ricin

Extremely poisonous extract from the seeds of the castor-oil plant. When incorporated into monoclonal antibodies, ricin can attack cancer cells, particularly in the treatment of lymphoma and leukaemia.

Ricin acquired notoriety in 1978, when Georgi Markov, an exiled Bulgarian dissident queuing at a London bus stop, was hit by an umbrella spiked with this poison and died within hours. A few similar cases have been reported since. Because of its extremely high toxicity – the LD50 or half-lethal dosis is estimated to lie in the milligram range – and its relatively straightforward production from the castor bean, this toxin is a major issue in crime prevention.

Ricin is a protein consisting of two polypeptide chains. The B chain is related to lectins, a large family of plant proteins known specifically to recognize cell surfaces, which can lead to blood clotting if one protein binds several cells. The castor bean seeds contain another toxin, Ricinus communis agglutinin, which acts in this way. In the case of ricin, however, each lectin-like B chain binds to just one cell and forces it to import the toxic A chain, which modifies the RNA of the ribosome and thereby efficiently blocks protein biosynthesis.