Ricin catalytic activities

It is generally accepted that ricin-induced apoptosis requires RTA, and is a downstream consequence of the inhibition of protein synthesis (Williams et al., 1997 Langer et al., 1999 Oda et al., 1999 Vervecken et al., 2000). There is recent evidence, however, to suggest that apoptosis in yeast cells exposed to ricin may require more than RTA catalytic activity, perhaps involving the subcellullar trafficking of the toxin (Tamura et al., 2002a Li et al., 2006). 

Likewise, the A chains of other plant [42-44] and bacterial [45-47] hetero-dimeric toxins are responsible for toxicity. These toxins contain a single A chain moiety which, in each case, has catalytic activity and efficiently inactivates its intracellular target [44]. The A chain is only toxic to intact cells when combined with B chain. The function of the B chain is to bind the toxin to cell-surface receptors, in the case of ricin to appropriate surface glycoproteins or glycolipids. This is the essential first step in the transfer of ricin A chain into the cytosol, where ribosome inactivation occurs [48]. Additionally, the B chain is believed to have a second function during the intoxication process in which it facilitates the transfer of the A chain across a membrane into the cytoplasm [49]. Separated A and B chains are essentially non-toxic, the toxic A chain lacking the ability to bind to and enter cells in the absence of the B chain. The toxicity of ricin therefore results from three sequential steps (1) binding of the whole molecule to the cell surface via the B chain (2) penetration of at least the A chain into the cytosol, and (3) inhibition of protein synthesis caused by the interaction of the A chain with the 60 S ribosomal subunit. 

Differences in cytotoxicity result from more than just the relative catalytic efficiencies of the toxin A-chains. For example, ricin is about 10-fold more cytotoxic than viscumin, despite the fact that the V-glycosidase activities of the two toxins are almost the same (Barbieri et al., 1993 Eck et al., 1999). Barbieri et al. (2004) systematically compared ricin and over a dozen other type 2 RlPs and confirmed that, despite 100- to 1000-fold differences in V-glycosidase activity, there was no apparent correlation between enzyme activity and overall cytotoxicity. 

The inhibitory effect of ricin on protein synthesis in cell-free systems is greatly increased in the presence of / -mercaptoethanol. This is because free A chain is the enzymatically active toxin, whereas A chain linked to B chain in whole ricin is not active [ 121]. In the absence of reducing agents, even high concentrations of intact toxin do not inactivate ribosomes. Presumably the catalytic site on the A chain is formed or exposed only when the A chain is released from the B chain. Free A chain, however, is non-toxic to intact cells since, in the absence of B chain, it lacks the ability to bind to and enter cells. Ricin A-chain preparations frequently show some level of toxicity, however, because the complete removal of contaminating B chain can be difficult to achieve [ 122]. 

RNA aptamer specifically binding and inhibiting the activity of the catalytic ricin A-chain (RTA) has been selected [57]. Initially 80 nucleotides long, it has been shortened to 31-nucleotide aptamer that contained all sequences and structures necessary for interacting with RTA. Inhibition properties of this aptamer exceeded known ricin inhibitors. This aptamer along with others was used to develop a multispecific sensor. Aptamers were immobilized on beads, introduced into micromachined chips on the electronic tongue sensor array, and used for the detection and quantification of proteins. The lowest detected ricin concentration using this sensor was 1 pM (320 ng/mL) [58]. 

Ricin toxin, found in the bean of the castor plant, Ricinis communis, is one of the most toxic and easily produced plant toxins. It is a lectin consisting of two polypeptide chains, the A-chain and the B-chain, linked by a disulfide bond. It is one of a group of dichain ribosome-inactivating proteins, which are specific for the depurination of a single adenosine in ribosomal ribonucleic acid (RNA).1 The active chain (ie, the A-chain) has the ability to modify catalytically the 28S subunit of... 

Poly(ADP-ribose) polymerase homology to other proteins. Sequence similarity comparison of the polymerase with the National Biomedical Center s protein data bases revealed no extensive identities with other proteins (> 4200 proteins searched). Although no extensive similarities were observed, the polymerase exhibits some short but interesting identities. The most statistictdly significant identity is with the catalytic site of the ricin A chain, a cytotoxic plant protein which inactivates the 60S ribosome. This may represent the active site of the enzyme, based upon the catalytic... 

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