Activities of trypsin and chymotrypsin inhibitors and their relationship in selected soybean germplasm

Abstract: Soybeans have been the number one source of alternative proteins for food and feed. However, the natural presence of proteinase inhibitors (namely, Kunitz inhibitor and Bowman-Birk inhibitor) in soybeans can reduce their nutritive values and cause undesirable health effects in animals. Historically, trypsin inhibitor activity (TIA) in soybean products has been of primary interest for measurement, but lately, there is a growing interest in monitoring chymotrypsin inhibitor activity (CIA) in these products as well, since trypsin and chymotrypsin are different digestive enzymes, and their inhibitory actions could have different nutritional and health implications. The objectives of the present study were to screen a total of 70 selected soybean accessions for TIA and CIA and delineate their relationships. For TIA assay, we used the latest AOCS Official Method Ba 12a-2020. For CIA assay, we used the optimized method developed at our USDA laboratory (J Food Sci. 2022, 87:2018–2033). TIA and CIA varied greatly with accessions. For the 44 cultivated soybeans (Glycine max), TIA ranged 17.03-53.27 mg trypsin inhibited (TId)/g sample while CIA ranged 13.41-38.13 mg chymotrypsin inhibited (CId)/g. The three accessions with the lowest TIA were Kunitz null variety or lines. For the 20 wild soybeans (G. soja), TIA ranged 34.37-62.17 mg TId/g while CIA varied 14.75-52.70 mg CId/g. For the 6 perennial soybeans (G. sp.), TIA ranged 5.84-20.94 mg TId/g while CIA from 11.99 to 40.79 mg CId/g. In general, CIA increased with increasing TIA. The two had moderate to strong linear correlations. Yet, with regards to the relative strength, the subgenus Glycine (perennials) showed a pattern of CIA > TIA, resulting from unusually lower TIA values. This pattern was just opposite from the subgenus Soja (cultivated and wild types). These findings will assist plant scientists to develop soybeans with altered protease inhibitions for improved food and feed proteins.