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 intes tinal brush-border membrane, cytosol, and homogenate was studied. Prot eolytic degradation of both compounds was pH dependent. At pH 7.5, rap id degradation was observed in brush-border membrane, cytosol, and hom ogenate. At pH 4.5, both compounds were stable in brush-border membran e, but were slowly degraded in mucosal homogenate of the proximal inte stine. In general, acetylneurotensin 8-13 was more stable than neurote nsin. Degradation of both compounds by 27 000 x g pellets, rich in bru sh-border membrane, was highest among the subcellular fractions. Studi es 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 a cetylneurotensin 8-13. At pH 4.5, degradation in homogenates was mainl y due to serine proteases. In cytosol, degradation of both compounds w as inhibited by a specific substrate of prolyl endopeptidase (EC 3.4.2 1.26) which is also called post-proline cleaving enzyme, an enzyme cle aving at the carboxyl end of proline. In summary, inhibitor studies su ggest that both compounds are degraded at pH 7.5 by endopeptidase-24.1 1 and ACE in brush-border membrane, by activities of prolyl endopeptid ase in cytosol, and at pH 4.5 by serine protease(s) in homogenate.