Specificity of isolectins of wheat germ agglutinin for sialyloligosaccharides: a 360-MHz proton nuclear magnetic resonance binding study.

The binding of three purified sialic acid containing oligosaccharides to two isolectins of wheat germ agglutinin (WGA I and WGA II) has been quantitated by measuring the broadening of a ligand resonance in the proton nuclear magnetic resonance (1H NMR) spectrum at 360 MHz. The ligands, isolated from bovine colostrum by using the procedure of Schneir and Rafelson [Schneir, M. L., & Rafelson, M. E., Jr. (1966) Biochim. Biophys, Acta 130, 1--11], were identified by 1H NMR as the alpha (2,3) and alpha (2,6) isomers of N-acetylneuraminyllactose, as well as the alpha (2,6) form of N,N'-diacetylneuraminyllactosamine. The dissociation constants, KD's, ranged from 0.7 to 10 mM (24 +/- 1 degree C). Two noteworthy features of WGA specificity emerge from an examination of the observed affinities: (1) both isolectins bind the alpha (2,3) isomer of N-acetylneuraminyllactose with higher affinity than the alpha (2,6) form and (2) WGA I binds two of the sialyloligosaccharides more tightly than does WGA II.