Proteolytic conversion of oxytocin by brain synaptic membranes: role of aminopeptidases and endopeptidases.

The proteolytic conversion of oxytocin and vasopressin by purified rat brain synaptic membranes was studied at 37 degrees C and physiological pH 7.4. The formed peptide fragments were isolated by high performance liquid chromatography and characterized by amino acid analysis. When oxytocin was incubated with synaptic membranes, either C- or N-terminal fragments were found. The most abundant were [Cyt6]oxytocin(4-9), [Cyt6]oxytocin(3-9), [Cyt6]oxytocin(2-9), oxytocin(1-8) and oxytocin(1-7). In contrast, only C-terminal fragments, [Cyt6-Arg8]vasopressin(4-9), [Cyt6-Arg8]vasopressin(3-9) and [Cyt6-Arg8]vasopressin(2-9), were found by incubating [Arg8]vasopressin. The formation of C-terminal oxytocin and vasopressin fragments was inhibited by the aminopeptidase inhibitors amastatin and bestatin, while the formation of oxytocin(1-7) and (1-8) was inhibited by the divalent cations Hg(2+) and Zn(2+). The formation of oxytocin(1-7) was also partially prevented by the endopeptidase inhibitor phosphoramidon. The formation of both C- and N-terminal fragments was inhibited by o-phenanthroline. The results suggest that, while [Arg8]vasopressin is metabolized only by membrane-bound aminopeptidases, oxytocin is also metabolized by membrane-bound endopeptidases.