Relevance of multiple soybean trypsin inhibitor forms to nutritional quality

Trypsin inhibitors contribute to the antinutritional component of raw soybean meal by inhibiting vertebrate pancreatic serine proteinases in the small intestine, resulting in a range of deleterious physiological effects in the animal. The variation in the nutritional quality of soybean cultivars stems partly from wide-ranging levels of trypsin inhibitor, and from varying proportions of trypsin inhibitors of two classes--the Kunitz and the Bowman-Birk inhibitor classes. The latter class is better able to survive heat processing and digestion in the stomach. Some variation in cultivars also arises from the array of isoinhibitors present in the seed. The three Kunitz isoinhibitors, Ti(a), Ti(b) and Ti(c) differ by as much as 1000-fold in their interaction with bovine trypsin. The Bowman-Birk isoinhibitors differ not only in their extent of interaction with trypsin, but in their spectrum of inhibition of the other pancreatic enzymes, chymotrypsin and elastase. In this chapter, we look at twenty-two Bowman-Birk inhibitors from ten soybean cultivars and find at least twelve which are different enough in amino acid composition and/or inhibitor activity to be distinct protein species. Of these, three pairs are related by proteolytic digestion. Quite ironically, the Bowman-Birk inhibitors, and to some extent the Kunitz inhibitors, contribute to the nutritional quality of soybeans by virtue of their high cystine content which supplements the low or negligible amounts of sulfur-containing amino acids in the storage proteins that comprise the bulk of the protein reserve in the seed.