Isolation of viscotoxins. Cytotoxic basic polypeptides from Viscum album L.

In the course of cytotoxicity-directed fractionation of extracts from Viscum album L., it appeared that crude viscotoxin, a basic polypeptide fraction, is responsible for cytotoxic properties of the plant examined. Separation of this fraction by ion exchange chromatography on carboxymethylcellulose led to isolation of five cytotoxic components. Four of them, named viscotoxins I, II, III and IVb were obtained as homogeneous substances. The isolated polypeptides exhibited cytotoxic activity against human tumor cells of KB and HeLa lines in tissue culture in the concentration range (ED50 values) of 0.2-1.7 microgram/ml. Considerably decreased activities were found when tissue culture was contaminated with mycoplasma. The estimated molecular weights (about 5000) and the amino acid compositions suggest the viscotoxins II, III and IVb are identical to viscotoxins B, A-2 and A-3, respectively. The latter three viscotoxins have been isolated previously by others as toxic (cardiotoxic) components of Viscum album. The fractionation of the crude viscotoxin was carried out by a new procedure, considerably simpler than that described for isolation of viscotoxins B, A-2 and A-3