Characterization of a nontrypsin cholecystokinin converting enzyme in mammalian brain

An enzyme has been partially purified from canine and porcine cerebral cortical extracts that differs from trypsin in that it manifests some degree of hormone specificity since it converts porcine cholecystokinin to smaller immunoreactive forms, i.e., the COOH-terminal dodecapeptide and octapeptide fragments, but fails to convert big gastrin (34 amino acids) to heptadecapeptide gastrin. This enzyme is distinguishable from trypsin not only in substrate specificity, but also in several physicochemical properties. It is not inhibited in the presence of concentrations of lima bean trypsin inhibitor sufficient to inhibit 1 mg of trypsin per ml of incubation mixture. It is inactivated when incubated with substrate at 45°C for 1 hr, whereas trypsin remains fully active when incubated under the same conditions at 55°C. The enzyme elutes in the void volume on Sephadex G-50 and G-75 gel filtration. On sucrose gradient centrifugation, the proteolytic activity associated with trypsin is recovered above albumin but that of the solubilized brain enzyme is recovered below gamma globulin. The enzyme is not detectable in splenic extracts, which do contain nonspecific proteases capable of completely degrading cholecystokinin. Further investigation is required to determine whether the enzyme in the gut that converts cholecystokinin to the bioactive and immunoactive COOH-terminal fragments resembles or is different from the brain converting enzyme.