Toxic effect of Ricinus lectin on hepatoma cells in relation to enzyme modification of the cell surface

Abstract

With regard to the toxic effects of Ricinus lectin, neuraminidase-treated hepatoma cells have been found to be the most sensitive, and untreated hepatoma cells the least. Cells treated with neuraminidase and galactose oxidase exhibited an intermediate sensitivity. At 37°C, the number of Ricinus lectin molecules bound to untreated, neuraminidase-treated and neuraminidase and galactose oxidase-treated cells required to bring about 30% toxicity within 2 h was 15 · 10 5, 7.5 · 10 5 and 11.5 · 10 5 molecules/cell, respectively. This difference was rather small and suggests that the additional binding sites exposed following enzyme treatment were as efficient in mediating lectin toxicity as those present before enzyme treatment. Positive cooperativity was observed during Ricinus lectin binding to enzyme-treated cells at 37°C and the apparent association constant increased with the increase of binding site occupancy. The binding sites on enzyme-treated cells appeared to be homogeneous since under different physical conditions (4°C) the shape of the Scatchard plot could be altered in such a way as to produce a single line of slope. In contrast to enzyme-treated cells, untreated cells did not exhibit a positive cooperative process either at 37°C or at 4°C. We found that the toxicity of Ricinus lectin paralleled the irreversible specific binding of lectin, suggesting that only this was able to mediate the toxic effect. Our results are discussed in terms of the possible entry into the cells of Ricinus lectin and this occurs more rapidly in enzyme-treated than in untreated cells. This difference agrees with the sequence of events proposed: (i) Binding of Ricinus lectin; (ii) Clustering of lectin binding sites; and (iii) Endocytosis.