Inactivation of soya bean trypsin inhibitors by thiols

Systematic studies of factors that inactivate soya bean trypsin inhibitors showed that thiols facilitate heat inactivation, as illustrated by the following examples. Heating soya flour in an aqueous medium in a water‐bath from 25 to 93°C for 1 h showed that heat alone does not begin to inactivate the inhibitor until about 55°C (around 10% inactivation). The corresponding value at 85 °C is 60%. In contrast, inactivation in the presence of N‐acetylcysteine proceeds more rapidly, as shown by 70% inactivation at 55 °C and 94% at 85 °C. These results, and additional data, illustrate the complementary co‐operative effect of heat and thiols in inactivating soya bean trypsin inhibitors in commercial soya flour. The thiol treatment also strikingly enhances the in‐vitro digestibility of soya proteins by trypsin. The results are discussed with reference to the role of disulphide bonds in the mechanism of enzyme inhibition.