Interactions of Soybean Lectin, Soyasaponins, and Glycinin with Rabbit Jejunal Mucosa In Vitro

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Abstract
Summary: Mucosal samples from rabbit jejunum were incubated (30 min, 25°C) with (125I) glycinin in the presence of buffer, soybean lectin (50 μg/ml) soyasaponins (1 mg/ml), or both lectin and saponins. The mueosal uptake of (125I) glycinin was negligible with buffer, and increased progressively with additions of soybean lectin (P < 0.05), soyasaponins (P < 0.005), and both (P < 0.0001). The stimulation of uptake by lectin and saponins together was greater than the sum of their individual effects (P < 0.0005). The effect of soybean lectin on glycinin uptake was concentration dependent, reaching a maximum at approximately 50 μg/ml for the stimulation of uptake in the presence of saponins, and was inhibited by D-GaINAc. Although the mechanisms involved in mucosal uptake of glycinin cannot be described from these data, we have assumed the presence of two independent pathways for lectin-stimulated and saponin-induced uptakes. In addition, we have proposed that soybean lectin, by binding to terminal galactoside sites at the enterocyte apical membrane, enhances a crenator effect of saponins that leads to increasing leakage of glycinin into the cell. Speculation: Soyasaponins are heat resistant and their cellular lytic and trypsin inhibiting activities are known to increase after heat processing. Soybean lectin is heat labile, but remains at appreciable concentrations in inadequately processed and crude soy extracts. Interactions and effects similar to those occurring with mueosal tissues in vitro could also take place in the jejunum in vivo upon ingestion of such soy products.

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