Abrin structure

Abrine, 484 Abrotine, 772 Abrtis precatorius, 484 Abuta spp., 371 Acacia spp., 771 Acetylcholine, 262, 518 Acetylomithine, 170, 171, 172 Achillea spp., 779 Achilleine achilletine, 779 Acolyctine, 686 Aconine, 673, 675, 679, 685 Aconines, nuclear structure, 693 Aconite alkaloids, 673 Aconitine, 673, 674, 775 oxidation products, 676 Aconitines, pharmacological action, 690 Aconitinone, 676 Aconitoline, 675... 

Diphtheria toxin is produced as a single polypeptide chain (Fig. la) yyhich is easily cleaved ("nicked") by trypsin and trypsin-like proteases into two disulfide-linked fragments, A and B (Pappenheimer, 1977). The structure of the nicked toxin resembles that of the plant toxins ricin, abrin, modeccin, viscumin and others (Olsnes and Sandvig, 1985). 

Chemical Abstracts Service Registry Number CAS 1393-62-0. Abrin is a toxalbumin similar in structure, absorption, and mechanism of action to ricin but is found not in castor beans but rather in jequirity beans. No reports of its use as a battlefield or terrorist agent exist, but in mice it is 75 times more potent than ricin. No specific treatment is available. Both ricin and abrin are type 2 ribosomal inhibitory proteins (RIPs) the other potent toxins in this class are Eranthis hyemalis lectin (EHL) from winter aconite, modeccin and volkensin from African succulents, and viscumin from mistletoe. 

Shiga toxin produced by Shigella dysenteriae has similar structural features. The toxin binds to a glycolipid (Gb3), undergoes endocytosis, and the enzymatie Ai fragment, which is a specific N-glycosidase, removes adenine from one particular adenosine residue in the 28S RNA of the 60S ribosomal subunit. Removal of the adenine inactivates the 60S ribosome, blocking protein synthesis. Ricin, abrin, and a number of related plant proteins inhibit eukaryotic protein synthesis in a similar manner (Chapter 25). 

Abrin is a plant toxin, which is closely related to ricin in terms of its structure and chemical properties. It is obtained from the seeds of Abrus pre-catorius (commonly known as jequirity bean or rosary pea ), a tropical vine cultivated as an ornamental plant in many locations. Jequirity beans are usually scarlet in colour with a black spot at one end (though less common different coloured varieties exist) and are approximately 3x8 mm in size. 

Like ricin, abrin is a type 2 ribosome inactivating protein with A and B chains linked by a disulphide bond. The abrin A chain comprises 251 amino acid residues compared to 267 in the ricin A chain, both having three folding structural domains and a molecular weight of approximately 30 kDa. The abrin B chain has a molecular weight of 35 kDa with 60% of its amino acid residues identical to those of ricin s B chain. Both B chains have two saccharide binding sites which are highly conserved between the two toxins. 

The structural similarity of abrin to ricin is reflected also in its toxicokinetic properties with... 

Olsnes S (1976). Abrin and ricin structure and mechanism of action of two toxic lectins. Bull Inst Pasteur, 74, 85-99. 

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. 

Mistletoe lectins have been extensively analyzed for their primary structure. Only the complete amino acid sequence of the A chain of mistletoe lectin 1 (VAA-1) has been determined thus far [25] but the data show a great homology with other ribosome inactivating proteins such as abrin a or ricin D with 111 and 103 amino acid residues conserved, respectively. 

Ricin is most toxic when inhaled and is therefore a potential aerosol threat. However, large quantities would he needed to cause toxicity. Ricin is much less lethal when ingested, suggesting poor absorption. The toxicity and lethality of parenteral exposure to ricin are well documented. Abrin which is closely related to ricin in structure is extremely toxic with an LD50 dose for humans of 2 pg/kg. 

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