Degradation of Neurotensin by Rat Brain Synaptic Membranes: Involvement of a Thermolysin-Like Metalloendopeptidase (Enkephalinase), Angiotensin-Converting Enzyme, and Other Unidentified Peptidases

Neurotensin was inactivated by membrane-bound and soluble degrading activities present in purified preparations of rat brain synaptic membranes. Degradation products were identified by HPLC and amino acid analysis. The major points of cleavage of neurotensin were the Arg8-Arg9, Pro10-Tyr11, and Tyr11-Ile12 peptide bonds with the membrane-bound activity and the Arg8-Arg9 and Pro10-Tyr11 bonds with the soluble activity. Several lines of evidence indicated that the cleavage of the Arg8-Arg9 bond by the membrane-bound activity resulted mainly from the conversion of neurotensin1-10 to neurotensin1-8 by a dipeptidyl carboxypeptidase. In particular, captopril inhibited this cleavage with an IC50 (5.7 nM) close to its K1 (7 nM) for angiotensin-converting enzyme. Thiorphan inhibited the cleavage at the Tyr11-Ile12 bond by the membrane-bound activity with an IC50 (17 nM) similar to its K1 (4.7 nM) for enkephalinase. Both cleavages were inhibited by 1,10-phenanthroline. These and other data suggested that angiotensin-converting enzyme and a thermolysin-like metalloendopeptidase (enkephalinase) were the membrane-bound peptidases responsible for cleavages at the Arg8-Arg9 and Tyr11-Ile12 bonds, respectively. In contrast, captopril had no effect on the cleavage at the Arg8-Arg9 bond by the soluble activity, indicating that the enzyme responsible for this cleavage was different from angiotensin-converting enzyme. The cleavage at the Pro10-Tyr11 bond by both the membrane-bound and the soluble activities appeared to be catalyzed by an endopeptidase different from known brain proline endopeptidases. The possibility is discussed that the enzymes described here participate in physiological mechanisms of neurotensin inactivation at the synaptic level.