Tryptophan metabolism in the joint diseases.

Tryptophan metabolites through kynurenine pathway and 5-hydroxytryptophan pathway in the synovial fluid, blood and urine of patients with rheumatoid arthritis and those with osteoarthritis were investigated in order to know the specificity of the tryptophan metabolism, especially the role of serotonin in the joint diseases. A metabolic map of tryptophan in the synovial fluid was also made. It was found that tryptophan in rheumatoid arthritis was metabolized mainly to anthranilic acid through kynurenine in the kynurenine pathway, whereas in osteoarthritis the pathway of kynurenic acid and nicotinic acid was characteristic. Otherwise in 5-hydroxytryptophan pathway there were no significant differences in the levels of 5-hydroxytryptamine and 5-hydroxyindole acetic acid in the synovial fluid between rheumatoid arthritis and osteoarthritis but the activity of monoamine oxidase in the synovial fluid was higher in osteoarthritis than in rheumatoid arthritis.