CRYSTALLINE SOYBEAN TRYPSIN INHIBITOR

Abstract

The isolation of a crystalline trypsin inhibitor from soybean has been reported in previous publications (1). This paper deals with some of the properties of the new crystalline protein and with the mechanism of its inhibiting action on trypsin and chymotrypsin. The soybean inhibitor is a protein of the globulin type. It is precipitated by trichloracetic acid and is non-diffusable through collodion or cellophane membranes. Its light absorption spectrum is that of a typical protein with a maximum at 280 m/ ~ and a minimum at 252 m#. The protein contains less than 0.01 per cent phosphorus and is free of carbohydrate. It acts as an inhibitor only when it is in its native state; denaturation of the soy protein by heat, acid, or alkali is accompanied by a loss in its inhibiting power. The action of the native soybean protein as a trypsin inhibitor is due to its combination with trypsin to form an irreversible stoichiometric compound. The combination is apparently instantaneous. The soy protein inhibits slightly the proteolytic action of chymotrypsin, but unlike that of trypsin the inhibition is due to the formation of a loose reversible compound of the type described by Northrop (2) for the combination between pepsin or trypsin with crude inhibitors. The reaction between chymotrypsin and the soybean inhibitor was found to agree with the law of mass action, for a reversible uni-unimolecular reaction. Crystalline soybean protein, if denatured, is readily digestible by pepsin, by chymotrypsin, or by trypsin. Crystalline soybean inhibitor has no inhibiting effect either on the proteolytic activity or on the milk-clotting ability of pepsin. EXPEI ~ rbtENTAL