Ricin: Difference between revisions

Ribosome inactivating protein (Ricin A chain)
Ribosome inactivating protein (Ricin A chain)
Identifiers
Symbol	RIP
Pfam	PF00161
InterPro	IPR001574
PROSITE	PDOC00248
SCOP2	1paf / SCOPe / SUPFAM
Pfam
Available protein structures:
Pfam	structures / ECOD
PDB	RCSB PDB; PDBe; PDBj
PDBsum	structure summary

Search for
Structures	Swiss-model
Domains	InterPro

Ricin
Ricin
	Ricin structure. The A chain is shown in blue and the B chain in orange.
Identifiers
Organism	Ricinus communis
Symbol	RCOM_2159910
Entrez	8287993
RefSeq (mRNA)	XM_002534603.1
RefSeq (Prot)	XP_002534649.1
UniProt	P02879
Other data
EC number	3.2.2.22
Chromosome	whole genome: 0 - 0.01 Mb
Structures
Search for
Structures	Swiss-model
Domains	InterPro

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Ricin /ˈraɪsɪn/, from the castor oil plant Ricinus communis, is a highly toxic, naturally occurring carbohydrate-binding protein of the type known as lectins. A dose the size of a few grains of table salt can kill an adult human.^[1] The median lethal dose (LD₅₀) of ricin is around 22 micrograms per kilogram (1.78 mg for an average adult, around 1⁄228 of a standard aspirin tablet/0.4 g gross) in humans if exposure is from injection or inhalation.^[2] Oral exposure to ricin is far less toxic and a lethal dose can be up to 20–30 milligrams per kilogram.

Abrin is a toxin found in the highly ornamental rosary pea. It is similar to ricin but even more toxic.

Toxicity

Ricin is poisonous if inhaled, injected, or ingested, acting as a toxin by the inhibition of protein synthesis.^[3] That is, it prevents the cell from assembling various amino acids into protein according to the messages it receives from messenger RNA. This process, conducted by the cell's ribosome, the protein-making machinery, is the most basic level of cell metabolism, essential to all living cells and thus to life itself. Ricin is resistant, but not impervious, to digestion by peptidases. By ingestion, the pathology of ricin is largely restricted to the gastrointestinal tract where it may cause mucosal injuries; with appropriate treatment, most patients will make a full recovery.^[4]^[5] Because the symptoms are caused by failure to make protein, they emerge only after a variable delay from a few hours to a full day after exposure. An antidote has been developed by the UK military, although it has not yet been tested on humans.^[6]^[7] Another antidote developed by the U.S. military has been shown to be safe and effective in lab mice injected with antibody-rich blood mixed with ricin, and has had some human testing.^[8] Symptomatic and supportive treatment are available. Long term organ damage is likely in survivors. Ricin causes severe diarrhea and victims can die of shock. Death typically occurs within 3–5 days of the initial exposure.^[9]

The seeds can be crushed in an oil press to extract castor oil. This leaves behind the spent crushed seeds, called variously the 'cake', 'oil cake' and 'press cake'. While the oil cake from coconut, peanuts and sometimes cotton seeds can be used either as cattle feed and/or fertilizer, the toxic nature of castor precludes them from being used as feed^[10] Accidental ingestion of Ricinus communis cake to be used as fertilizer has been reported to be responsible for fatal ricin poisoning in animals.^[3]^[11]

Deaths from ingesting castor plant seeds are rare, partly because of their indigestible capsule, and because the body can, although only with difficulty, digest ricin.^[12] The pulp from eight beans is considered dangerous to an adult.^[13] Rauber and Heard have written that close examination of early 20th century case reports indicates that public and professional perceptions of ricin toxicity "do not accurately reflect the capabilities of modern medical management".^[14]

Overdose

Most acute poisoning episodes in humans are the result of oral ingestion of castor beans, 5–20 of which could prove fatal to an adult. Victims often manifest nausea, diarrhea, tachycardia, hypotension and seizures persisting for up to a week.^[3] Blood, plasma, or urine ricin concentrations may be measured to confirm diagnosis.^[15]

Biochemistry

Ricin-type beta-trefoil lectin domain (Ricin B chain)

Identifiers

Symbol

?

