Purification of melibiose-binding lectins from two cultivars of Chinese black soybeans

A dimeric 50 kDa melibiose-binding lectin was isolated from the seeds of the cultivar of soybean (Glycine max), called the small glossy black soybean. The isolation procedure comprised ion exchange chromatography on Q Sepharose, SP Sepharose and Mono Q followed by gel filtration on Superdex 75. The lectin was adsorbed on all three ion exchangers, and it exhibited an N-terminal sequence identical to that of soybean lectin. Of all the sugars tested, melibiose most potently inhibited the hemagglutinating activity of the lectin, which was stable between pH 3-12 and 0-70 °C. The lectin evoked maximal mitogenic response at about the same molar concentration as Con A. However, the response was much weaker. The soybean lectin inhibited the activity of HIV-1 reverse transcriptase as well as the proliferation of breast cancer MCF7 cells and hepatoma HepG2 cells with an IC₅₀ of 2.82 μM, 2.6 μM and 4.1 μM, respectively. There was no antifungal activity. Another lectin was isolated from a different cultivar of soybean called little black soybean. The lectin was essentially similar to small glossy black soybean lectin except for a larger subunit molecular mass (31 kDa), a more potent mitogenic activity and lower thermostability. The results indicate that different cultivars of soybean produce lectins that are not identical in every aspect.