Soyasaponins: the relationship between chemical structure and colon anticarcinogenic activity

Soyasaponins are bioactive compounds found in many legumes. Although crude soyasaponins have been shown to have anti-colon carcinogenic activity, there have been no structure-activity studies. In this study, therefore, purified soyasaponins and soyasapogenins were tested for their ability to suppress the growth of HT-29 colon cancer cells, as determined by the WST-1 assay, over a concentration range of 0-50 ppm. Soyasaponin I and III, soyasapogenol B monoglucuronide, soyasapogenol B, soyasaponin A1, soyasaponin A2, and soyasapogenol A were evaluated. Also tested were mixtures comprising acetylated group A soyasaponins, deacetylated group A soyasaponins, and group B soyasaponins. The most potent compounds were the aglycones soyasapogenol A and B, which showed almost complete suppression of cell growth. The glycosidic soyasaponins by comparison were largely inactive. Soyasaponin A(1), A(2), and I, group B and deacetylated and acetylated group A fractions had no effect on cell growth. Soyasaponin III and soyasapogenol B monoglucuronide were marginally bioactive. These results suggested that the bioactivity of soyasaponins increased with increased lipophilicity. Results from in vitro fermentation suggested that colonic microflora readily hydrolyzed the soyasaponins to aglycones. These observations suggest that the soyasaponins may be an important dietary chemopreventive agent against colon cancer, after alteration by microflora.