Pfam clan

1abr / SCOPe / SUPFAM

CAZy

CBM13

Available protein structures:
Pfam	structures / ECOD
PDB	RCSB PDB; PDBe; PDBj
PDBsum	structure summary

Ricin is classified as a type 2 ribosome inactivating protein (RIP). Whereas type 1 RIPs are composed of a single protein chain that possesses catalytic activity, type 2 RIPs, also known as holotoxins are composed of two different protein chains that form a heterodimeric complex. Type 2 RIPs consist of an A chain that is functionally equivalent to a type 1 RIP, covalently connected by a single disulfide bond to a B chain that is catalytically inactive, but serves to mediate transport of the A-B protein complex across the cell membrane into the cytosol. Both type 1 and type 2 RIPs are functionally active against ribosomes in vitro, however only type 2 RIPs display cytoxicity due to the lectin-like properties of the B chain. In order to display its ribosome-inactivating function, the ricin disulfide bond must be reductively cleaved.^[16]

Biosynthesis

Ricin is synthesized in the endosperm of castor oil plant seeds.^[17] The ricin precursor protein is 576 amino acid residues in length and contains a signal peptide (residues 1 – 35), the ricin A chain (36 – 302), a linker peptide (303 – 314), and the ricin B chain (315 – 576).^[18] The N-terminal signal sequence delivers the prepropolypeptide to the endoplasmic reticulum (ER) and then the signal peptide is cleaved off. Within the lumen of the ER the propolypeptide is glycosylated and a protein disulfide isomerase catalyzes disulfide bond formation between cysteines 294 and 318. The propolypeptide is further glycosylated within the Golgi apparatus and transported to protein storage bodies. The propolypeptide is cleaved within protein bodies by an endopeptidase to produce the mature ricin protein that is composed of a 267 residue A chain and a 262 residue B chain that are covalently linked by a single disulfide bond.^[17]

Structure

The quaternary structure of ricin was shown to be a globular, glycosylated heterodimer of approximately 60–65 kDa.^[12] Ricin toxin A chain and ricin toxin B chain are of similar molecular weights, approximately 32 kDa and 34 kDa, respectively.

Ricin A chain (RTA) is an N-glycoside hydrolase composed of 267 amino acids.^[19] It has three structural domains with approximately 50% of the polypeptide arranged into alpha-helices and beta-sheets.^[20] The three domains form a pronounced cleft that is the active site of RTA.
Ricin B chain (RTB) is a lectin composed of 262 amino acids that is able to bind terminal galactose residues on cell surfaces.^[21] RTB forms a bilobal, barbell-like structure lacking alpha-helices or beta-sheets where individual lobes contain three subdomains. At least one of these three subdomains in each homologous lobe possesses a sugar-binding pocket that gives RTB its functional character.

Many plants such as barley have the A chain but not the B chain. People do not get sick from eating large amounts of such products, as ricin A is of extremely low toxicity as long as the B chain is not present.

Entry into the cytosol

The ability of ricin to enter the cytosol depends on hydrogen bonding interactions between RTB amino acid residues and complex carbohydrates on the surface of eukaryotic cells containing either terminal N-acetylgalactosamine or beta-1,4-linked galactose residues. Additionally, the mannose-type glycans of ricin are able to bind cells that express mannose receptors.^[22] Experimentally, RTB has been shown to bind to the cell surface on the order of 10⁶-10⁸ ricin molecules per cell surface.^[23]

The profuse binding of ricin to surface membranes allows internalization with all types of membrane invaginations. Experimental evidence points to ricin uptake in both clathrin-coated pits, as well as clathrin-independent pathways including caveolae and macropinocytosis.^[24]^[25] Vesicles shuttle ricin to endosomes that are delivered to the Golgi apparatus. The active acidification of endosomes are thought to have little effect on the functional properties of ricin. Because ricin is stable over a wide pH range, degradation in endosomes or lysosomes offer little or no protection against ricin.^[26] Ricin molecules are thought to follow retrograde transport via early endosomes, the trans-Golgi network, and the Golgi to enter the lumen of the endoplasmic reticulum (ER).^[27]

For ricin to function cytotoxically, RTA must be reductively cleaved from RTB in order to release a steric block of the RTA active site. This process is catalysed by the protein PDI (protein disulphide isomerase) that resides in the lumen of the ER.^[28] Free RTA in the ER lumen then partially unfolds and partially buries into the ER membrane, where it is thought to mimic a misfolded membrane-associated protein.^[29] Roles for the ER chaperones GRP94^[30] and EDEM^[31] have been proposed prior to the 'dislocation' of RTA from the ER lumen to the cytosol in a manner that utilizes components of the endoplasmic reticulum-associated protein degradation (ERAD) pathway. ERAD normally removes misfolded ER proteins to the cytosol for their destruction by cytosolic proteasomes. Dislocation of RTA requires ER membrane-integral E3 ubiquitin ligase complexes,^[32] but RTA avoids the ubiquitination that usually occurs with ERAD substrates because of its low content of lysine residues, which are the usual attachment sites for ubiquitin.^[33] Thus RTA avoids the usual fate of dislocated proteins (destruction that is mediated by targeting ubiquitinylated proteins to the cytosolic proteasomes). In the mammalian cell cytosol, RTA then undergoes triage by cytosolic molecular chaperones that results in its folding to a catalytic conformation^[30] that de-purinates ribosomes, thus halting protein synthesis.

