ENKEPHALIN METABOLISM BY MICROGLIA AMINOPEPTIDASE-N (CD13)

Rat microglia in culture showed a high capacity to degrade neuropeptid es compared with other glial cells. Leu-enkephalin was readily hydroly zed to free tyrosine and Gly-Gly-Phe-Leu. Inhibition experiments and i mmunostaining revealed that aminopeptidase N (CD13) on the surface of microglia was responsible for enkephalin cleavage. Endopeptidase-24.11 (''enkephalinase''), angiotensin-converting enzyme, or carboxypeptida ses could not be detected on microglia. Aminopeptidase N activity in m icroglia was considerably higher than in rat peripheral monocytes and macrophages, which both also exhibited low endopeptidase 24.11 activit ies. Activity of aminopeptidase N was upregulated by culture of microg lia on astrocytes and downregulated by exposure of microglia to lipopo lysaccharide. The occurrence of aminopeptidase N on microglia is in li ne with the view that they originate from the monocytic lineage.