Characterization of a toxic lectin in Iscador, a mistletoe preparation with alleged cancerostatic properties.

Iscador, a mistletoe preparation widely used in cancer chemotherapy, was found to contain a toxic component which is structurally and functionally closely related to viscumin. When antiviscumin was present in the medium, Vero cells were strongly, but not completely protected against Iscador. The main cytotoxic component in Iscador appears, like viscumin, to bind to terminal nonreducing galactose residues. Thus, addition to the medium of galactose, lactose or melibiose blocked the cytotoxic activity. The main cytotoxic protein in Iscador was retained on a desialylated fetuin column and on an antiviscumin column. This material was found to have a molecular weight close to, but not identical to, that of viscumin.