Ribosome inactivation

RTA has rRNA N-glycosylase activity which is responsible for the cleavage of a glycosidic bond within the large rRNA of the 60S subunit of eukaryotic ribosomes.^[34] RTA specifically and irreversibly hydrolyses the N-glycosidic bond of the adenine residue at position 4324 (A4324) within the 28S rRNA, but leaves the phosphodiester backbone of the RNA intact.^[35] The ricin targets A4324 that is contained in a highly conserved sequence of 12 nucleotides universally found in eukaryotic ribosomes. The sequence, 5’-AGUACGAGAGGA-3’, termed the sarcin-ricin loop, is important in binding elongation factors during protein synthesis.^[36] The depurination event rapidly and completely inactivates the ribosome, resulting in toxicity from inhibited protein synthesis. A single RTA molecule in the cytosol is capable of depurinating approximately 1500 ribosomes per minute.

Depurination reaction

Within the active site of RTA, there exist several invariant amino acid residues involved in the depurination of ribosomal RNA.^[26] Although the exact mechanism of the event is unknown, key amino acid residues identified include tyrosine at positions 80 and 123, glutamic acid at position 177, and arginine at position 180. In particular, Arg180 and Glu177 have been shown to be involved in the catalytic mechanism, and not substrate binding, with enzyme kinetic studies involving RTA mutants. The model proposed by Mozingo and Robertus,^[20] based x-ray structures, is as follows:

Sarcin-ricin loop substrate binds RTA active site with target adenine stacking against tyr80 and tyr123.
Arg180 is positioned such that it can protonate N-3 of adenine and break the bond between N-9 of the adenine ring and C-1’ of the ribose.
Bond cleavage results in an oxycarbonium ion on the ribose, stabilized by Glu177.
N-3 protonation of adenine by Arg180 allows deprotonation of a nearby water molecule.
Resulting hydroxyl attacks ribose carbonium ion.
Depurination of adenine results in a neutral ribose on an intact phosphodiester RNA backbone.

Manufacture

Ricin is easily purified from castor oil manufacturing waste. The aqueous phase left over from the oil extraction process is called waste mash. It would contain about 5–10% ricin by weight, but heating during the oil extraction process denatures the protein, making the resultant seed cake safe for use as animal feed. Ricin can be isolated from fresh seed by chromatographic techniques similar to those routinely used for purification of many other plant proteins.

Patented extraction process

A process for extracting ricin has been described in a patent.^[37] The described extraction method is very similar to that used for the preparation of soy protein isolates.

The patent was removed from the United States Patent and Trademark Office (USPTO) database sometime in 2004.^[38]^[39] Modern theories of protein chemistry cast doubt on the effectiveness of the methods disclosed in the patent.^[40]

Research

Some researchers ^[who?] have speculated about using ricins in the treatment of cancer, as a so-called "magic bullet" to destroy targeted cells.^[26] Because ricin is a protein, it can be linked to a monoclonal antibody to target malignant cells recognized by the antibody. The major problem with ricin is that its native internalization sequences are distributed throughout the protein. If any of these native internalization sequences are present in a therapeutic, then the drug will be internalized by, and kill, untargeted epithelial cells as well as targeted cancer cells.

Some researchers ^[who?] hope that modifying ricin will sufficiently lessen the likelihood that the ricin component of these immunotoxins will cause the wrong cells to internalize it, while still retaining its cell-killing activity when it is internalized by the targeted cells. Generally, however, ricin has been superseded for medical purposes by more practical fragments of bacterial toxins, such as diphtheria toxin, which is used in denileukin diftitox, an FDA-approved treatment for leukemia and lymphoma. No approved therapeutics contain ricin.

A promising approach is also to use the non-toxic B subunit (a lectin) as a vehicle for delivering antigens into cells, thus greatly increasing their immunogenicity. Use of ricin as an adjuvant has potential implications for developing mucosal vaccines.

Regulation

In the U.S., ricin appears on the select agents list of the Department of Health and Human Services,^[41] and scientists must register with HHS to use ricin in their research. However, investigators possessing less than 100 mg are exempt from regulation.^[42]

Chemical or biological warfare agent

The United States investigated ricin for its military potential during World War I.^[43] At that time it was being considered for use either as a toxic dust or as a coating for bullets and shrapnel. The dust cloud concept could not be adequately developed, and the coated bullet/shrapnel concept would violate the Hague Convention of 1899 (adopted in U.S. law at 32 Stat. 1903), specifically Annex §2, Ch.1, Article 23, stating "... it is especially prohibited ... [t]o employ poison or poisoned arms".^[44] World War I ended before the United States weaponized ricin.

