metabolites as peripheral biomarkers of brain dysfunction in schizophrenia^3,22.

Schizophrenia is characterized by disruptions in the peripheral KYN pathway, including elevated KYN levels and reduced TRP levels, which suggest accelerated TRP metabolism via the KYN pathway^22,23. Additionally, schizophrenia is associated with decreased levels of neuroprotective metabolites like KYNA and XA and increased levels of neurotoxic metabolites such as 3-HK and QUIN in peripheral blood^23–25. Elevated levels of KYN and its downstream metabolites in serum were associated with psychotic symptoms and cognitive deficits in schizophrenia²⁶. Notably, peripheral QUIN levels were correlated with the severity of negative symptoms²⁵, and elevated peripheral 3-HK levels were reduced with antipsychotic treatment, reflecting an improvement in psychotic symptoms²⁷. These findings suggest a pathological shift towards increased neurotoxic metabolite production in schizophrenia.

Although peripheral blood metabolite levels provide insights into overall metabolic states, urinary metabolite levels—filtered and partially excreted by the kidneys—offer additional perspectives, contributing to a more comprehensive understanding of metabolic status. Previous urinary metabolomics studies have revealed disruptions in KYN metabolism in psychiatric disorders, emphasizing the significance of the TRP-KYN pathway in these conditions^28,29. However, research has predominantly focused on blood TRP-KYN metabolites, with limited studies examining both peripheral blood and urinary TRP-KYN metabolites in schizophrenia.

In this study, we aimed to measure serum and urine TRP-KYN metabolites in patients with first-episode schizophrenia (FES) and healthy controls (HCs). By comparing metabolite levels between these groups and exploring their correlations with clinical symptoms, we sought to identify potential biomarkers for schizophrenia. We hypothesized that alterations in serum and urine TRP-KYN metabolites reflected an imbalance between neurotoxic and neuroprotective factors and correlated with psychotic symptoms, thereby elucidating their role in schizophrenia pathophysiology.

Materials and methods

Participants

This study enrolled thirty-eight patients with schizophrenia, diagnosed according to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), and forty-three demographically matched healthy controls (HCs). All participants were right-handed and aged between 18 and 45 years. The patient cohort consisted of drug-naive individuals with a disease duration of less than two years. See Supplementary Information for further details. Informed consent was obtained from all participants or their legal guardians. This study was performed in accordance with the principles of the Declaration of Helsinki and approved by the Ethics Committee of the Affiliated Mental Health Centre & Hangzhou Seventh People’s Hospital, Zhejiang University School of Medicine (approval number: 2022KY990). All methods were conducted in accordance with relevant guidelines and regulations.

Clinical interview and measurement

Participants underwent face-to-face clinical interviews conducted by trained psychiatrists. Psychotic symptoms were assessed using the Positive and Negative Syndrome Scale (PANSS). All psychiatrists received comprehensive training before the study to ensure consistency in PANSS ratings. The inter-rater reliability for PANSS total scores was confirmed with an intraclass correlation coefficient exceeding 0.75.

Measurement of serum and urine TRP-KYN metabolite levels

Peripheral blood samples (4 mL) were collected from fasting participants after a 4-hour fasting period, using vacuum-sealed tubes. The samples were immediately processed with an Eppendorf 5430R centrifuge at 1500 rpm and 4 °C for 15 min. The serum was aliquoted into Eppendorf tubes and stored at -80 °C. Concurrent midstream urine (5 mL) was collected, centrifuged at 3500 rpm for 5 min at room temperature, and the supernatant was also stored at -80 °C. TRP-KYN metabolites were quantified using ultra-high performance liquid chromatography-tandem triple quadrupole mass spectrometry (UHPLC-QTRAP). Separation was performed on an Agilent 1290 Infinity UHPLC system, and detection was carried out using a SCIEX 5500 QTRAP mass spectrometer in both positive and negative ion modes. Quantification was carried out using Multiquant 3.0.2 software. The metabolites analyzed included TRP, KYN, KYNA, QUIN, PIC, and XA. Five metabolite ratios (KYN/TRP, QUIN/KYN, KYNA/KYN, QUIN/KYNA, and PIC/QUIN) were calculated to estimate the activity of specific enzymatic steps and branches of the pathway.

