INTERACTION OF WHEAT-GERM AGGLUTININ WITH SIALIC-ACID

Wheat germ agglutinin (WGA) specifically binds N-acetylneuraminic acid (NeuNAc) and N-acetyl-D-galactosamine (D-GalNAc) in addition to N-acetyl-D-glucosamine (D-GlcNAc) and its .beta.-(1 .fwdarw. 4)-linked oligosaccharides. Both ovine submaxillary mucin (OSM) and asialo ovine submaxillary mucin, glycoconjugates lacking D-GlcNAc units, precipitate WGA. The precipitation of WGA by OSM is inhibited completely by D-GlcNAc as well as by the following saccharides (potency relative to GlcNAc (1.0) in parentheses): N,N''-diacetylchitobiose (32); NeuNAc (0.30); NeuNAc methyl ester (0.27); NeuNAc-7, 5-acetamido-3,5-dideoxy-L-arabino-heptulosonic acid (0.80); D-GalNAc (0.20). Selective conversion of the NeuNAc groups of fetuin and of OSM to NeuNAc-7 (2 fewer C atoms) by periodate oxidation-borohydride reduction yielded analog glycoproteins that reacted more strongly with WGA than the parent glycoproteins. Analog fetuin precipitated 2.5 times more WGA from solution than native fetuin. A 3-fold higher concentration of N,N''-diacetylchitobiose also was required to inhibit the analog fetuin-WGA interaction than to inhibit equivalently the native fetuin-WGA interaction. Sialic acid groups appear to be immunodominant in fetuin-WGA interaction. Removal of sialic acid from fetuin or analog fetuin by mild acid hydrolysis abolished the ability of the resulting substrates to precipitate WGA but had no effect on their interaction with the phytohemagglutinin from Phaseolus vulgaris. These results are interpreted in terms of the configurational similarity of NeuNAc and D-GalNAc with DGlcNAc at positions C-2 (N-acetamido group) and C-3 (hydroxyl group) of the pyranose ring; these are the positions critical to productive contact with the WGA combining site.