Prolyl endopeptidase, also known as PREP, belongs to a distinct class of serine peptidases. It is a large cytosolic enzyme which was first described in the cytosol of rabbit brain as an oligopeptidase. Prolyl endopeptidase degrades the nonapeptide bradykinin at the Pro-Phe bond. It is involved in the maturation and degradation of peptide hormones and neuropeptides such as alpha-melanocyte-stimulating hormone, luteinizing hormone-releasing hormone (LH-RH), thyrotropin-releasing hormone, angiotensin, neurotensin, oxytocin, substance P and vasopressin. Prolyl endopeptidase's activity is confined to action on oligopeptides of less than 1 kD and it has an absolute requirement for the trans-configuration of the peptide bond preceding proline. It cleaves peptide bonds at the C-terminal side of proline residues.