Thermal inactivation of tepary bean (Phaseolus acutifolius), soybean and lima bean protease inhibitors: effect of acidic and basic pH

Thermal stability of the protease inhibitor of tepary bean (Phaseolus acutifolius) was studied by comparing the antitryptic activities of a crude extract and the purified inhibitor (TBPI) to those of three purified protease inhibitors, soybean trypsin inhibitor (Kunitz; KSTI), soybean trypsin and chymotrypsin inhibitor (BBI) and lima bean trypsin inhibitor (LBTI). Three purified inhibitors, TBPI, BBI and LBTI, were found to be heat-stable when subjected to boiling (100 °C) at either neutral or acidic pH, or to autoclaving at neutral pH. The tepary bean crude extract and KSTI were found to be heat labile when boiled or autoclaved at neutral pH. At alkaline pH, all the purified protease inhibitors were heat labile, losing antitryptic activity rapidly upon heating to 100 °C. These data suggest that the trypsin inhibitor of TBPI is heat-stable, and similar to the Bowman–Birk family of inhibitors, and can be rapidly inactivated by heat under alkaline conditions.