Pepsin-mediated processing of synthetic precursor-like sequence yields neurotensin-like peptide

A 15 amino acid synthetic peptide, which spanned the dibasic cleavage site C-terminal to neurotensin (NT), in its 170-residue canine precursor, was synthesized by solid-phase methods. Using this substrate in combination with a radioimmunoassay specific for the C-terminal region of NT, a simple assay was developed to monitor protease-mediated cleavage of the Leu8-Lys9 bond in the substrate. Hog pepsin and the related enzymes, rhizopus pepsin, bovine cathepsin D, and mouse renin, were found to be effective in this assay, pepsin cleaving only this bond to liberate the NT-like sequence. The pH dependence of the reaction indicated that pepsin, cathepsin D, and renin exhibited significant activity at pH's characteristic for secretory vesicles (pH 5.5-6.5). In addition, pepsin and cathepsin D were shown to process the native precursor at pH's as high as 5.5. These results, although not proof, are consistent with the idea that endoproteases with pepsin-like specificity may be involved in the processing of the NT precursor in neural/endocrine cells.