Trypsin Inhibitor Assay: Expressing, Calculating, and Standardizing Inhibitor Activity in Absolute Amounts of Trypsin Inhibited or Trypsin Inhibitors

Abstract For expressing trypsin inhibitor activity (TIA), trypsin units inhibited (TUI), trypsin inhibited, and trypsin inhibitors have been used. Although the last two units are preferred, their calculations in current practices require refinement. With the proposed AOCS method Ba 12a‐2020, four experiments were conducted, using four trypsin preparations having specific activity of 11,625, 12,602, 13,728, and 14,926 Nα ‐benzoyl‐L‐arginine ethyl ester (BAEE) units/mg protein, respectively. Experiment 1 determined the relationship between absorbance at 410 nm (A410) and trypsin concentration. Experiment 2 involved assaying raw and heated soybeans, expressing TIA as TUI/mg sample and μg trypsin inhibited/mg sample, and determining conversion factors between the two units. Experiment 3 resembled Experiment 2 except for using purified soybean Kunitz inhibitor (KTI) and Bowman‐Birk inhibitor (BBI). Conversion factors determined correlated highly with trypsin‐specific activity (R 2 = 0.9789). After standardizing against a reference trypsin having 15,000 BAEE units/mg protein, a standardized conversion factor of 0.03 A410 (1.5 TUI) = 1 μg trypsin inhibited was determined. It remained consistent regardless of trypsin specific activity, with or without inhibitors, and type of inhibitor samples. By using purified inhibitors (Experiment 3), conversion values between TUI and μg trypsin inhibitor and between μg trypsin inhibited and μg trypsin inhibitor could also be calculated, enabling expression of TIA in amounts of pure KTI, BBI or their equivalents. Furthermore, when the AOCS method was modified with half substrate concentration, half trypsin concentration or half both (Experiment 4), TIA values in TUI could change with modifications but values in mg trypsin inhibited (standardized) or trypsin inhibitor remained consistent.