STABILITY OF NEUROTENSIN AND ACETYLNEUROTENSIN-8-13 IN BRUSH-BORDER MEMBRANE, CYTOSOL, AND HOMOGENATE OF RAT SMALL-INTESTINE

The stability of neurotensin and acetylneurotensin 8-13 in small intestinal brush-border membrane, cytosol, and homogenate was studied. Proteolytic degradation of both compounds was pH dependent. At pH 7.5, rapid degradation was observed in brush-border membrane, cytosol, and homogenate. At pH 4.5, both compounds were stable in brush-border membrane, but were slowly degraded in mucosal homogenate of the proximal intestine. In general, acetylneurotensin 8-13 was more stable than neurotensin. Degradation of both compounds by 27 000 x g pellets, rich in brush-border membrane, was highest among the subcellular fractions. Studies using enzyme inhibitors suggested that both compounds were degraded by brush-border endopeptidase-24.11 and angiotensin-converting enzyme (ACE), and that endopeptidase-24.11 was' the major enzyme degrading acetylneurotensin 8-13. At pH 4.5, degradation in homogenates was mainly due to serine proteases. In cytosol, degradation of both compounds was inhibited by a specific substrate of prolyl endopeptidase (EC 3.4.21.26) which is also called post-proline cleaving enzyme, an enzyme cleaving at the carboxyl end of proline. In summary, inhibitor studies suggest that both compounds are degraded at pH 7.5 by endopeptidase-24.11 and ACE in brush-border membrane, by activities of prolyl endopeptidase in cytosol, and at pH 4.5 by serine protease(s) in homogenate.