During World War II the United States and Canada undertook studying ricin in cluster bombs.^[45] Though there were plans for mass production and several field trials with different bomblet concepts, the end conclusion was that it was no more economical than using phosgene. This conclusion was based on comparison of the final weapons, rather than ricin's toxicity (LCt₅₀ ~40 mg·min/m³). Ricin was given the military symbol W or later WA.^{[citation needed]} Interest in it continued for a short period after World War II, but soon subsided when the U.S. Army Chemical Corps began a program to weaponize sarin.

The Soviet Union also possessed weaponized ricin. There were speculations that the KGB used it outside the Soviet bloc; however, this was never proven. In 1978, the Bulgarian dissident Georgi Markov was assassinated by Bulgarian secret police who surreptitiously "shot" him on a London street with a modified umbrella using compressed gas to fire a tiny pellet contaminated with ricin into his leg.^[4]^[46] He died in a hospital a few days later; his body was passed to a special poison branch of the British Ministry of Defence (MOD) that discovered the pellet during an autopsy. The prime suspects were the Bulgarian secret police: Georgi Markov had defected from Bulgaria some years previously and had subsequently written books and made radio broadcasts which were highly critical of the Bulgarian communist regime. However, it was believed at the time that Bulgaria would not have been able to produce the pellet, and it was also believed that the KGB had supplied it. The KGB denied any involvement although high-profile KGB defectors Oleg Kalugin and Oleg Gordievsky have since confirmed the KGB's involvement. Earlier, Soviet dissident Aleksandr Solzhenitsyn also suffered (but survived) ricin-like symptoms after an encounter in 1971 with KGB agents.^[47]

Given ricin's extreme toxicity and utility as an agent of chemical/biological warfare, it is noteworthy that the production of the toxin is rather difficult to limit. The castor bean plant from which ricin is derived is a common ornamental and can be grown at home without any special care, and the major reason ricin is a public health threat is that it is easy to obtain.^{[citation needed]}

Under both the 1972 Biological Weapons Convention and the 1997 Chemical Weapons Convention, ricin is listed as a schedule 1 controlled substance. Despite this, more than 1 million tonnes of castor beans are processed each year, and approximately 5% of the total is rendered into a waste containing negligible concentrations of undenatured ricin toxin.^[48]

Ricin is several orders of magnitude less toxic than botulinum or tetanus toxin, but the latter are harder to come by. Compared to botulinum or anthrax as biological weapons or chemical weapons, the quantity of ricin required to achieve LD₅₀ over a large geographic area is significantly more than an agent such as anthrax (tons of ricin vs. only kilogram quantities of anthrax).^[49] Ricin is easy to produce, but is not as practical nor likely to cause as many casualties as other agents.^[4] Ricin is inactivated (the protein changes structure and becomes less dangerous) much more readily than anthrax spores, which may remain lethal for decades. Jan van Aken, a Dutch expert on biological weapons, explained in a report for The Sunshine Project that Al Qaeda's experiments with ricin suggest their inability to produce botulinum or anthrax.^[50]

A biopharmaceutical company called Soligenix, Inc. has licensed a vaccine called RiVax™ from Vitetta et al. at UT Southwestern. The vaccine is safe and immunogenic in mice, rabbits and humans. It has completed two successful clinical trials.^[51]

Incidents

Ricin has been involved in a number of incidents, including the high-profile assassination of Georgi Markov in 1978 using a weapon disguised as an umbrella.^[3]

Several terrorists and terrorist groups have experimented with ricin, and several incidents of the poison being mailed to U.S. politicians have occurred in the 21st century.

On May 29, 2013 two anonymous letters sent to New York City Mayor Michael Bloomberg contained traces of the deadly poison ricin. ^[52] Another was sent to the offices of Mayors Against Illegal Guns in Washington DC. A letter containing ricin was also alleged to have been sent to US President Barack Obama at the same time. An actress Shannon Richardson was later charged with the crime.^[53]

June 7th, 2013 news report^[54] (in reference to the above May 29th, 2013 letters) states:

"A pregnant Texas actress who told FBI agents her husband had sent ricin-tainted letters to President Barack Obama and New York Mayor Michael Bloomberg has been arrested for allegedly sending the letters herself, law enforcement officials said Friday."

