and RCA are synthesized in the endosperm cells of maturing seeds, and are
stored in an organelle called the "protein body", a vacuolar
compartment. When the mature seed germinates, the toxins are destroyed by
hydrolysis within a few days.
Ricin begins synthesis as a prepropolypeptide
that contains both A and B chains. The signal sequence of the
Nh3-terminus targets the nascent chain to the endoplasmic reticulum (ER)
and is then cleaved off. As the proricin polypeptide elongates it is
N-glycosylated within the lumen of the ER. Protein disulfide isomerases
catalyze disulfide bond formation as the proricin molecule folds itself.
Proricin undergoes further oligosaccharide modifications within the Golgi
complex and then is transported within vesicles to the protein bodies.
Ricin is not catalytically active until it is
proteolytically cleaved by an endopeptidase within the protein bodies. This splits
the polypeptide into the A chain and the B chain still linked by a single
disulfide bond. Since ricin is inactive until then, the plant avoids
poisoning its own ribosomes in case some proricin accidentally passes into the
cytosol during synthesis and transport.
In 1978, ricin was used to assassinate Georgi
Markov in 1978, a Bulgarian journalist who spoke out against the Bulgarian
government. He was stabbed with the point of an umbrella while waiting
at a bus stop near Waterloo Station in London.
They found a perforated metallic pellet embedded in his leg that had presumably
contained the ricin toxin. Castor beans are used as an ingredient in some animal
feeds after the oil has been extracted or inactivated by heating for 20
minutes at 140oC. Attempts to use castor beans in feed for livestock involve
different methods of inactivating ricin while maintaining nutritional value.
Some studies have shown that even after such heat treatment, toxicity remains.
For example, it was lethal to mallard ducks given the feed. "The
toxicity of the meal could be due to either a heat stable or growth inhibiting
factor or due to minute residues of ricin"(Okoye et al.)
A study with sheep showed that
autoclaved castor-bean-meal can be incorporated to 10% of sheep rations without
any ill effect.
of livestock usually occurs by accidental incorporation of castor beans in
their feed. Horses are particularly vulnerable.