About the Lectin

Although now recognized as widespread occurrence, the presence of a lectin in the bark was considered unusual when SNA from elderberry bark was first isolated. Initial inhibition experiments indicated that of the simple sugars tested, lactose and GalNAc are the most potent inhibitors of agglutination ¹ . Galactose is only slightly less inhibitory than either lactose or GalNAc, while melibiose, raffinose, and fucose are 3-4 times less inhibitory than lactose or GalNAc. These data suggest that SNA actually has a broad specificity for both α– and β–linked galactose, as well as unrelated sugars such as fucose. It has subsequently been determined that the lectin does not bind to glycoproteins or glycolipids containing only terminal α(2,3)-linked sialic acid (NANA) residues. Purified SNA, a tetrameric protein (140 kDa) composed of two different subunits, is highly specific for NANA α(2,6)galactose, as found in N-linked oligosaccharides, and for NANA α(2,6)GalNAc, as found in O-linked oligosaccharides ² . SNA is not blood group specific, but it does agglutinate type A erythrocytes more strongly than either B or O cells. Neuraminidase treated erythrocytes will still react with SNA due to the presence of terminal galactose residues. Trypsin treated erythrocytes react with the lectin better than untreated cells. SNA activity is stable up to 50°C, but the lectin is inactivated when heated above 70°C. The lectin is also inactivated in the presence of higher than 2.0 M NaCl ¹ . Immobilized SNA has been used successfully for affinity purification of glycoproteins ³ and the separation of molecular species that differ only in the linkage of NANA to the penultimate sugar, i.e. α2,6 or α2,3 to Gal/GalNAc. Thus, a clean separation of Neu5Ac α2,6 lactitol (bound) and Neu5Ac α2,3 lactitol (unbound) was achieved. Roth and his colleagues ⁶ have demonstrated the presence of α(2,6)-linked NANA in human colon carcinoma cells ⁴ . The lectin has also been used for localization of the α(2,6) sialyltransferase ^5,6 , and for the detection of sialic acid containing glycoproteins blotted onto nitrocellulose ⁷ .

REFERENCES

1. Broekaert, W. F., et al. (1984) Biochem. J. 221 : 163-169.
2. Shibuya, N., et al. (1987) J. Biol. Chem. 262 : 1596.
3. Shibuya, N., et al. (1987) Arch. Biochem. Biophys. 254 : 1-8.
4. Sata, T., et al. (1991) Am. J. Pathol. 139 : 1435-1448.
5. Roth, J., et al. (1985) Cell. 43 : 287-295.
6. Roth, J., et al. (1986) J. Biol. Chem. 261 : 14307-14312.
7. Taatjes, D. J., et al. (1988) Histochem. J. 20 : 478-490.