EFFECT OF THIOREDOXIN-LINKED REDUCTION ON THE ACTIVITY AND STABILITY OF THE KUNITZ AND BOWMAN-BIRK SOYBEAN TRYPSIN-INHIBITOR PROTEINS

Protease inhibitors, including the Kunitz and Bowman-Birk trypsin inhibitors of soybean, can be reduced by the NADP/thioredoxin system (NADPH, thioredoxin, and NADP-thioredoxin reductase) from either Escherichia coli or wheat germ. In the present study, we report that, when reduced by thioredoxin, the Kunitz and Bowman-Birk soybean trypsin inhibitors lose their ability to inhibit trypsin. Moreover, the reduced form of the inhibitors showed increased susceptibility to heat and proteolysis by either subtilisin or a protease preparation from germinating wheat seeds. These finding, on the one hand, pave the way for complete in vitro inactivation of trypsin inhibitory activity in soybean products and, on the other, provide new evidence that thioredoxin functions in vivo in mobilizing proteins during seed germination.