Prolyl endopeptidase (PEP)[EC 3.4. 21.261 is a serine protease that specifically cleaves peptidyl proline bonds.'This enzyme hydrolyzes many biologically active peptides, including thyrotropin-releasing hormone, substance P, angiotensin 11, oxytocin, and bradykinin.-The enzyme also degrades arginine vasopressin which may facilitate learning and mem~ ry.~-~ Yoshimoto et al. reported that the PEP inhibitor 1-[1-(benzyloxycarbonyl)-~-prolyl] prolinal ...