Degradation of the neuropeptide somatostatin by cultivated neuronal and glial cells.

The enzymatic degradation of the neuropeptide somatostatin was investigated in cultivated cells and subcellular fractions from rat brain. Dissociated neurones, astrocytes, and oligodendrocytes obtained from rat cerebral cortex were of more than 85-98% purity as evidenced by immunostaining with antisera to cell specific markers. All of these cell types were able to cleave radiolabeled somatostatin to smaller fragments, especially cultivated astrocytes with the highest specific activity. The neuroblastoma cell line N1E-115 did not measureably cleave somatostatin. The somatostatin-degrading proteases of the cultivated brain cells could be differentiated by their sensitivity to protease inhibitors and by the fragments produced: astrocytes contain a metallo-endoprotease sensitive to phenanthroline which cleaves somatostatin at the Phe6-Phe7 and Thr10-Phe11 bonds, whereas the endoprotease(s) of neurones and oligodendrocytes was insensitive to chelating agents but strongly inhibited by the antibiotic bacitracin. In accordance with this, the bacitracin-sensitive activity was mainly recovered in the synaptic plasma membrane and myelin subcellular fractions obtained by differential centrifugation of rat cerebral cortex homogenate. However, the highest total and specific somatostatin-degrading activity was detected in the cytosolic fraction.