HYDROLYSIS OF SOYBEAN TRYPSIN INHIBITOR BY THE γ SUBUNIT OF 7SNGF AND EGF‐BP: Demonstration of Proteolytic Activity

Although soybean trypsin inhibitor (STI) does not inhibit the esterase activity of either epidermal growth factor binding protein (EGF BP) or the gamma subunit of 7SNGF, it does behave as a substrate for proteolysis. Cleavage of the active site peptide bond of STI does occur when incubated in the presence of either EGF-BP or the gamma subunit of 7SNGF. The hydrolysis id pH dependent with maximum proteolysis at pH 6.0-7.0. the newly formed C-terminal arginine residue in modified STI can be released by carboxypeptidase B digestion. Both enzymes are inhibited by low concentrations (2-4 microgram/ml) of the microbial protease inhibitors leupeptin and antipain. These inhibitors are specific for trypsin-like proteases. Since both enzymes can be found as part of high molecular weight complexes with growth factors these results confirm the hypothesis that they are involved during a postranslational modification event.