ISOLATION AND CHEMICAL PROPERTIES OF A RICIN VARIANT FROM CASTOR BEAN

A ricin variant was isolated and purified from defatted castor bean [Ricinus communis] meal by extraction with water at pH 3.8, precipitation with sodium chloride, fractionation with ammonium sulfate followed by gel filtration through Sephadex G-75 and chromatography on DEAE-cellulose. The preparation thus obtained appeared homogeneous on gel disc electrophoresis (pH 4.0 and 8.3), electrofocusing and ultracentrifugation. The ricin variant which was isoelectric at pH 7.25 proved to be a glycoprotein of 54,000 dalton MW. When injected i.p., it was found to have an LD50 in mice of 0.031 .mu.g of protein/g of body wieght. The amino acid composition as well as the amino terminal amino acids (isoleucine and alanine) of this protein were determined. After reduction and carboxymethylation, the ricin variant was shown by gel disc electrophoresis to consist of 2 dissimilar subunits. Isolation and partial characterization of these subunits revealed that the A chain of the ricin variant and that of ricin D are very similar, whereas their B chains differed considerably in their chemical composition.