Catalyzes the sequential release of Tyr-Pro, Phe-Pro and Gly-Pro from the N-terminus of peptides and proteins. Is able to cleaves bioactive peptide beta-casomorphin. Release of an N-terminal dipeptide, Xaa-Yaa-|-Zaa-, from a polypeptide, preferentially when Yaa is Pro, provided Zaa is neither Pro nor hydroxyproline. Completely inhibited by the serine protease inhibitor diisopropyl fluorophosphate (DFP) and moderately by N-tosyl-L-phenyl-alanyl chloromethyl ketone (TPCK). Somewhat inhibited by phenylmethanesulfonyl fluoride (PMSF). Activity is not affected by thiol- or metalloprotease inhibitors, such as iodoacetate (IAA), EDTA, N-tosyl-L-lysyl chloromethyl ketone (TLCK), o-phenanthlorine, N-ethylmaleimide (NEM) or dithiothreitol (DTT). Optimum pH is 8.0 for the hydrolysis of Gly-Pro-pNA. No hydrolysis is detected at a pH below 5.5 or above 11.0. Optimum temperature is between 40 and 50 degrees Celsius for the hydrolysis of Gly-Pro-pNA. Stable for at least 30 min below 30 degrees Celsius. Homodimer. Belongs to the peptidase S9B family.