References

^ "What Makes Ricin So Deadly[1] - What Makes Ricin So Deadly". Anthony Sabella. Retrieved 2013-04-24.
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^ ^a ^b ^c ^d Ujváry I (2010). Krieger R (ed.). Hayes´ Handbook of Pesticide Toxicology (Third ed.). Elsevier, Amsterdam. pp. 119–229. ISBN 978-0-12-374367-1.
^ ^a ^b ^c Schep LJ, Temple WA, Butt GA, Beasley MD (2009). "Ricin as a weapon of mass terror--separating fact from fiction". Environ Int. 35 (8): 1267–71. doi:10.1016/j.envint.2009.08.004. PMID 19767104. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
^ Kopferschmitt J, Flesch F, Lugnier A, Sauder P, Jaeger A, Mantz JM (1983). "Acute voluntary intoxication by ricin". Hum Toxicol. 2 (2): 239–42. doi:10.1177/096032718300200211. PMID 6862467. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)
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^ "Human trial proves ricin vaccine safe, induces neutralizing antibodies; further tests planned". University of Texas Southwestern Medical Center. 2006-01-30. Retrieved 2012-05-07.
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^ Endo Y, Tsurugi K (1988). "The RNA N-glycosidase activity of ricin A-chain. The characteristics of the enzymatic activity of ricin A-chain with ribosomes and with rRNA" (PDF). J. Biol. Chem. 263 (18): 8735–9. PMID 3288622. {{cite journal}}: Unknown parameter |month= ignored (help)
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^ "Preparation of Toxic Ricin", U.S. patent 3,060,165, assigned to the U.S. Secretary of the Army, inventors: Harry L. Craig, O. H. Alderks, Alsoph H. Corwin, Sally H. Dieke, and Charlotte Karel (granted October 23, 1962)
^ "Harry L. Craig, O. H. Alderks, Alsoph H. Corwin, Sally H. Dieke, and Charlotte Karel, US Patent 3,060,165, "Preparation of Toxic Ricin", granted October 23, 1962". V3.espacenet.com. Retrieved 2010-09-01.
^ "Ricin Patent". Cryptome.org. 2004-03-12. Retrieved 2010-09-01.
^ John Pike. "THE RECIPE FOR RICIN, Part III: US patent, "the production of toxic ricin," intellectual property of the US Army". Globalsecurity.org. Retrieved 2010-09-01.
^ HHS and USDA Select Agents and Toxins 7 CFR Part 331, 9 CFR Part 121, and 42 CFR Part 73. http://www.cdc.gov/od/sap/docs/salist.pdf
^ "Permissible Toxin Amounts". National Select Agent Registy. Retrieved 24 April 2013.
^ Augerson, William S.; Spektor, Dalia M.; United States Dept. of Defense, Office of the Secretary of Defense, National Defense Research Institute (U.S.) (2000). A Review of the Scientific Literature as it Pertains to Gulf War Illnesses. Rand Corporation, ISBN 978-0-8330-2680-4^{[page needed]}
^ "The Avalon Project — Laws of War : Laws and Customs of War on Land (Hague II); July 29, 1899". Avalon.law.yale.edu. Retrieved 2010-09-01.
^ Gupta R (2009). Handbook of Toxicology of Chemical Warfare Agents. Boston: Academic Press. ISBN 978-0-12-374484-5.^{[page needed]}
^ "Ricin and the umbrella murder". CNN. January 7, 2003. Retrieved 2008-03-15.
^ Thomas DM (1998). Alexander Solzhenitsyn: A Century in His Life (First ed.). St. Martin's Press. pp. 368–378. ISBN 978-0756760113.
^ "Cornell University Department of Animal Science". Ansci.cornell.edu. Retrieved 2012-05-07.
^ Kortepeter MG, Parker GW (1999). "Potential biological weapons threats". Emerging Infect. Dis. 5 (4): 523–7. doi:10.3201/eid0504.990411. PMC 2627749. PMID 10458957.
^ van Aken J (2001). "Biological Weapons: Research Projects of the German Army". Backgrounder Series #7. The Sunshine Project.
^ "RiVax™ Ricin Toxin Vaccine". Soligenix, Inc.
^ http://news.msn.com/us/letter-to-nyc-mayor-bloomberg-contained-ricin
^ Paul Harris (June 8, 2013). "Bit-part actor charged over plot to frame husband for ricin letters". Guardian.
^ ABC News http://abcnews.go.com/US/wireStory/ap-sources-texas-woman-arrested-ricin-case-19349834. {{cite news}}: Missing or empty |title= (help)

External links

[1] "What Makes Ricin So Deadly[1] - What Makes Ricin So Deadly". Anthony Sabella. Retrieved 2013-04-24.

