Serotonin metabolism and its enzymic activities in joint diseases

In order to clearly understand the synovial metabolic pathway of 5-HT, which is interesting as an inflammatory mediator and a pain producing substance, in patients with RA and with OA, determinations were made of 5-HIAA levels and of the activities of MAO-A and MAO-B, in the synovial fluid and the blood. With respect to 5-HIAA levels, there was no significant difference in the synovial levels between patients with RA and those with OA, although higher values were obtained in the plasma of patients with RA. A correlation was found between synovial and plasma levels in both diseases. In patients with RA, 5-HIAA levels tended to increase in both levels of the synovial fluid and the plasma in higher stages. The activities of MAO-A and MAO-B in the synovial fluid were found to be lower in patients with RA than in those with OA. The MAO-B activity in the synovial fluid increased in higher stages and correlated with ESR in patients with RA. In patients with RA the efflux of 5-HIAA from the synovial fluid was reduced. No remarkable changes occurred in the permeability of 5-HIAA. The ability to inactivate 5-HT was lower than in OA. The determination of synovial MAO-B activity is useful in diagnosing the status of the patient with respect to rheumatoid arthritis.