PEPTIDE-AMIDATING ENZYMES ARE EXPRESSED IN THE STELLATE EPITHELIAL-CELLS OF THE THYMIC MEDULLA

C-terminal amidation is a post-translational processing step necessary to convey biological activity to a large number of regulatory peptide s. In this study we have demonstrated that the peptidyl-glycine alpha- amidating monooxygenase enzyme complex (PAM) responsible for this acti vity is located in the medullary stellate epithelial cells of the thym us and in cultured epithelial cells bearing a medullary phenotype, usi ng Northern blot, immunocytochemistry, in situ hybridization, and enzy me assays. Immunocytochemical localization revealed a granular pattern in the cytoplasm of the stellate cells, which were also positive for cytokeratins and a B-lymphocyte-associated antigen. The presence of PA M activity in medium conditioned by thymic epithelial cell lines sugge sts that PAM is a secreted product of these cells. Among the four epit helial cell lines examined, there was a direct correlation between PAM activity and content of oxytocin, an amidated peptide. Taken together , these data provide convincing evidence that thymic epithelial cells have the capacity to generate amidated peptides that may influence T-c ell differentiation and suggest that the amidating enzymes could play an important role in the regulation of thymic physiology.