Dimeric derivatives of concanavalin A (Con A) such as acetyl- and succinyl-Con A have been used for years as probes of cellular membranes. The altered binding and biological activities of these derivatives relative to native tetrameric Con A have generally been attributed to their reduced valence. However, the present study shows that acetyl- and succinyl-Con A possess lower affinities than tetrameric Con A toward certain oligomannose-type glycopeptides which are found on the surface of cells. It has previously been shown that native tetrameric Con A possesses 5-30-fold enhanced affinities toward Man7-Man9 oligomannose-type glycopeptides, respectively, relative to Man5 and Man6 oligomannose-type glycopeptides [Bhattacharyya, L., & Brewer, C.F. (1989) Eur. J. Biochem. 178, 721-726]. Using titration microcalorimetry and hemagglutination inhibition measurements, methyl alpha-D-mannopyranoside, methyl 3,6-di-O-(alpha-D-mannopyranosyl)-alpha-D-mannopyranoside (which binds with about 60-fold higher affinity than methyl alpha-D-mannopyranoside and is the major Con A binding epitope on oligomannose-type carbohydrates), and a Man5 oligomannose-type oligosaccharide are shown to bind to underivatized dimeric Con A at pH 5.2 and acetyl- and succinyl-Con A at pH 7.2 with affinities equal to those of native tetrameric Con A. However, a mixture of Man7 and Man8 glycopeptides and a Man9 oligomannose-type glycopeptide were shown to bind to underivatized dimeric Con A and acetyl- and succinyl-Con A with affinities only about 2-fold higher than the Man5 oligosaccharide, in contrast to the higher affinities of native tetrameric Con A for these carbohydrates. Thus, Man7-Man9 oligomannose-type glycopeptides bind with approximately 4- and 10-fold lower affinities, respectively, to dimeric Con A and its derivatives relative to tetrameric Con A. Differences in the affinities of dimeric and tetrameric Con A for the larger oligomannose-type glycopeptides are ascribed to the ability of the longer alpha(1-3) and alpha(I-6) arms of the Man7-Man9 glycopeptides to ''jump'' between adjacent monomer binding sites of the tetramer before dissociating from the protein and the absence of this effect in the dimer where the binding sites are further separated. The present findings indicate that acetyl- and succinyl-Con A can not be used as mere ''divalent'' derivatives of the lectin in studies of cell membranes which possess Man7-Man9 oligomannose-type carbohydrates.
