Soybean Proteinase Inhibitors and Human Proteolytic Enzymes: Selective Inactivation of Inhibitors by Treatment with Human Gastric Juice

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Abstract
The stability of soybean proteinase inhibitors to 0.1 M HCl and to human gastric juice was investigated. Purified Kunitz soybean inhibitor (SBTI), purified Lima bean inhibitor (LBI), and crude, aqueous extract (1:5 w/v) of three varieties of soybeans were incubated for 24 hours at 37°. The SBTI, the LBI, and the soybean extracts were mixed with gastric juice or 0.1 M HCl in amounts corresponding to 4.0 g SBTI, 2.4 g LBI, and 130 g soybean meal per liter. The trypsin and chymotrypsin inhibitor activities of the incubation mixtures were measured after 0.0, 0.5, 1.0, 2.0, 4.0, 8.0, and 24.0 hours of incubation. Human pancreatic juice was used as enzyme source. SBTI was rapidly inactivated by the incubation with gastric juice. LBI was hardly affected by the incubation. The trypsin and chymotrypsin inhibitor activities of the soybean extracts were reduced to 60–70%, indicating that SBTI contributed ⅓ of the inhibitor activity of these varieties. Thus, the compact, rigid inhibitors, such as the Bowman-Birk, were responsible for the remainder. The latter group of inhibitors, when ingested with the soybeans, will not be inactivated by passage through the stomach. They probably reach the duodenum in an active form.

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