Microcystin toxicity

Chemoprotectant studies have indicated that membraneactive antioxidants such as vitamin E may offer protection against microcystin toxicity. 

Gehringer, M.M., Govender, S., Shah, M., and Downing, T.G. 2003. An investigation of the role of vitamin E in the protection of mice against microcystin toxicity. Wiley Periodicals Environ Toxicol 18 142—148. 

Imanishi S, Harada K. Proteomics approach on microcystin binding proteins in mouse hver for investigation of microcystin toxicity. Toxicon 2004 43(6) 651-9. 

Tencalla, F., Dietrich, D., 1997. Biochemical characterization of microcystin toxicity in rainbow trout Oncorhynchus mykiss). Toxicon 35 (4), 583-596. 

In the Slimmer of 1989, Rutland Water, the largest man-made lake in Western Europe and which supplies potable water to approximately 500 000 people in the East of England, contained a heavy bloom of Microcystis aeruginosa. By the end of the summer, a number of sheep and dogs had died after drinking from the bloom and concentrated scum. Analysis revealed that the cyanobacterial bloom material was toxic to laboratory mice, and that rumen contents from a poisoned sheep contained fivemicrocystin variants.Microcystins were detected in waters used for recreation in Australia at concentrations greater than 1 mg per... 

Daphnia assay, the brine shrimps are exposed to different concentrations of toxicant, and the toxicity is expressed as the LCjo value. Extracts of cyanobacterial blooms and laboratory cultures, containing microcystins or anatoxin-a, have been found to be toxic towards brine shrimp," and fractionation of such extracts resulted in brine shrimp fatalities only with fractions containing microcystins." " ... 

In vitro cytotoxicity assays using isolated cells have been applied intermittently to cyanobacterial toxicity testing over several years." Cells investigated for suitability in cyanobacterial toxin assays include primary liver cells (hepatocytes) isolated from rodents and fish, established permanent mammalian cell lines, including hepatocytes, fibroblasts and cancerous cells, and erythrocytes. Earlier work suggested that extracts from toxic cyanobacteria disrupted cells of established lines and erythrocytes," but studies with purified microcystins revealed no alterations in structure or ion transport in fibroblasts or erythrocytes,... 

The ability to identify and quantify cyanobacterial toxins in animal and human clinical material following (suspected) intoxications or illnesses associated with contact with toxic cyanobacteria is an increasing requirement. The recoveries of anatoxin-a from animal stomach material and of microcystins from sheep rumen contents are relatively straightforward. However, the recovery of microcystin from liver and tissue samples cannot be expected to be complete without the application of proteolytic digestion and extraction procedures. This is likely because microcystins bind covalently to a cysteine residue in protein phosphatase. Unless an effective procedure is applied for the extraction of covalently bound microcystins (and nodiilarins), then a negative result in analysis cannot be taken to indicate the absence of toxins in clinical specimens. Furthermore, any positive result may be an underestimate of the true amount of microcystin in the material and would only represent free toxin, not bound to the protein phosphatases. Optimized procedures for the extraction of bound microcystins and nodiilarins from organ and tissue samples are needed. 

It is obvious from the provisional risk assessment values for microcystins, and, being of the same order of magnitude of mammalian toxicity, similar values may be calculated for the cyanobacterial neurotoxins, that sensitive detection methods are required to detect these low concentrations of toxins. Of the biological methods of detection discussed earlier, the mouse and invertebrate bioassays are not sensitive enough without concentration of water samples, in that they are only able to detect mg of microcystins per litre. Only the immunoassays (ng-/rg 1 and the protein phosphatase inhibition assays (ng O... 

Rapid-acting cytotoxin that disrupts cell membranes in the liver (hepatoxin) causing an accumulation of blood in the liver. It is the most toxic of the Microcystins. It is a solid obtained from freshwater blue-green cyanobacteria (Microcystis aeruginosa, Microcystis cyanea). It is heat stable and water soluble. Aqueous solutions are "probably stable" and resistant to chlorine at 100 ppm. It is also soluble in alcohol and acetone. 

One role of cyanobacterial allelochemicals may be to alter the motility and distribution of competing photoautotrophs. In a recent study, Kearns and Hunter (2001) examined the effects of toxic metabolites from the filamentous cyanobacterium A. flos-aquae on a unicellular phytoplankton species, Chlamydomonas rein-hardtii. A. flos-aquae synthesizes both microcystins as well as anatoxins, providing the authors with an ecologically relevant opportunity to assess the individual and combinatorial effects of these toxins on an alga. 

