Methionine‐Enkephalin in Bone and Joint Tissues

Methionine‐enkephalin (met‐enk), an endogenous opiate, mimics many of the effects of morphine by binding to opiate receptors, thereby eliciting similar cellular and behavioral effects. Using biochemical and immunohistochemical techniques, several peptides have been identified in bone and joint tissues. Here we report, for the first time, the presence as well as concentration of met‐enk in bone and joint tissues. Immunohistochemistry using electron and immunoflourescence microscopy showed cellular and neuronal distribution of met‐enk in bone and joint tissues. The concentration of met‐enk analyzed by high performance liquid chromatography electrochemical detection or radioimmunoassay was high in bone marrow, periosteum, ankle joint tissue, and cortical bone. Analysis by fast atom bombardment mass spectrometry suggested that the recovered fragment was met‐enk. Administration of met‐enk inhibits osteoblast cell growth in culture, which is reversible by naltrexone. In arthritic rats, the concentration of met‐enk was significantly decreased in ankle joints compared with controls, suggesting a role for met‐enk in the pathophysiology of adjuvant arthritis.