Ricin

Ricin /ˈraɪsɪn/ is a highly toxic, naturally occurring lectin (a carbohydrate-binding protein) produced in the seeds of the castor oil plant, Ricinus communis. A dose of purified ricin powder the size of a few grains of table salt can kill an adult human. The median lethal dose (LD₅₀) of ricin is around 22 micrograms per kilogram of body weight if the exposure is from injection or inhalation (1.78 milligram for an average adult). For comparison, a standard paracetamol (acetaminophen) tablet contains 500 mg of acetaminophen and a low dose aspirin contains 81 mg of aspirin. Oral exposure to ricin is far less toxic as some of the poison is inactivated in the stomach. An estimated lethal oral dose in humans is approximately 1 milligram per kilogram.

Biochemistry

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 from the cell surface, via vesicle carriers, to the lumen of the endoplasmic reticulum (ER). Both type 1 and type 2 RIPs are functionally active against ribosomes in vitro; however, only type 2 RIPs display cytotoxicity 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.