Statistical analysis

Statistical analysis was conducted using SPSS version 22. Data normality was assessed using Q-Q plots and the Shapiro-Wilk test. Variables including education level, disease duration, and metabolite concentrations showed deviations from normality. Demographic differences between patients with FES and HCs were compared using two-sample t-tests, Mann-Whitney U tests, or chi-square tests, as appropriate. Normally distributed variables are reported as means with standard deviations (SD), while non-normally distributed variables are reported as medians with interquartile ranges (IQR; 25th-75th percentiles). Partial correlation analysis was conducted to investigate the relationships between serum and urine TRP-KYN metabolites and psychotic symptoms, controlling for age, sex, body mass index (BMI), education level, and disease duration. Logistic regression analysis was employed to evaluate the impact of serum and urine TRP-KYN metabolite levels with significant differences between groups, adjusting for age, sex, BMI, and education as covariates. The results are reported as odds ratios (OR) with 95% confidence intervals (CI). The significance level was set at α = 0.05 for all statistical tests (two-tailed).

Results

Demographic and clinical characteristics

The FES and HC groups were matched for age, sex, and BMI (see Table 1). A significant difference in education level was observed (p < 0.05), with the FES group exhibiting a lower education level compared to the HC group.

Table 1.

Demographic and clinical characteristics between groups.

Variables	Patients (n = 38)	Controls (n = 43)	Statistics
Age (years), Mean (SD)	24.79 (4.36)	26.09 (3.55)	t = 1.48
Sex (M/F)	23/15	24/19	χ2 = 0.18
BMI (kg/m2), Mean (SD)	21.83 (2.54)	21.55 (2.50)	t = -0.50
Education (years), Median (IQR)	13 (9–14)	16 (14–16)	Z = -5.79*
Disease duration (months), Median (IQR)	6 (1–12)	NA	NA
PANSS
Positive symptoms, Mean (SD)	18.89 (5.52)	NA	NA
Negative symptoms, Mean (SD)	17.68 (5.40)	NA	NA
General psychopathology, Mean (SD)	26.87 (4.25)	NA	NA
Total scores, Mean (SD)	63.45 (11.81)	NA	NA

BMI, body mass index. PANSS, Positive and Negative Syndrome Scale. SD, standard deviation. IQR, interquartile range. NA, not applicable. ^*p < 0.05.

Dysregulation of serum and urine TRP-KYN metabolites in schizophrenia

Table 2 presents the differences in serum and urine TRP-KYN metabolites between patients with FES and HCs. Conversely, levels of neuroprotective metabolites, including serum KYNA, XA, and urine PIC, were significantly reduced, alongside a decreased urine PIC/QUIN ratio compared to the HC group (p < 0.05). See Fig. 1. No significant differences were found between the two groups in TRP, KYN, QUIN, QUIN/KYN ratio, or KYNA/KYN ratio. Additionally, no significant differences were observed in the serum/urine ratios of TRP-KYN metabolites, which may suggest no differences in excretion between the groups (Table S1 in the Supplementary Information).

Table 2.

Serum and urine TRP-KYN metabolites between groups.

Metabolites	Serum (ng/ml)			Urine (ng/ml)
	Patients (n = 38)	Controls (n = 43)	Z	Patients (n = 38)	Controls (n = 43)	Z
TRP	7946.68 (7283.19–9475.62)	8324.27 (7218.98–9427.72)	-0.48	12472.06 (6658.54–23027.40)	15026.57 (8541.06–23352.03)	-0.80
KYN	286.54 (219.91–425.20)	349.23 (209.00–440.95)	-0.69	710.39 (350.09–1228.97)	550.90 (350.08–1092.33)	-0.61
KYNA	0.14 (0.11–0.15)	0.16 (0.12–0.18)	-2.69**	0.60 (0.31–1.29)	0.79 (0.45–1.21)	-1.07
QUIN	47.55 (43.94–50.68)	47.55 (45.17–50.94)	-0.13	3059.83 (1859.58–5183.54)	3658.28 (2158.12–4712.76)	-0.59
PIC	42.59 (32.76–54.64)	38.65 (29.16–53.47)	-0.60	162.38 (98.85–206.90)	231.73 (148.28–291.00)	-3.13**
XA	2.84 (2.33–3.94)	3.96 (2.98–5.34)	-3.57**	648.31 (344.32–1168.88)	895.01 (508.05–1527.46)	-1.16
KYN/TRP	0.04 (0.03–0.06)	0.04 (0.03–0.05)	-0.61	0.05 (0.04–0.06)	0.04 (0.03–0.06)	-2.77**
QUIN/KYN	0.17 (0.12–0.21)	0.14 (0.10–0.23)	-0.73	4.15 (2.68–8.22)	5.85 (3.24–8.74)	-1.25
KYNA/KYN	0.0004 (0.0003–0.0006)	0.0005 (0.0003–0.0007)	-0.84	0.0008 (0.0004–0.0021)	0.0014 (0.0007–0.0022)	-1.66
QUIN/KYNA	346.88 (302.77–456.02)	299.36 (269.04–366.78)	-2.44*	6547.52 (3027.95–10081.04)	4294.33 (3248.22–7334.07)	-0.90
PIC/QUIN	0.86 (0.71–1.12)	0.76 (0.62–1.16)	-0.73	0.04 (0.03–0.06)	0.07 (0.04–0.09)	-2.32*

