ENDOPEPTIDASE-24.16 AND ENDOPEPTIDASE-24.15 ARE RESPONSIBLE FOR THE DEGRADATION OF SOMATOSTATIN, NEUROTENSIN, AND OTHER NEUROPEPTIDES BY CULTIVATED RAT CORTICAL ASTROCYTES

Several neuropeptides, including neurotensin, somatostatin, bradykinin , angiotensin II, substance P, and luteinizing hormone-releasing hormo ne but not vasopressin and oxytocin, were actively metabolized through proteolytic degradation by cultivated astrocytes obtained from rat ce rebral cortex. Because phenanthroline was an effective degradation inh ibitor, metalloproteases were responsible for neuropeptide fragmentati on. Neurotensin was cleaved by astrocytes at the pro(10)-Tyr(11) and A rg(8)-Arg(9) bonds, whereas somatostatin was cleaved at the Phe(6)-Phe (7) and Thr(10)-Phe(11) bonds. These cleavage sites have been found pr eviously with endopeptidases 24.16 and 24.15 purified from rat brain. Addition of specific inhibitors of these proteases, the dipeptide Pro- lie and rboxy-3-phenylpropyl]-Ala-Ala-Phe-4-aminobenzoate, significant ly reduced the generation of the above neuropeptide fragments by astro cytes. The presence of endopeptidases 24.16 and 24.15 in homogenates o f astrocytes could also be demonstrated by chromatographic separations of supernatant solubilized cell preparations. Proteolytic activity fo r neurotensin eluted after both gel and hydroxyapatite chromatography at the same positions as found for purified endopeptidase 24.16 or 24. 15. In incubation experiments or in chromatographic separations no pho sphoramidon-sensitive endopeptidase 24.11 (enkephalinase) or captopril -sensitive peptidyl dipeptidase A (angiotensin-converting enzyme) coul d be detected in cultivated astrocytes. Because astrocytes embrace the neuronal synapses where neuropeptides are released, we presume that t he endopeptidases 24.16 and 24.15 on astrocytes are strategically loca ted to contribute significantly to the inactivation of neurotensin, so matostatin, and other neuropeptides in the brain.