Stability and specificity of rice bran trypsin inhibitor

The stability and inhibitory specificity of rice bran trypsin inhibitor (RBTI) was investigated in an attempt to understand its nutritional significance. RBTI retained about 100% of its original activity over a pH range from 4 to 10 during 24-h incubation at 37 degrees C. In heat treatment, RBTI at acidic and neutral pH values still possessed about 50% of its initial activity after 30-min incubation at 100 degrees C, although it was completely inactivated during 15-min incubation at pH 10 and 100 degrees C. The effects of metal ions and some reagents on RBTI were examined and it was found that Hg ion reduced RBTI's inhibitory activity: The inhibitor lost 30-100% of its original activity upon incubation with a reducing, an oxidizing or a thiol reagent. Digestion tests on RBTI indicated that alpha-chymotrypsin did not affect the inhibitory activity and pepsin caused only a 30% loss of the initial inhibitory activity after 24-h digestion. To determine inhibitory specificity, bovine, hog, rat, and human trypsins were used as target enzymes bound to an immobilized RBTI column. Titrations of the purified enzymes with RBTI showed that bovine, hog, and rat trypsins were powerfully inhibited by the inhibitor, while human trypsin was only weakly inhibited.