Activation and inhibition of cerebral prolidase

Purified of prolidase from calf brain (acetone and [NH4]2SO4 fractionation) separated this enzyme from proteases, leucine aminopeptidase, master dipeptidase, and Gly-Gly dipeptidase. Prolidase was tested with peptidase and protease inhibitors, used at higher levels (35 times or more) than their ID50 for peptidases and proteases. Bacitracin, leupeptin, chymostatin, and antipain had no effect; pepstatin slightly increased activity, and only bestatin was inhibitory. Antibiotics that affect protein synthesis did not inhibit prolidase. Peptides with proline at the NH2 end activated prolidase, whereas those with proline at the carboxyl end inhibited it. Di, tri, and tetra-Pro peptides increased prolidase activity. Thyrotropin-releasing hormone had no effect on prolidase; its analog Pro-His-Pro-NH2 gave high activation and decreased the Km from 20 mM to 1.54 mM. Pro-peptide inhibitors and activators were not themselves split by prolidase. The results indicate influences of specific peptides, for both inhibition and activation, on prolidase activity.