The amino acid sequences of two soybean double headed proteinase inhibitors and evolutionary consideration on the legume proteinase inhibitors.

Two proteinase inhibitors, C-II and D-II, were isolated from soybeans. C-II was shown to be an inhibitor of bovine trypsin [EC 3.4.21.4], bovine alpha-chymotrypsin [EC 3.4.21.1], and porcine elastase [EC 3.4.21.11], whereas D-II inhibited only trypsin. The complete amino acid sequences of the two inhibitors establishors. On the basis of the specificities of the inhibitors and their homologies with other double-headed inhibitors, the reactive sites of C-II seems to be alanine-22 for elastase and arginine-49 for trypsin (and probably also for chymotrypsin). D-II was quite unique because its both reactive sites are arginine residues and it only inhibits trypsin. It is suggested that D-II might be a primitive form of double-headed inhibitor and that the prototype single-headed inhibitor was a trypsin inhibitor with an arginine residue as the reactive site.