Rapid Degradation of Neurotensin by Intact Murine Neuroblastoma Cells (Clone N1E‐115)

Murine neuroblastoma clone N1E‐115, which possesses receptors for neurotensin mediating the formation of intracellular cyclic GMP and the stimulation of inositol phospholipid hydrolysis, exhibited only partial desensitization to neurotensin. This result led to the observation that neurotensin was very rapidly degraded by intact N1E‐115 cells. In experiments measuring the time course of [³H]neurotensin degradation, a minimum of six major tritiated products were found, with the breakdown peptides formed and the degree of proteolysis of [³H]neurotensin being dependent upon the length of incubation and the concentration of cells. Clone N1E‐115 degraded [³H]neurotensin in an apparently sequential fashion; the primary initial cleavage of intact neurotensin was at the peptide bond between residues Arg⁸ and Arg⁹. Initial degradation peptides from the active carboxyl‐terminal portion of neurotensin were more rapidly degraded, after formation, than were the peptides from the inactive amino‐terminal half of neurotensin. The final two degradation products found were tyrosine, from the carboxyl‐terminal portion of neurotensin, and an as yet unidentified peptide from the amino‐terminal half of neurotensin. [³H]Neurotensin(8–13) was more rapidly hydrolyzed under identical conditions than was [³H]neurotensin itself. A combination of the protease inhibitors 1, 10‐phenanthroline and Z‐Pro‐Prolinal was able to inhibit almost completely the degradation of neurotensin by clone N1E‐115.