Survey of the Proteolytic Activities Degrading the Kunitz Trypsin Inhibitor and Glycinin in Germinating Soybeans (Glycine max).

The cotyledons of the soybean (Glycine max [L.] Merrill cv Amsoy 71) were examined for proteolytic activities capable of degrading soybean seed proteins. Three distinct activities were identified that attack the native Kunitz soybean trypsin inhibitor of Amsoy 71, Ti(a). Protease K1 cleaves Ti(a) to Ti(a) (m), the inhibitor form lacking the five carboxyl-terminal amino acid residues relative to Ti(a). Protease K1 is a cysteine protease that peaks in activity on day 4 after the beginning of imbibition, with maximal activity toward Ti(a) at pH 4. The characteristics of protease K1 are consistent with the involvement of this protease in the initial proteolysis of the Kunitz inhibitor during germination. Protease K2 also degrades Ti(a) at pH 4 but produces no electrophoretically recognizable products. It peaks later in seedling growth, at day 8. Protease K3 degrades Ti(a) to products other than Ti(a) (m). However, it is active at pH 8. Two proteolytic activities were identified that attack the major storage protein, glycinin. Protease G1 (which appears by 4 days after imbibition) specifically cleaves the acidic polypeptides of glycinin at pH 4, yielding a product approximately 1.5 kilodaltons smaller. Protease G1 is inhibited by metal chelators as well as by reagents reactive toward thiols. Protease G2 also degrades the glycinin acidic chains at pH 4, but without the appearance of electrophoretically recognizable products. Protease G2, while present at low levels in the dry seed, is found primarily in the cotyledons after 8 days of growth.