Interaction of trypsin, beta-factor XIIa, and plasma kallikrein with a trypsin inhibitor isolated from barley seeds: a comparison with the corn inhibitor of activated Hageman factor

A trypsin inhibitor was purified from barley seeds by a modification of published procedures. We determined the dissociation constant, Ki, for the complexes of the barley inhibitor with trypsin, beta-Factor XIIa, and plasma kallikrein. We compared these constants for those of the same enzymes with the corn Hageman Factor inhibitor, which is a homolog of the barley inhibitor. The strength of interaction of the barley inhibitor with the three enzymes was: trypsin greater than beta-Factor XIIa greater than plasma kallikrein. In contrast, the corn inhibitor inhibits beta-Factor XIIa most strongly and does not inhibit plasma kallikrein at all. A possible structural basis for the difference in inhibition specificity is discussed.