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

The catabolism of tryptophan (Trp) in the kynurenine (Kyn) pathway is thought to have a critical immunosuppressive effect. To evaluate plasma Trp and metabolite levels to identify their diagnostic potential in rheumatoid arthritis (RA). 50 RA patients and 41 control were included in this study. The Trp and Kyn were analyzed and the Kyn ratio was calculated to estimate Indolamine 2,3 dioxygenase (IDO) enzyme activity involved in Trp degradation. 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 control (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 stiffness (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). Our study showed that there is a significant relationship between RA and Kyn and Trp levels and IDO enzyme activity.