Studies on soybean trypsin inhibitors, XII. Linear sequences of two soybean double-headed trypsin inhibitors, D-II and E-I.

Soybean inhibitor D-II is an inhibitor of bovine trypsin. Sequence analysis was carried out on the reduced and S-carboxymethylated protein by conventional methods to establish the complete amino acid sequence. The sequence of D-II indicated high homology with other legume inhibitors, but it was unique because of the occurrence of identical residues (arginine) at both of the reactive sites. This structure is thought to reflect that of a prototype double-headed inhibitor. The possible evolutionary process of the legume double-headed inhibitors is discussed on this basis. Comparison with another soybean inhibitor C-II suggested that a single methionine (C-II)-glutamine (D-II) replacement at the P2'position resulted in the loss of alpha-chymotrypsin inhibitory activity of D-II. The results of a hydrogen peroxide oxidation experiment on C-II supported this suggestion. The sequence of the amino-terminal 21 residues of inhibitor E-I was determined using a sequentor. It was shown that this inhibitor lacks the amino-terminal nine residues of D-II.