Isolation and Comparison of Galactose-binding Lectins from Abrus precatorius and Ricinus communis

Abstract

The agglutinins present in the seeds of Abrus precatorius and Ricinus communis have been separated from the toxins, abrin and ricin, and extensively purified. The procedure involves ion exchange chromatography on DEAE- and CM-cellulose columns, affinity chromatography on Sepharose 4B columns, and sucrose gradient centrifugation. The purified agglutinins which possessed almost no toxic activity were found by analytical ultracentrifugation to have molecular weights of 134,000 (abrus agglutinin) and 120,000 (ricinus agglutinin). After treatment with sodium dodecyl sulfate and β-mercaptoethanol the agglutinins were split into four peptide chains, as revealed by polyacrylamide gel electrophoresis. The binding of the agglutinins and the toxins to human erythrocytes was inhibited by galactose or galactose-containing carbohydrates, like lactose. Equilibrium dialysis experiments indicate that the agglutinins possess two binding sites for lactose, while the nonagglutinating toxins, abrin and ricin, have only one binding site. The agglutinins as well as the toxins were found to consist of polypeptide chains of nearly the same size (30,000 to 35,000). Abrin and ricin consist of only two polypeptide chains, one of which is involved in the binding to cells. The agglutinins consist of four polypeptide chains, only two of which appear to be involved in the binding to cell surfaces. The data suggest that, within one kind of seed, the polypeptide chains involved in the binding of the agglutinins and the toxins to cell surfaces are identical or very similar.