TRP-KYN metabolites are presented as median (IQR) and were compared between groups using the Mann-Whitney U test. TRP, tryptophan. KYN, kynurenine. KYNA, kynurenic acid. QUIN, quinolinic acid. PIC, picolinic acid. XA, xanthurenic acid. ^*p < 0.05. ^**p < 0.01.

Fig. 1.

Serum and urine TRP-KYN metabolites between groups. (A)-(F) histograms for comparisons of serum and urine TRP-KYN metabolites between patients and controls. (A) serum and urine kynurenic acid (KYNA). (B) serum and urine picolinic acid (PIC). (C) serum and urine xanthurenic acid (XA). (D) serum and urine KYN/TRP. KYN, kynurenine. TRP, tryptophan. (E) serum and urine QUIN/KYNA. QUIN, quinolinic acid. (F) serum and urine PIC/QUIN. * p < 0.05. ** p < 0.01. NS, not significant.

Binary logistic regression analysis of serum and urine TRP-KYN metabolites

After adjusting for age, sex, BMI, and education level, binary logistic regression analysis revealed that lower serum XA levels (OR = 0.64, 95% CI: 0.42–0.99, p = 0.04) and lower urine PIC levels (OR = 0.98, 95% CI: 0.97–0.99, p = 0.01) were associated with an increased risk of schizophrenia.

Correlation of serum and urine TRP-KYN metabolites with PANSS

In the FES group, the urine KYNA/KYN ratio was negatively correlated with PANSS general psychopathology scores (r = -0.35, p = 0.04) and PANSS total scores (r = -0.35, p = 0.046) (see Fig. 2). No significant correlations were found between serum TRP-KYN metabolites and PANSS scale or subscale scores (p > 0.05).

Fig. 2.

Partial correlation between urine KYNA/KYN ratio and PANSS in the FES group. KYNA, kynurenic acid. KYN, kynurenine. PANSS, Positive and Negative Syndrome Scale. FES, first-episode schizophrenia.

Discussion

This exploratory study investigated serum and urine levels of TRP-KYN pathway metabolites in patients with FES and examined their correlation with psychotic symptoms. Our findings revealed significant dysregulation of KYN metabolites in both serum and urine, indicating disturbances in the TRP-KYN pathway associated with FES. Specifically, patients with FES showed an elevated serum QUIN/KYNA ratio, reduced levels of the neuroprotective metabolites KYNA and XA in serum, an increased urine KYN/TRP ratio, and decreased levels of the neuroprotective metabolite PIC, along with a reduced PIC/QUIN ratio in urine. These results suggest a metabolic shift towards neurotoxic pathways in schizophrenia. Alterations in the ratios of neuroprotective metabolites correlated with the severity of psychotic symptoms, indicating the potential of these metabolites as biomarkers for schizophrenia.

This study found a significant elevation of the serum QUIN/KYNA ratio in patients with FES, reflecting an imbalance between neurotoxic and neuroprotective metabolites within the KYN metabolic pathway. QUIN is known to be neurotoxic, whereas KYNA is neuroprotective^30,31. An elevated QUIN/KYNA ratio suggests a shift towards neurotoxic metabolite production, potentially impairing neuroprotective mechanisms and resulting in increased levels of neurotoxic 3-HK and QUIN, while levels of neuroprotective KYNA and XA are reduced^12,32. KYNA and XA are metabolized from KYN via the KAT enzymes, whereas QUIN is produced from KYN via the KMO enzyme^7,8. Consistent with previous research, our results indicated severe disruption of the KAT-mediated metabolic pathway in FES, as evidenced by the significant reduction in serum KYNA and XA levels^33,34. However, these findings appear to contrast with cerebrospinal fluid studies reporting significantly elevated KYNA levels^16,17. The discrepancy between peripheral and central KYNA levels may arise from factors such as altered enzyme activity, activation of neuronal cell types, inflammation, disease stage, antipsychotic treatment, and diet^3,22. This contrast underscores the complexity of the KYN metabolic pathway and necessitates further investigation into the mechanisms driving these divergent observations. Additionally, unmedicated patients with FES exhibited significantly lower serum levels of TRP, KYN, and KYNA compared to HCs, with a downward trend in serum QUIN and 3-HK levels^35,36. Persistently low serum XA levels, even after 12 months of antipsychotic treatment, suggest a potentially enduring biochemical trait associated with schizophrenia²⁴.

