What makes them so toxic?
The toxins bind to the phospholipid bilayer of the cell membrane. Being amphipathic— partly hydrophobic— they embed in the membrane’ s hydrophobic core, effectively becoming an integral protein. There they form pores, like channel proteins but without any control over what passes through, causing an ion imbalance as Ca2 + ions flood uncontrollably into the cell.
That rise in Ca2 + activates enzymes— proteases that break down proteins, phospholipases that damage membranes, and endonucleases that fragment DNA— so the cell begins to digest itself from within. It also causes mitochondrial damage and osmotic problems, drawing water in until the cell swells and bursts( lysis). Phoratoxins are more efficient at pore formation, producing a greater ion imbalance, whereas viscotoxins increase membrane permeability, leading to lysis.
A Christmas kiss sure is sweet— but a mistletoe snack will be far from holly and jolly.
The evidence
A study from the Instituto de Biología Molecular y Celular investigated how two viscotoxins, VtA3 and VtB— both cationic proteins— interact with model membranes built from negatively charged phospholipids, aiming to explain why the isoforms differ in cytotoxicity. The researchers used three techniques: infrared spectroscopy to assess the toxins’ secondary structure when bound to membranes( comparing native and reduced proteins with their disulphide bonds broken); fluorescence spectroscopy to monitor changes in membrane order and fluidity; and a functional membrane-perturbation test, in which large artificial vesicles loaded with fluorescent dye released that dye as the membrane was disrupted.
VtA3 proved the Grinch, stealing more of the membrane’ s integrity and causing more leakage than VtB, though both increased permeability. Crucially, reduced viscotoxins caused no leakage at all— showing that the disulphide-bonded structure is essential to disrupting the membrane. Once bound, the viscotoxins became restricted in motion, forming strong, stable bonds with the negatively charged phospholipids, with strength varying by lipid composition. Because both toxins coexist in real mistletoe, the effect may be heightened if the plant is eaten, speeding the influx of ions and the rate of cell lysis. The findings also suggest viscotoxins act on the membrane surface, disturbing lipid organisation, rather than inserting as pores.
This was, however, a model system. Real membranes contain other proteins and cholesterol that influence structure and fluidity, and artificial vesicles may not fully replicate a living cell. Mistletoe is genuinely poisonous and can be fatal, but the precise mechanism in living cells may not match the model exactly.
Conclusion
Mistletoe’ s viscotoxins and phoratoxins are both highly toxic, increasing membrane permeability and letting Ca2 + ions flood in— harming the body, especially the brain, gut and heart. The two differ slightly: viscotoxins disrupt the phospholipid organisation, while phoratoxins insert as pores and act through the membrane. The studies behind these conclusions have limits, since they were not conducted in a living body with its many protective factors— such as the glycocalyx, a carbohydrate shield against cationic proteins— and testing the toxins separately ignores how they act together. The true effect on a living cell has not been fully evaluated. The takeaway is simple: do not eat mistletoe.
CATALYST 15