Interactions with endogenous receptors

The existence of functional endogenous receptors for lectins in plants has often been suggested. Such receptors, by definition, are good inhibitors of lectins from the same plant and therefore their presence ought to be relatively easy to demonstrate. Indeed, the occurrence of endogenous inhibitors in the seeds of Vicia cracca had been suggested as early as 1960 (Renkonnen, 1960) and similar observations have been made for the seeds of Dioclea sclerocarpa (Pusztai & Moreira, unpublished observations).

Most legume seeds contain appreciable amounts of α-galactosidic oligosaccharides, such as raffinose, stachyose and verbascose, which are good inhibitors of galactose-specific lectins. In seeds such as soyabean, which contain both such lectins and also α-galactosides, the oligosaccharides may be regarded as natural lectin-binders. The lectin, which is a major example of the N-acetyl-D-galactosamine/galactose-specific class of lectins co-exists with raffinose-type oligosaccharides in the seeds. These naturally occurring sugars are about 14% as effective in inhibiting the haemagglutination activity of the lectin as N-acetyl-D-galactosamine (Pereira et al., 1974). It is thus not surprising that less than 20% of the lectin can be absorbed on to guar gum-based affinity absorbents from non-dialysed extracts of soyabean. However, both by dialysis and/or precipitation of the proteins with ammonium sulphate followed by dialysis, most of the natural lectin-binders are removed from the soyabean extracts. The lectin in the extract is then selectively and quantitatively absorbed by the affinity column. Similar results can be obtained if the seed meal is first washed by 60% aqueous ethanol to remove the bulk of low molecular weight glycosides before the extraction of the lectin (Pusztai, Stewart & Watt, in press).