CHANGES IN KUNITZ TRYPSIN INHIBITOR DURING GERMINATION OF SOYBEANS: AN IMMUNOELECTROPHORESIS ASSAY SYSTEM

Implications of antinutritional components in soybean foods

There are a number of components present in soybeans that exert a negative impact on the nutritional quality of the protein. Among those factors that are destroyed by heat treatment are the protease inhibitors and lectins. Protease inhibitors exert their antinutritional effect by causing pancreatic hypertrophy/hyperplasia, which ultimately results in an inhibition of growth. The lectin, by virtue of its ability to bind to glycoprotein receptors on the epithelial cells lining the intestinal mucosa, inhibits growth by interfering with the absorption of nutrients. Of lesser significance are the antinutritional effects produced by relatively heat stable factors, such as goitrogens, tannins, phytoestrogens, flatus-producing oligosaccharides, phytate, and saponins. Other diverse but ill-defined factors appear to increase the requirements for vitamins A, B12, D, and E. The processing of soybeans under severe alkaline conditions leads to the formation of lysinoalanine, which has been shown to damage the kidneys of rats. This is not generally true, however, for edible soy protein that has been produced under milder alkaline conditions. Also meriting consideration is the allergenic response that may sometimes occur in humans, as well as calves and piglets, on dietary exposure to soybeans.

Aflatoxin contamination in soybeans: role of proteinase inhibitors, zinc availability, and seed coat integrity

Soybean trypsin inhibitors are thought to ward off pathogens. Studies with aflatoxigenic strains of Aspergillus flavus and A. parasiticus, frequent soybean contaminants, revealed that trypsin inhibitors do not affect the growth of these fungi and aflatoxin production. Further, the availability of zinc, an essential mineral for aflatoxin synthesis that was thought to explain increased aflatoxin accumulation in cooked compared with raw soybeans, was shown to decrease upon cooking. Seed coat integrity, ensuring limited access and a low moisture content, is responsible for the slow colonization of the seed by A. flavus.

Antigenicity of native and modified Kunitz soybean trypsin inhibitors

Food provides a continuous antigenic stimulus to the immune system and the antigenicity of processed food proteins should be considered in toxicological evaluations. The antigenicity of the Kunitz trypsin inhibitor was studied using antibodies prepared by inoculating rabbits with native, heat-denatured, and N-acetylcysteine-treated Kunitz soybean trypsin inhibitors. Immunochemical studies using a competitive solid-phase enzyme immunoassay and two groups of sera revealed two patterns of antigenicity. Antibodies elicited with the denatured inhibitor were specific for the denatured conformation of the protein. In contrast, native inhibitor elicited antibodies that selectively recognized determinants in both native and heat-treated protein, but that did not bind trypsin inhibitors treated with N-acetylcysteine. These results imply that: the disulfide bonds must be intact to maintain the native antigenic conformation and the cysteine treatment may suppress allergic manifestations of soybean trypsin inhibitors and possibly other food proteins. These studies were extended by analyzing a panel of monoclonal antibodies prepared against native Kunitz trypsin inhibitor. The inhibitor has at least two distinct antigenic sites (epitopes), one of which is retained under denaturing conditions. The measurement of native Kunitz trypsin inhibitor in food samples by immunoassay appears practical. The relevance of these findings to food processing, food safety, and health is also discussed.

Isolation and characterization of genetic variants of the Kunitz soybean trypsin inhibitor

Three genetically related forms of the Kunitz soybean trypsin inhibitor have been isolated by ion-exchange chromatography. The three variants have different electrophoretic mobilities and have been designated Tia, Tib and Tic. An enzyme assay system was used to determine the association equilibrium constant of each variant for trypsin at pH 8.1 and values of 2 x 10(9) M-1, 2 x 10(7) M-1 and 3 x 10(10) M-1 were found for Tia, Tib and Tic respectively. The hydrolysis equillibrium constant at pH 6.0 for the reactive site peptide bond hydrolysis for each variant was also determined and found to be 4, 9 and 3 for Tia, Tib and Tic, respectively.