Toxalbumins are toxic plantproteins that disableribosomes and thereby inhibitprotein synthesis, producing severecytotoxic effects in multiple organ systems. They aredimers held together by adisulfide bond and comprise alectin (carbohydrate-binding protein) part which binds to the cell membrane and enables the toxin part to gain access to the cell contents. Toxalbumins are similar in structure toAB toxins found incholera,tetanus,diphtheria,botulinum and others;[1] and their physiological and toxic properties are similar to those ofviperinesnake venom.[citation needed]
Toxalbumins were first described in about 1890 by Ludwig Brieger (1849–1919) andSigmund Fraenkel (1868–1939), associates of the organic chemistEugen Baumann. Brieger first used the termtoxin.[2]
Toxalbumins notably are present in the plant familiesLeguminosae andEuphorbiaceae, occurring for instance inRobinia pseudoacacia,Abrus precatorius,Jatropha curcas,Croton gratissimus andRicinus communis. Typical toxalbumins areabrin andricin.[3][self-published source?][4] Ingestion of seed containing toxalbumins is not necessarily fatal as the hard seed coat will withstand digestion, unless the seed has been pierced, as would happen in the making of necklaces, prayer beads or bracelets, and even then the toxalbumin is likely to be digested and thereby rendered harmless. Toxalbumins injected intravenously or subcutaneously or inhaled in powdered form, though, are highly toxic. A latent period of hours to days may follow with no sensible signs of distress, after which symptoms of nausea, vomiting and diarrhoea will appear, followed by delirium, seizures, coma, and death. From an evolutionary viewpoint, toxalbumins developed as a deterrent to consumption of seeds, foliage, bark and roots. Ripe fruits having a fleshy pulp are usually tasty and edible and lacking toxalbumins, encourage ingestion and the consequent distribution of seeds that have a coat sufficiently durable to survive a passage through the digestive system of a herbivore or fructivore.[5][6]
Being soluble in water, ricin is not present in extracted oils. As with most proteins it breaks down after heat treatment, such as cooking or steaming, and after the oil is extracted, the resulting pomace is often used as animal feed. There is an enormous variation in sensitivity to the toxin, and a lethal dose may be as little as two-millionths of body weight. Since ricin is a protein, antibodies may be produced by inoculation, allowing resistance of up to 800 times a normal lethal dose. Ricin has been used in assassinations, a notorious case being the use of a 1.53 mm pellet holding a few hundred millionths of a gram of ricin to kill the Bulgarian broadcaster,Georgi Markov, who died 4 days after being attacked.[7]
The toxins present in poisonous mushrooms such asAmanita phalloides are quite different from toxalbumins and are mostlysecondary metabolites oramatoxins which do not readily break down under applied heat. They are potent inhibitors ofRNA polymerase II, an enzyme vital in the synthesis ofmessenger RNA (mRNA),microRNA, andsmall nuclear RNA (snRNA). Without mRNA, the template for protein synthesis, cell metabolism stops. In this respect, their metabolic effect is similar to that of toxalbumins.