[2] "EFSA Scientific Opinion: Ricin (from Ricinus communis) as undesirable substances in animal feed [1] - Scientific Opinion of the Panel on Contaminants in the Food Chain". Efsa.europa.eu. Retrieved 2010-09-01.

[Ujváry_2010-3] Ujváry I (2010). Krieger R (ed.). Hayes´ Handbook of Pesticide Toxicology (Third ed.). Elsevier, Amsterdam. pp. 119–229. ISBN 978-0-12-374367-1.

[Schep-4] Schep LJ, Temple WA, Butt GA, Beasley MD (2009). "Ricin as a weapon of mass terror--separating fact from fiction". Environ Int. 35 (8): 1267–71. doi:10.1016/j.envint.2009.08.004. PMID 19767104. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid6862467-5] Kopferschmitt J, Flesch F, Lugnier A, Sauder P, Jaeger A, Mantz JM (1983). "Acute voluntary intoxication by ricin". Hum Toxicol. 2 (2): 239–42. doi:10.1177/096032718300200211. PMID 6862467. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[6] Rincon P (2009-11-11). "Ricin 'antidote' to be produced". BBC News. Retrieved 2010-09-01.

[7] "Human trial proves ricin vaccine safe, induces neutralizing antibodies; further tests planned". University of Texas Southwestern Medical Center. 2006-01-30. Retrieved 2012-05-07.

[8] Karen Fleming-Michael (2005-09-01). "Vaccine for ricin toxin developed at Detrick lab". Dcmilitary.com. Retrieved 2010-09-01.

[9] "The Emergency Response Safety and Health Database: Biotoxin: RICIN". Centers for Disease Control and Prevention (CDC). 2008-08-01. Retrieved 2011-02-17.

[10] ttp://www.britannica.com/EBchecked/topic/426145/oil-cake

[pmid12046967-11] Soto-Blanco B, Sinhorini IL, Gorniak SL, Schumaher-Henrique B (2002). "Ricinus communis cake poisoning in a dog". Vet Hum Toxicol. 44 (3): 155–6. PMID 12046967. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[aplin-12] Aplin PJ, Eliseo T (1997). "Ingestion of castor oil plant seeds". Med. J. Aust. 167 (5): 260–1. PMID 9315014. {{cite journal}}: Unknown parameter |month= ignored (help)

[pmid3964368-13] Wedin GP, Neal JS, Everson GW, Krenzelok EP (1986). "Castor bean poisoning". Am J Emerg Med. 4 (3): 259–61. doi:10.1016/0735-6757(86)90080-X. PMID 3964368. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid4082461-14] Rauber A, Heard J (1985). "Castor bean toxicity re-examined: a new perspective". Vet Hum Toxicol. 27 (6): 498–502. PMID 4082461. {{cite journal}}: Unknown parameter |month= ignored (help)

[isbn0-9626523-7-7-15] Baselt RC (2008). Disposition of Toxic Drugs and Chemicals in Man (Eighth ed.). Foster City, California: Biomedical Pubilcations. pp. 1381–3. ISBN 0-9626523-7-7.

[pmid3606124-16] Wright HT, Robertus JD (1987). "The intersubunit disulfide bridge of ricin is essential for cytotoxicity". Arch. Biochem. Biophys. 256 (1): 280–4. doi:10.1016/0003-9861(87)90447-4. PMID 3606124. {{cite journal}}: Unknown parameter |month= ignored (help)

[Lord_Roberts_2005-17] Lord MJ, Roberts LM (2005). "Ricin: structure, synthesis, and mode of action". In Raffael S, Schmitt M (ed.). Microbial Protein Toxins. Topics in Current Genetics. Vol. 11. Berlin: Springer. pp. 215–233. doi:10.1007/b100198. ISBN 3-540-23562-0.

[urlRicin_precursor_-_Ricinus_communis_(Castor_bean)-18] "Ricin precursor - Ricinus communis (Castor bean)". P02879. UniProt Consortium.

[pmid4730499-19] Olsnes S, Pihl A (1973). "Different biological properties of the two constituent peptide chains of ricin, a toxic protein inhibiting protein synthesis". Biochemistry. 12 (16): 3121–6. doi:10.1021/bi00740a028. PMID 4730499. {{cite journal}}: Unknown parameter |month= ignored (help)