In the TRP-KYN pathway, kynureninase converts 3-HK to 3-hydroxyanthranilic acid (3-HAA), which is subsequently metabolized to QUIN via 2-amino-3-carboxymuconate-6-semialdehyde or to PIC via 2-amino-3-carboxymuconate-6-semialdehyde decarboxylase (ACMSD)³⁷. ACMSD is critical in regulating the balance between neurotoxic QUIN and neuroprotective PIC, with its activity or expression being indirectly measurable in blood through PIC and QUIN levels³⁸. Specifically, the PIC/QUIN ratio serves as a key indicator of this metabolic balance, where a higher ratio generally reflects a predominance of neuroprotective metabolism, and a lower ratio indicates a shift towards neurotoxic metabolism³⁹. Our findings showed reduced urine PIC levels and a lower urine PIC/QUIN ratio in patients with FES, indicating increased metabolic flux towards QUIN production and highlighting a neurotoxic shift within the KYN pathway. High ACMSD activity is considered protective because it promotes the conversion of KYN to PIC, thereby reducing QUIN production³⁸. Therefore, the observed decrease in PIC levels and PIC/QUIN ratio in patients with FES implies that the protective mechanisms may be impaired, contributing to the neurotoxicity observed in schizophrenia. This shift towards a neurotoxic pathway may play a role in the pathophysiology of schizophrenia.

IDO is a key enzyme in the TRP metabolism to KYN, and its activity typically monitored using the KYN/TRP ratio^5,40. An elevated KYN/TRP ratio reflects systemic inflammation and upregulation of IDO⁴¹. While previous research has shown reduced TRP levels and an elevated KYN/TRP ratio in the plasma in schizophrenia²², this study found a significantly elevated urine KYN/TRP ratio in patients with FES, suggesting enhanced conversion to KYN prior to renal excretion. Furthermore, a lower urine KYNA/KYN ratio was correlated with more severe clinical manifestations, suggesting that reduced peripheral KYNA levels may be linked to the exacerbation of psychiatric symptoms in schizophrenia^42,43, potentially due to an imbalanced ratio between neuroprotective and neurotoxic metabolites within the KYN pathway.

Limitations

This study has several limitations. First, its cross-sectional design and relatively small sample size may restrict the generalizability of the findings. Second, it focused exclusively on peripheral TRP-KYN metabolism and did not include metabolite levels in the cerebrospinal fluid, which could provide additional insights into central nervous system involvement. Third, uncontrolled factors such as inflammation, diet, and physical activity may influence TRP-KYN metabolism. Even with a 4-hour fasting period before sample collection, the potential effects of food intake cannot be fully controlled. Future research should control for these variables to better understand their impact on the TRP-KYN pathway.

Conclusions

This study identified peripheral dysregulation of the TRP-KYN pathway in patients with FES, which was associated with psychotic symptoms. This dysregulation was characterized by an increase in neurotoxic metabolite production and a decrease in neuroprotective metabolites. These results underscore the importance of KYN metabolites in the pathogenesis of schizophrenia and suggest their potential utility in clinical practice, including the development of novel therapeutic targets and personalized treatment strategies.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Author contributions

S.L. and M.L. designed the study. Y.W., Y.X., X.H., K.G., N.H., S.Z. and C.W. were responsible for the data collection. M.L. performed the statistical analyses. S.L. and M.L. wrote a first draft. All authors contributed to the article and approved the submitted version. All authors read and approved by the final manuscript.

Funding

This research was partly supported by the Project for Hangzhou Medical Disciplines of Excellence & Key Project for Hangzhou Medical Disciplines, the Zhejiang Provincial Natural Science Foundation (LTGY24H090012), and the Zhejiang Provincial Medical and Health Science and Technology Plan Project (2022KY990). The funders had no role in the study design, data collection, and analysis, decision to publish, or preparation of the manuscript.

Data availability

The data are available from the corresponding author upon reasonable request.

Declarations

Competing interests

The authors declare no competing interests.