Tryptophan degradation increases with stage in patients with rheumatoid arthritis

Immune system activation is known to be involved in the progression of rheumatoid arthritis (RA). The proinflammatory cytokine interferon-gamma in various cells, including monocytes, induces the enzyme indoleamine (2,3)-dioxygenase (IDO), which converts tryptophan to kynurenine. In sera of 22 patients (17 women and 5 men) with RA stages 1 to 4 according to Steinbrocker, the concentrations of tryptophan and kynurenine were measured by high-pressure liquid chromatography. To estimate IDO activity, the kynurenine to tryptophan ratio (kyn/trp) was calculated. In parallel, concentrations of the macrophage activation marker neopterin were determined by enzyme-linked immunosorbent assay. Tryptophan concentrations were lower in patients with RA, and the decrease in serum tryptophan correlated with increase in stage (p