A Protease Inhibitor from Penicillium cyclopium

Part III. The Mode of Action of the Inhibitor

Abstract

When the protease inhibitor from Penicillium cyclopium was mixed with the acid protease of the mold at acid pH formed a precipitate consisting of a enzyme-inhibitor complex. The precipitation occurred maximally at pH 3.0 and was interfered with by increasing amounts of salts and other protein. Subsequent incubation of the complex brought about inactivation of the enzyme and the inactivation was found to be accompanied by modification of the enzyme so that less was precipitable with trichloroacetic acid. Paper chromatography revealed that the enzyme on complete inactivation had been degraded to several fragments or polypeptides. The inhibitor acted on the enzyme in a catalytic fashion, bringing about degradation of more than a stoichiometric amount of enzyme. The proposed mechanism of the inhibitor action involved acceleration of auto-digestion of the enzyme which splits the molecule into small fragments and abolishes the activity.