Sementes de Dioclea Sclerocarpa Ducke: um sistema endógeno lectina-receptor

Abstract

The lectins constitute a special class of proteíns with the ability to interact reversibbly and specifically with cells and glycoconjugates, eventually inducing physiological modifications to the cells with while they interact, with no modificatons in the carbohydrate covalent structure. Although being widely found in plant seeds, their roles are yet a case of speculation. The ability of the lectins to interact with sugar containing structures suggests a relevant in vivo biological role. This hypothesis demands the existence of a specific receptor in the locus where it is found. The search for this receptor is one of the most intriguing subjects in lectinology. Although a great number of lectins have already been isolated and characterized, well identified and characterized structures, that should interact with the lectin is to be found. When Dioclea sclerocarpa Ducke seed flour is extracted in saline (0,15M NaCI) and the activity tested with red blood cells, no activity was found, although several lectins from the same genus are well known. This fact suggests that receptors shoud be present in the seed, reacting with the lectin, making it unavailable. When the flour was extracted at low pH or in the presence of the binding sugar, the lectins could be extracted. In order to obtain the lectins, the flour was exhaustively extracted with 0,15 M NaCI, followed by 0,1 M B-alanine, pH 2,6, buffer. The clear supernatant after dialysis against 0,15 M NaCI containing CaCl2 and MnCI2 5 mM, showed a high activity. Moreover, the residue, after a thorough protein extraction, was able to interact with the lectin, suggesting the presence of an insoluble lectin receptor. On the other hand, when the saline supernatant was applied to a column containing the immobilized Dioclea altissima lectin (in Sepharose-4B-CNBr activated), a fraction containing a glicoprotein was obtained. The lectin isolated was showed to be similar to the other Diocleinae lectins. The results suggest the presence, not only of a lectin but also of two receptors, one soluble that could be the responsible for the lack of activity in the saline extract, and one insoluble that retains the lectin in the seed "residue".