Cyanobacteria toxins are toxins produced by certain species of blue-green algae that have become a major environmental and public health concern. The behavior of cyanotoxins during chlorination treatment has been recently reviewed by Merel et al. [129]. Chlorination DBFs have been reported only for the hepatotoxins microcystin-LR and cylindrospermopsin. Other cyanotoxins, such as nodularins, saxitoxins, and anatoxins, have yet to be investigated. Different isomers of six chlorination products of microcystin-LR have been characterized dihydroxy-microcystin, monochloro-microcystin, monochloro-hydroxy-microcystin, monochloro-dihydroxy-microcystin, dichloro-dihydroxy-microcystin, and trichloro-hydroxy-microcystin. Only two chlorination DBFs have been reported so far for cylindrospermopsin 5-chloro-cylindros-permopsin and cylindrospermopsic acid [129]. Chlorination of microcystin, cylindrospermopsin, and nodularins seems to reduce the mixture toxicity however, this aspect has not been extensively studied [129]. 

The main genera responsible for freshwater toxic blooms are Microcystis, Anabaena, Aphanizomenon and Oscillatoria. Toxins produced include 1. anatoxins, alkaloids and peptides of Anabaena 2. the peptide microcystin and related peptides of Microcystis 3. aphantoxins, compounds of Aphanizomenon with properties similar to some paralytic shellfish poisons. Properties of Oscillatoria toxin suggest they are peptides similar to those of Microcystis. Microcystis toxins are peptides (M.W. approx. 1200) which contain three invariant D-amino acids, alanine, erythro-3-methyl aspartic and glutamic acids, two variant L-amino acids, N-methyl dehydro alanine and a 3-amino acid. Individual toxic strains have one or more multiples of this peptide toxin. The one anatoxin characterized is a bicylic secondary amine called anatoxin-a (M.W. 165). The aphantoxin isolated in our laboratory contains two main toxic fractions. On TLC and HPLC the fractions have the same characteristics as saxitoxin and neosaxitoxin. 

The potent toxin microcystin [blue-green algae inhibit one of the phosphatases (PP2A)] increases mitogenic activity and so is a tumor promoter at low-level exposures. The liver toxicity at high levels of acute exposure is probably also due to disturbances in protein phosphorylation. 

Microcystins are extremely potent inhibitors of protein phosphatases, active at the level of 0.2 nM (2 x 10-10 M). Coupled with the active uptake into the liver, this means that microcystins are extremely toxic. As well as being hepatotoxic, they are also tumor promoters. 

Describe the cellular target and the specific mechanism of toxicity of microcystin LR. 

Cyanobacteria - the Jekyll and Hyde of marine organisms - are a novel source of potential new pharmaceutical compounds (2618-2620, 2662). On the other hand, toxic cyanobacterial blooms in lakes, rivers, and water storage reservoirs have occurred worldwide (2621, 2663, 2664). For example, 60 patients in a Brazil hemodialysis unit died after drinking water from a lake contaminated with cyanobacterial microcystins (2622), not unlike the toxicity of red tides (2623). Cyanobacteria also produce the highly toxic neurotoxin, p-N-methylamino-L-alanine, which may be produced by all cyanobacteria (2624, 2665). 

Our research group is working on the development of electrochemical biosensors for the detection of microcystin and anatoxin-a(s), based on the inhibition of protein phosphatase and acetylcholinesterase, respectively. These enzyme biosensors represent useful bioanalytical tools, suitable to be used as screening techniques for the preliminary yes/no detection of the toxicity of a sample. Additionally, due to the versatility of the electrochemical approach, the strategy can be applied to the detection of other cyanobacterial toxins. 

However, problems associated with the reproducibility between electrodes derived from the screen-printing process and the partial electrode fouling have compromised the sensitivity of the biosensors. Work is in progress to improve both the reproducibility and the limits of detection by the use of new types of electrodes. The toxin overestimation observed with the amperometric biosensor, in the case of the microcystin analysis, suggests the use in parallel to other analytical techniques in order to minimise the risk of false-positive results. Nevertheless, the electrochemical strategy is appropriate to discriminate between toxic/non-toxic samples. 

S. Nagata, H. Soutome, T. Tsutsumi, A. Hasegawa, M. Sekijima, M. Sugamata, K.-I. Harada, M. Suganuma and Y. Ueno, Novel monoclonal antibodies against microcystin and their protective activity for hepato-toxicity, Nat. Toxins, 3 (1995) 78-86. 

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