| Abstract: | Using a radioimmunoassay directed toward the COOH-terminal, biologically active region of mammalian neurotensin (NT), the rapid (within seconds) generation of immunoreactive NT (iNT) during acid extraction of mammalian and avian gastric tissues has been demonstrated. Levels of iNT were shown to increase 25-200-fold in time. The reaction occurred in 0.1 N HCl and 2 N acetic acid and was prevented by raising the pH above 5 or by adding acetone. The temperature and pH dependence and the ability of pepstatin A to inhibit the reaction suggested the involvement of a pepsin-related acid protease. Furthermore, the reaction could be mimicked by incubating a stabilized gastric extract with hog pepsin at pH 2. Size exclusion chromatography demonstrated the presence of a precursor-like substance with an apparent Mr of 60,000. Although iNT generated in avian and mammalian gastric extracts could be distinguished chromatographically from NT in that species, the partially purified gastric iNT was active in a bioassay for NT which quantitates changes in vascular permeability after intradermal injection into rats. One might suggest that iNT serves as a signal within the gastric lumen, being generated at low pH by secreted pepsin. It is also possible that iNT could be formed within blood or gastric interstitial fluid by the action of pepsin-related (cathepsin or renin-like) enzymes at normal physiologic pH. |
