Clinical significance of plasma tryptophan, kynurenine, and kynurenine/tryptophan ratio in rheumatoid arthritis patients

Abstract

Background: The catabolism of tryptophan (Trp) in the kynurenine (Kyn) pathway is thought to have a critical immunosuppressive effect.Aim of the work: To evaluate plasma Trp and metabolite levels to identify their diagnostic potential in rheumatoid arthritis (RA).Patients and methods: 50 RA patients and 41 control were included in this study. The Trp and Kyn were analyzed and the Kyn\textbackslash{}Trp ratio was calculated to estimate Indolamine 2,3 dioxygenase (IDO) enzyme activity involved in Trp degradation.Results: The 50 patients had a mean age of 58.5 +/- 10.6 years

37 females and 13 males, F:M 2.8:1 and median disease duration was 10.1 (7-16) years. Rheumatoid factor was positive in 64 \%. The median Trp value was significantly lower in RA (11120.7

3259-16352 ng/ml) compared to control (12372.3

7217-31,936 ng/ml) (p = 0.001). Kyn/Trp was significantly higher in RA (4.04

2.5-12.3) compared to con-trol (3.2

2.1-4.7) (p < 0.0001). The median Kyn value was higher in RA (451.02

264-1292 ng/ml) than in control (391.4

236-1494 ng/ml) (p = 0.04). Trp significantly inversely correlated with the morning stiff-ness (r =-0.32, p = 0.025), Kyn significantly correlated with the C-reactive protein (CRP) (r = 0.31, p = 0.028) and erythrocyte sedimentation rate (ESR) (r = 0.41, p = 0.003) and the Kyn/Trp ratio correlated with the morning stiffness, CRP and ESR (r = 0.26, p = 0.045

r = 0.32, p = 0.025 and r = 0.32, p = 0.024 respectively). Kyn/Trp, Kyn and Trp significantly predicted RA at a cut-off value of 4.72, 589.1 ng/ml and 9921.1 ng/ml (p < 0.0001, p = 0.04 and p = 0.001 respectively).Conclusion: Our study showed that there is a significant relationship between RA and Kyn and Trp levels and IDO enzyme activity.(c) 2022 THE AUTHORS. Publishing services by ELSEVIER B.V. on behalf of The Egyptian Society of Rheu-matic Diseases. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/ licenses/by-nc-nd/4.0/).