Pyroglutamyl peptidase-1 (PGPEP-1) is an omega peptidase which removes pyroglutamyl residues from the amino termini of peptides and proteins. It is a cytosolic cysteine peptidase that is expressed in most cell types. The enzyme requires a thiol-reducing agent for activity. PGPEP-1 is potentially involved in the inactivation of biologically active peptides that possess an amino terminal pyroglutamyl group. Examples of such peptides include neurotensin, luteinizing hormone releasing hormone, and thyrotropin-releasing hormone.