The Toxin Protein toxins are among the most potent poisons known to man. Because of its incredible toxicity, ricin was used in probably the most famous political assassination in history. A minuscule amount of one toxin, ricin, was placed in the tip of an umbrella and used to assassinate Georgi Markov, a Bulgarian dissident. As little as 1 mg can kill an adult, and it has been extrapolated that as little as a single toxin molecule is enough to kill a cell. Toxins are produced in some bacteria and in low levels in common plants such as pokeweed and the castor bean plant.
Toxins work by catalytically inactivating protein synthesis.
Because it is an enzyme, one toxin molecule can work on many substrate molecules, having a devastating effect on the cell. The mechanism by which immunotoxins work to kill diseased cells in the body is quite simple. Once an AIDS patient has been treated, the immunotoxin floats around in the bloodstream until it binds to a GP120 molecule on the outside of an infected cell. Inside these organelles, the linker holding the toxin to the antibody is cleaved . Usually the linker is made with an internal disulfide bond, so that it is stable in the oxidizing atmosphere outside the cell and cleaved by reduction in the reducing environment inside the cell.