Abstract: The effects of ultrasonic fields on the activities and secondary structures of Kunitz trypsin inhibitor (KSTI) andBowman-Birk trypsin inhibitor (BBTI) from soybean were studied by measuring the changes of inhibitory activities, sulfhydrylbonds and far-UV circular dichroism (CD) spectra. It was found that the far-UV CD spectra of both SKTI and BBTI show asingle negative peak at around 200nm. The negative minimum [θ]200nm of SKTI was measured as -2545 deg•cm2•d/mol andthe secondary structure was composed of β-sheet (22.5%), β-turn (16.2%) and random (61.4%). The negative minimum [θ]200nm of BBTI was measured as -797 deg•cm2•d/mol and the secondary structure was composed only of β-sheet (52.6%)and random (47.4%). By the treatment at 65% amplitude of ultrasonic field and 11min period, [θ]200nm of KSTI was changedas-1827 deg•cm2•d/mol. When the treatment period was extended to 20 min, The β- turn and random coil content of KSTIwere decreased to 10.8% and 54% respectively and β-sheet was increased to 35.2%. About 71.5% of disulfide bonds werefound transformed to sulfhydryl bonds and 55% of inhibitory activity inactivated. Whereas BBTI show more stable than KSTIunder ultrasonic fields in its inhibitory activity and secondary structure. With 20min period treatment at 65% amplitude ofultrasonic field.The [θ]200nm of BBTI was only changed as -700 deg•cm2•d/mol. The β-sheet, random structures and theinhibitory activity of BBTI were kept unchanged basically and only about 5.29% of disulfide bonds were transformed. Hence itis suggested that ultrasonic fields lead the reduction and breakdown of disulfide bonds of SKTI, and then lead the alteration of thesecondary structures of KSTI, and finally, inactivate the inhibitory activity of KSTI.

Key words: soybean trypsin inhibitors, ultrasound, circular dichroism, secondary structure