Mistletoe lectins and their A and B chains.

Mistletoe lectins are of high biological activity. The mistletoe lectin I (ML I) is a naturally occurring conjugate of an enzyme (A chain) and a lectin (B chain). Its cytotoxicity is caused by inhibiting the protein synthesis on the ribosomal level. Prominent properties of the A chain are mitogenicity and inhibition of the protein synthesis in cell-free systems. The A chain is also a candidate for the construction of immunotoxins. The B chain as well as the intact lectins activate macrophages and release lymphocytes. They both inhibit the allergen-induced histamine release from leukocytes and the collagen-induced serotonin release from platelets. It cannot be excluded that the combination of selectively cytotoxic and immunopotentiating properties of mistletoe lectins and their chains are decisive for the therapeutic effects of mistletoe preparations.