Sunday 7 September 2014

Death of a Bulgarian

It's been a while since I wrote about the quantification of lethal doses but today's anniversary gives me the chance to fill in a gap in the table of increasing toxicity.
On this day 7th September in 1978  Георги Иванов Марков, a Bulgarian living in London and working for BBC World Service was stabbed in the back of the leg by a James Bond inspired weapon: an umbrella that delivered a fatal dose of ricin. Georgi Markov died three days later in hospital.  This was a decade before the fall of the Berlin Wall and the Cold War was at its height. Markov was a satirist who exposed the Bulgarian government and their masters in the Kremlin to ridicule, which is often a more effective weapon in a propaganda war than bullets. If you pissed off the Bulgarian government they'd send an assassin after you with (probably) the help of the KGB.

What's ricin?  Why, it is a type 2 ribosome-inactivating protein (RIP) isolated from the castor bean Ricinus communis: an enzyme which is really effective at preventing protein synthesis, which it does by knobbling the ribosomes. Structure [L] ripped from decodedscience. Ribosomes are the tiny, cottage-loaf shaped, sub-cellular organelles that translate RNA into protein. Because it is an enzyme, a single molecule of ricin can destroy the activity of 1500 ribosomes a minute without itself degrading at all. The fact that stopping the production of protein is so fatally damaging gives a very real insight into the dynamics of cellular processes. We never stop turning over the bits and pieces which make cells metabolically active.  It is important to remember that all enzymes are proteins and that enzymes do everything that a cell needs to do. How much ricin do you need to kill a man like Markov?  The LD50 is about 20µg/kg or 1.50mg for a chap about Markov's size.  On that calculation it's about 100x more toxic than the nerve-gas sarin but 10,000x less toxic than Botox (another enzyme).  And that's only if you inject or inhale it.  Ricin is much less toxic if swallowed, presumably because the digestive enzymes [hey, enzymes again] help to break it down before it is absorbed by the bloodstream.

The rosary pea Abrus precatorius, produces another RIP called Abrin, which is between 5-10x more toxic by inhalation than ricin.  Abrus is a true leguminous plant related to peas, beans and the locust tree but the Castor Oil plant Ricinus communis is not, belonging to the Euphorbiaceae which toaxon includes rubber Hevea brasiliensis. poinsettia Euphorbia pulcherrima, and cassava Manihot esculenta.  The common name rosary pea comes from the fact that these bright red robust seeds are handy to make rosaries, to fill maraccas and other folksy uses.  You want to be real careful not to tell your beads when you have a cut on your fingers, because you could be up at the Pearly Gates long before you expected.

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