Ricin biosynthesis

Injected ricin kills laboratory animals in a concentration- and time-dependent manner with steep lethality curves (Fodstad et al., 1976, 1979 Olsnes and Pihl, 1977). After administration of ricin to experimental animals by injection, there is a characteristic time delay before signs of intoxication appear. The delay time decreases with increasing amounts of toxin, but it is always several hours, perhaps reflecting the time required for sufficient toxin to reach the target ribosome and disrupt protein synthesis (Olsnes and Pihl, 1977 Fodstad et al., 1979). In laboratory rats, for example, liver protein biosynthesis is unchanged compared with control levels for the first 3 h after injection (i.p.) with 500 pg/kg ricin, but steadily declines to approximately 15% of that of control groups by 10 h (Lin et al., 1971). 

Ko, T.S. and Kaji, A. (1975) Effect of ricin on biosynthesis of myeloma protein (IgA) and general cellular proteins in MOPC-315 cells. Biochim Biophys Acta, 414, 155-160. 

CgH8N202, Mr 164.16, mp. 201 °C. A pyridine alkaloid from Ricinus communis (Euphorbiaceae). Its content in this plant is 1 % and together with the extremely toxic protein ricin R. is responsible for the toxicity of Ricinus seeds. The biosynthesis of R. proceeds through nicotinamide as an intermediate. 

Ricin a toxalbumin phytotoxin from Ricinus seeds. M, 66,000,493 amino acid residues. R. inhibits protein biosynthesis (causes dissociation of polysomes) and has antitumor properties. It consists of an A-chain (M, 32,000) and a B-chain (M, 34,000) joined by disulfide bridges. After reductive separation by 2-mercap-toethanol, both chains show increased inhibitor activity but markedly decreased toxicity. Toxic activity is carried by the A-chain (effectomer), while the B-chain (haptomer) binds the toxin to the cell surface. Similar action and structure are possessed by abrin (M, 65,000 A-chain 30,000, B-chain 35,000), a toxalbumin from the red seeds of Abrus precatorius. Abrin is used in opthalmology. 

Known plant toxins are 1. the homologous viscotoxins (Af, 4,840 46 amino acid residues of known sequence 3 disulfide bridges) from leaves and branches of the European mistletoe, which have hypotensive activity and cause a slowing of the heart beat 2. the toxalbumins Ricin (see) and abrin, which inhibit protein biosynthesis. 

FIGURE 27.5 Biosynthesis of the ricin toxin. Source Rlustration taken from EHSO (2014). 

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