Proteolytic conversion of arginine-vasopressin and oxytocin by brain synaptic membranes. Characterization of formed peptides and mechanisms of proteolysis.

This study concerned the fragmentation of the nonapeptides arginine-vasopressin (AVP-(1-9)) and oxytocin (OXT-(1-9)) by proteolytic enzymes present in a brain synaptic membrane preparation. The peptides formed during digestion of arginine-vasopressin and oxytocin were isolated by high pressure liquid chromatography and chemically characterized by amino acid composition, NH2-terminal amino acid residues, and the presence of 14C radioactivity in tyrosine-2 and glycinamide-9. The major peptide fragments of arginine-vasopressin were [Cyt6]-AVP-(2-9), [Cyt6]-AVP-(3-9), [less than Glu4, Cyt6]-AVP-(4-9), and a peptide having the AVP-(4-8) sequence. The characterized fragments of oxytocin were [Cyt6]-OXT-(2-9), [Cyt6]-OXT-(3-9), [Cyt6]-OXT-(4-9), [less than Glu4, Cyt6]-OXT-(4-9), and [Cyt6] OXT-(5-9). Employing differentially 14C-labeled arginine-vasopressin and oxytocin, the proteolysis of the two peptides into fragments was followed with time. The results showed the sequential formation of peptide fragments by proteolytic cleavage from the NH2 terminus onward, demonstrating the action of an aminopeptidase-like enzyme. Arginine-vasopressin was converted significantly more rapidly by the amino-peptidase activity than oxytocin. In contrast to known brain aminopeptidases, the synaptic membrane-associated activity cleaved the nonapeptides without prior reduction of the disulfide bridge. From the present data it is concluded that aminopeptidases predominate in the proteolytic mechanism by which brain synaptic membranes convert arginine-vasopressin and oxytocin. The role of the proteolytic events and the significance of formed peptide fragments is discussed in view of the concept that arginine-vasopressin and oxytocin are precursors for neuropeptides in brain.