[pmid7990130-20] Weston SA, Tucker AD, Thatcher DR, Derbyshire DJ, Pauptit RA (1994). "X-ray structure of recombinant ricin A-chain at 1.8 A resolution". J. Mol. Biol. 244 (4): 410–22. doi:10.1006/jmbi.1994.1739. PMID 7990130. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid1717462-21] Wales R, Richardson PT, Roberts LM, Woodland HR, Lord JM (1991). "Mutational analysis of the galactose binding ability of recombinant ricin B chain". J. Biol. Chem. 266 (29): 19172–9. PMID 1717462. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid8453986-22] Magnusson S, Kjeken R, Berg T (1993). "Characterization of two distinct pathways of endocytosis of ricin by rat liver endothelial cells". Exp. Cell Res. 205 (1): 118–25. doi:10.1006/excr.1993.1065. PMID 8453986. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid7657599-23] Sphyris N, Lord JM, Wales R, Roberts LM (1995). "Mutational analysis of the Ricinus lectin B-chains. Galactose-binding ability of the 2 gamma subdomain of Ricinus communis agglutinin B-chain". J. Biol. Chem. 270 (35): 20292–7. doi:10.1074/jbc.270.35.20292. PMID 7657599. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)

[pmid2862151-24] Moya M, Dautry-Varsat A, Goud B, Louvard D, Boquet P (1985). "Inhibition of coated pit formation in Hep2 cells blocks the cytotoxicity of diphtheria toxin but not that of ricin toxin". J. Cell Biol. 101 (2): 548–59. doi:10.1083/jcb.101.2.548. PMC 2113662. PMID 2862151. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid11567873-25] Nichols BJ, Lippincott-Schwartz J (2001). "Endocytosis without clathrin coats". Trends Cell Biol. 11 (10): 406–12. doi:10.1016/S0962-8924(01)02107-9. PMID 11567873. {{cite journal}}: Unknown parameter |month= ignored (help)

[pmid14579547-26] Lord MJ, Jolliffe NA, Marsden CJ, Pateman CS, Smith DC, Spooner RA, Watson PD, Roberts LM (2003). "Ricin. Mechanisms of cytotoxicity". Toxicol Rev. 22 (1): 53–64. doi:10.2165/00139709-200322010-00006. PMID 14579547.{{cite journal}}: CS1 maint: multiple names: authors list (link)

[pmid16603059-27] Spooner RA, Smith DC, Easton AJ, Roberts LM, Lord JM (2006). "Retrograde transport pathways utilised by viruses and protein toxins". Virol. J. 3: 26. doi:10.1186/1743-422X-3-26. PMC 1524934. PMID 16603059.{{cite journal}}: CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)

[pmid15225124-28] Spooner RA, Watson PD, Marsden CJ, Smith DC, Moore KA, Cook JP, Lord JM, Roberts LM (2004). "Protein disulphide-isomerase reduces ricin to its A and B chains in the endoplasmic reticulum". Biochem. J. 383 (Pt 2): 285–93. doi:10.1042/BJ20040742. PMC 1134069. PMID 15225124. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid19211561-29] Mayerhofer PU, Cook JP, Wahlman J, Pinheiro TT, Moore KA, Lord JM, Johnson AE, Roberts LM (2009). "Ricin A chain insertion into endoplasmic reticulum membranes is triggered by a temperature increase to 37 {degrees}C". J. Biol. Chem. 284 (15): 10232–42. doi:10.1074/jbc.M808387200. PMC 2665077. PMID 19211561. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link) CS1 maint: unflagged free DOI (link)

[Spooner-30] Spooner RA, Hart PJ, Cook JP, Pietroni P, Rogon C, Höhfeld J, Roberts LM, Lord JM (2008). "Cytosolic chaperones influence the fate of a toxin dislocated from the endoplasmic reticulum". Proc. Natl. Acad. Sci. U.S.A. 105 (45): 17408–13. Bibcode:2008PNAS..10517408S. doi:10.1073/pnas.0809013105. JSTOR 25465291. PMC 2580750. PMID 18988734. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid16452630-31] Slominska-Wojewodzka M, Gregers TF, Wälchli S, Sandvig K (2006). "EDEM is involved in retrotranslocation of ricin from the endoplasmic reticulum to the cytosol". Mol. Biol. Cell. 17 (4): 1664–75. doi:10.1091/mbc.E05-10-0961. PMC 1415288. PMID 16452630. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid20519439-32] Li S, Spooner RA, Allen SC, Guise CP, Ladds G, Schnöder T, Schmitt MJ, Lord JM, Roberts LM (2010). "Folding-competent and folding-defective forms of ricin A chain have different fates after retrotranslocation from the endoplasmic reticulum". Mol. Biol. Cell. 21 (15): 2543–54. doi:10.1091/mbc.E09-08-0743. PMC 2912342. PMID 20519439. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid11876649-33] Deeks ED, Cook JP, Day PJ, Smith DC, Roberts LM, Lord JM (2002). "The low lysine content of ricin A chain reduces the risk of proteolytic degradation after translocation from the endoplasmic reticulum to the cytosol". Biochemistry. 41 (10): 3405–13. doi:10.1021/bi011580v. PMID 11876649. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[pmid3036799-34] Endo Y, Tsurugi K (1987). "RNA N-glycosidase activity of ricin A-chain. Mechanism of action of the toxic lectin ricin on eukaryotic ribosomes" (PDF). J. Biol. Chem. 262 (17): 8128–30. PMID 3036799. {{cite journal}}: Unknown parameter |month= ignored (help)

