Ricin cytotoxic effect

RIPs are plant protein toxins that are able to inhibit enzymatically ribosomal activity and are therefore highly cytotoxic [98]. RIPs are taken up in the cells by means of endocytosis, and only a small fraction (5% or less) are translocated to the cytosol where the toxins inhibit the protein synthesis and eventually kill the cell. PCI may be used to increase both the efficacy and specificity of these toxins. RIPs are divided into two groups, type I and type II. Type II RIPs, like ricin, consists of two polypeptide chains, one cytotoxic A-chain with /V-glycosidase activity and one B-chain which binds to the cell surface. Type I RIPs, like gelonin, agrostin, and saporin, lack the B chain, which make them poorly transported over the cell- and intracellular membranes to the cell cytosol. Hence, the cytotoxic effect of these protein toxins is usually absent or very low. A considerable cytotoxic effect of type I RIPs has been shown in combination with PCI, both in vitro and in vivo [25, 99]. 

Complete elucidation of the mechanisms by which ricin kills target cells remains an area of active study, but it is clear that the A -glycosidase activity of RTA is the essential triggering event. Inhibition of protein synthesis precedes other detectable alterations in target cell biochemistry. Ricin blocks amino acid incorporation in cmde microsome preparations before changes occur in energy metabohsm or oxidative phosphorylation the toxin has essentially no effect on mitochondrial respiration in isolated mitochondria or tumor cells (Waller et al., 1966 Dirheimer et al., 1968 Lin et al., 1971). Likewise, the first observable cytotoxic effect of ricin in cell culture is typically the inhibition of protein synthesis, followed by a reduction in DNA synthesis (Lin et al., 1970 Lin et al., 1971 Onozaki et al., 1972 Refsnes et al., 1974 Nicolson et al., 1975 Olsnes et al., 1976 Refsnes et al., 1977). 

At present the scientific literature contains a large and rapidly expanding number of accounts of the conjugation of cell-reactive antibodies to ricin or its A chain and their cytotoxic effects on cultured cells or whole animal models. In the present account, we will not attempt to catalogue these reports but will briefly and simply outline emerging conclusions and present constraints. 

There is normally a delay of several hours between the exposure to ricin and the onset of clinical symptoms, but the cytotoxic effects may not occur until two to five days after ingestion. In contrast, certain individuals may display allergic reactions almost immediately after exposure, and this is presumed to be related to the glycoproteins that the beans contain. Episodes of asthma have been linked to the inhalation of ricin dust at castor bean mills — in one incident in 1952 at Bauru in Brazil, 150 people living close to a bean mill were affected by sudden asthma attacks. 

As in the case of MBS, discussed previously, SMPB was found to be more effective than aliphatic crosslinkers in producing immunotoxin conjugates with ricin that have high yields of cytotoxicity (Myers et al., 1989). This was attributed to the reagent s aromatic ring structure. A comparison with SPDP produced immunotoxin conjugates concluded that SMPB formed more stable complexes that survive in serum for longer periods (Martin and Papahadjopoulos, 1982). 

Viscum contain lectins that are cytotoxic by inhibiting protein synthesis on the ribosomal level in a manner similar to the toxalbumins ricin and abrin. Viscotoxin and phoratoxin are cardiac toxins and vasoconstrictors. Both produced reflex bradycardia, negative inotropic effects, and, in high doses, vasoconstriction of skin and skeletal muscle vessels in animals. 

Da. The toxins are made up of two polypeptide chains (A and B) connected by a disulfide bond. The cytotoxicity of ricin is due to inhibition of protein synthesis, caused when the B chain binds to cell-surface receptors and the toxin-receptor complex is taken into the cell, and the A chain that has endonuclease activity and, at extremely low concentrations, will inhibit DNA replication and protein synthesis (USAMRICD, 2005). Ricin is stable under ambient conditions and can be detoxified by heat at 80°C for 10 min, or 50°C for an hour at a pH of 7.8. Chlorine inactivates over 99.4% by 100 mg/L FAC in 20 min. Low chlorine concentrations, such as 10 mg/L FAC, as well as iodine at up to 16 mg/L will have no effect on ricin (USAMRICD, 2005). 

The ability of PAAs to mediate the delivery of proteic macromolecules such as toxins has been investigated. With gelonin and ricin A-chain as payloads, it was shown that these polymers are able to permeabilize the endosomal membrane, and thus aid cytoplasmic entry, displaying good results in both in vitro and in vivo delivery. In particular, PAAs were able to restore toxin cytotoxicity, whereas neutral polymers such as dextran were unable to mediate this effect. Moreover, PAAs were able to escape the reticuloendothelial system (clearance after intravenous administration), allowing tumor targeting by the enhanced permeability and retention... 

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