Enkephalinase: a physiologic neuroimmunomodulator detected in the synovial fluid.

Enkephalinase (endopeptidase 24.11) is a metallopeptidase that is able to cleave not only neuropeptides and hormones but also immune mediators. The enzyme was quantified in synovial fluid obtained from 36 swollen joints. Its concentration correlated with the synovial fluid cell count, mainly the polymorphonuclear cells and lymphocytes, and with the erythrocyte sedimentation rate. No statistically significant difference in enkephalinase levels was demonstrated between the groups of patients with rheumatoid arthritis, seronegative spondylarthropathy, microcrystalline arthritis, or osteoarthritis. The presence of enkephalinase in the synovial fluid could reflect the intensity of the inflammatory process, or it could represent a physiologic regulator of inflammation and pain within the joint.