DIFFERENCES IN THE BINDING AFFINITIES OF DIMERIC CONCANAVALIN-A (INCLUDING ACETYL AND SUCCINYL DERIVATIVES) AND TETRAMERIC CONCANAVALIN-A WITH LARGE OLIGOMANNOSE-TYPE GLYCOPEPTIDES

Dimeric derivatives of concanavalin A (Con A) such as acetyl- and succ inyl-Con A have been used for years as probes of cellular membranes. T he altered binding and biological activities of these derivatives rela tive to native tetrameric Con A have generally been attributed to thei r reduced valence. However, the present study shows that acetyl- and s uccinyl-Con A possess lower affinities than tetrameric Con A toward ce rtain oligomannose-type glycopeptides which are found on the surface o f cells. It has previously been shown that native tetrameric Con A pos sesses 5-30-fold enhanced affinities toward Man7-Man9 oligomannose-typ e glycopeptides, respectively, relative to Man5 and Man6 oligomannose- type glycopeptides [Bhattacharyya, L., & Brewer, C.F. (1989) Eur. J. B iochem. 178, 721-726]. Using titration microcalorimetry and hemaggluti nation inhibition measurements, methyl alpha-D-mannopyranoside, methyl 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 un derivatized 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. Howe ver, a mixture of Man7 and Man8 glycopeptides and a Man9 oligomannose- type glycopeptide were shown to bind to underivatized dimeric Con A an d 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 olig omannose-type glycopeptides bind with approximately 4- and 10-fold low er affinities, respectively, to dimeric Con A and its derivatives rela tive to tetrameric Con A. Differences in the affinities of dimeric and tetrameric Con A for the larger oligomannose-type glycopeptides are a scribed to the ability of the longer alpha(1-3) and alpha(I-6) arms of the Man7-Man9 glycopeptides to ''jump'' between adjacent monomer bind ing sites of the tetramer before dissociating from the protein and the absence of this effect in the dimer where the binding sites are furth er separated. The present findings indicate that acetyl- and succinyl- Con A can not be used as mere ''divalent'' derivatives of the lectin i n studies of cell membranes which possess Man7-Man9 oligomannose-type carbohydrates.