[pmid3288622-35] Endo Y, Tsurugi K (1988). "The RNA N-glycosidase activity of ricin A-chain. The characteristics of the enzymatic activity of ricin A-chain with ribosomes and with rRNA" (PDF). J. Biol. Chem. 263 (18): 8735–9. PMID 3288622. {{cite journal}}: Unknown parameter |month= ignored (help)

[pmid4360718-36] Sperti S, Montanaro L, Mattioli A, Stirpe F (1973). "Inhibition by ricin of protein synthesis in vitro: 60 S ribosomal subunit as the target of the toxin" (PDF). Biochem. J. 136 (3): 813–5. PMC 1166019. PMID 4360718. {{cite journal}}: Unknown parameter |month= ignored (help)CS1 maint: multiple names: authors list (link)

[37] "Preparation of Toxic Ricin", U.S. patent 3,060,165, assigned to the U.S. Secretary of the Army, inventors: Harry L. Craig, O. H. Alderks, Alsoph H. Corwin, Sally H. Dieke, and Charlotte Karel (granted October 23, 1962)

[http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=US3060165&QPN=US3060165-38] "Harry L. Craig, O. H. Alderks, Alsoph H. Corwin, Sally H. Dieke, and Charlotte Karel, US Patent 3,060,165, "Preparation of Toxic Ricin", granted October 23, 1962". V3.espacenet.com. Retrieved 2010-09-01.

[39] "Ricin Patent". Cryptome.org. 2004-03-12. Retrieved 2010-09-01.

[http://www.globalsecurity.org/org/nsn/nsn-040723.htm-40] John Pike. "THE RECIPE FOR RICIN, Part III: US patent, "the production of toxic ricin," intellectual property of the US Army". Globalsecurity.org. Retrieved 2010-09-01.

[41] HHS and USDA Select Agents and Toxins 7 CFR Part 331, 9 CFR Part 121, and 42 CFR Part 73. http://www.cdc.gov/od/sap/docs/salist.pdf

[42] "Permissible Toxin Amounts". National Select Agent Registy. Retrieved 24 April 2013.

[Augerson2000-43] Augerson, William S.; Spektor, Dalia M.; United States Dept. of Defense, Office of the Secretary of Defense, National Defense Research Institute (U.S.) (2000). A Review of the Scientific Literature as it Pertains to Gulf War Illnesses. Rand Corporation, ISBN 978-0-8330-2680-4^{[page needed]}

[44] "The Avalon Project — Laws of War : Laws and Customs of War on Land (Hague II); July 29, 1899". Avalon.law.yale.edu. Retrieved 2010-09-01.

[Gupta_2009-45] Gupta R (2009). Handbook of Toxicology of Chemical Warfare Agents. Boston: Academic Press. ISBN 978-0-12-374484-5.^{[page needed]}

[46] "Ricin and the umbrella murder". CNN. January 7, 2003. Retrieved 2008-03-15.

[47] Thomas DM (1998). Alexander Solzhenitsyn: A Century in His Life (First ed.). St. Martin's Press. pp. 368–378. ISBN 978-0756760113.

[http://www.ansci.cornell.edu/plants/toxicagents/ricin.html-48] "Cornell University Department of Animal Science". Ansci.cornell.edu. Retrieved 2012-05-07.

[pmid10458957-49] Kortepeter MG, Parker GW (1999). "Potential biological weapons threats". Emerging Infect. Dis. 5 (4): 523–7. doi:10.3201/eid0504.990411. PMC 2627749. PMID 10458957.

[vanaken2001-50] van Aken J (2001). "Biological Weapons: Research Projects of the German Army". Backgrounder Series #7. The Sunshine Project.

[51] "RiVax™ Ricin Toxin Vaccine". Soligenix, Inc.

[52] ttp://news.msn.com/us/letter-to-nyc-mayor-bloomberg-contained-ricin

[53] Paul Harris (June 8, 2013). "Bit-part actor charged over plot to frame husband for ricin letters". Guardian.

[54] ABC News http://abcnews.go.com/US/wireStory/ap-sources-texas-woman-arrested-ricin-case-19349834. {{cite news}}: Missing or empty |title= (help)

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