Thermal stability and digestibility of alpha-amylase inhibitor from Phaseolus vulgaris cultivar Tora

Tora bean alpha-amylase inhibitor (TAI) was a proteinaceous alpha-amylase i nhibitor from Phaseolus vulgaris cultivar Tora, a common kidney bean in Jap an, and it was active against pancreatic and salivary alpha-amylases. We in vestigated the behavior of this inhibitor during heat treatment and enzymat ic digestion in order to evaluate the nutritional significance of kidney be an alpha-amylase inhibitor. The inhibition against pancreatic alpha-amylase increased with a rise in preincubation temperature from 25 to 45 degrees C . TAI was relatively stable in pH 7.0 after 20 min incubation at 80 degrees C, while it was rapidly inactivated in pH 3.0 and 5.0. Soaking of tora bea ns in water for 15 h at 20 degrees C caused an de crease of soluble protein content, but hardly affect the inhibitory activity, while boiling treatmen t for 15 min inactivated nearly 90% of the inhibitor. The inhibitor was qui te resistant to the proteolysis by pepsin and trypsin, while it was relativ ely susceptible to hydrolysis by chymotrypsin and lost about 90% of the ori ginal activity during 2 h digestion.