An endopeptidase associated with bovine neurohypophysis secretory granules cleaves pro-ocytocin/neurophysin peptide at paired basic residues.

The octacosapeptide sequence [Tyr18] pro-ocytocin/neurophysin (1-18)NH2 [pro-OT/Np(1-18)NH2] was synthesized and used as substrate to detect endoprotease(s) possibly involved in the processing of this precursor in bovine hypothalamo-neurohypophyseal tract. An endopeptidase (58 Kda) was detected in Lysates made from highly purified neurosecretory granules. This protease which cleaves the peptide bond on the carboxyl side of the Lys-Arg doublet, and no single basic residue, generates both OT-Gly10-Lys11-Arg12+Ala13-Val-Leu-Asp-Leu-Tyr18 (NH2) from the octacosapeptide substrate. In addition, a carboxypeptidase B-like activity converting OT-Gly10-Lys11-Arg12 into OT-Gly10 was detected in the same granule Lysates. It is hypothesized that a combination of these endoprotease and carboxypeptidase B-like activities together with the amidating enzyme of secretory granules might participate in the cleavage and processing of pro-OT/Np in vivo.