Subclassification of soybean Bowman-Birk isoinhibitors

Trypsin inhibitors are proteins in the soybean which inhibit vertebrate pancreatic proteinases. Multiple forms of the two inhibitor classes, the Kunitz and the Bowman Birk inhibitors, are found in the soybean. The three Kunitz isoinhibitors are well characterized with regard to amino acid sequence, genetic inheritance and affinity for trypsin. Enough Bowman‐Birk trypsin inhibitors have been purified and characterized that identities can be established and inhibitors classified into subgroups. The first subgroup includes the classical doubleheaded inhibitor of bovine trypsin and chymotrypsin, BBSTI‐E; its proteolytic derivative, BBSTI‐D, and BBSTI‐E. Subgroup II has the double‐headed weak trypsin inhibitors, BBSTI‐C′, C and A″, related by proteolysis at the amino terminus. Subgroup III has the even weaker trypsin inhibitor BBSTI‐B′ and its apparent proteolytic derivative, BBSTI‐B. Subgroup I, II and III inhibitors have 70–80 residues with high half‐cystine and low glycine content. Subgroup IV consists of the strong trypsin inhibitor BBSTI‐A and its apparent proteolytic derivative BBSTI‐A′. These have about 200 residues, only two half‐cystine residues per molecule and 25 residue percent glycine. They crossreact better than do Subgroup III inhibitors with anti BBSTI‐E antibodies. While the cystine‐rich BBSTI‐E and BBSTI‐C are predominant in the cotyledon, the storage organ of the plant, the glycine‐rich trypsin inhibitors are predominant in the vegetative tissues of the seedling.