Microcystins hepatotoxicity

Gehringer M M, Downs K S, Downing T G, et al. (2003). An investigation into the effect of selenium supplementation on microcystin hepatotoxicity. Toxicon. 41 451-458. 

The toxicity exerted by PTX in several cell types indicates that the hepatocytes are not the only cellular model sensitive to this group of toxins. Therefore, unlike the fresh water toxin microcystin, hepatotoxicity of which is due to unique transport mechanisms present in the hepa-tocyte plasma membrane [40,41], PTXs hepatotoxicity in vivo seems to be determined in part by... 

Fastner, J. et al., Microcystins (Hepatotoxic heptapeptides) in German fresh water bodies. Environ. 

Strongly hepatotoxic cyclic heptapeptides produced by some species of freshwater cyanobacteria (blue-green algae) (28). These microcystins represent a health risk to humans through drinking water, since they have been found to act as tumor promoters (29). Several chromatographic analytical procedures for microcystins have been... 

Fig. 5.1 Common cyanobacterial hepatotoxins. (a) Generalized structure of microcystin, a cyclic heptapeptide. Note that X and Z are L-amino acids. For example, microcystin-LR possesses lysine and arginine residues at X and Z, respectively, (b) Cylindrospermopsin, a hepatotoxic alkaloid from Cylindrospermopsis raceborskii...

Zurawell RW, Chen H, Burke JM, Prepas EE (2005) Hepatotoxic cyanobacteria a review of the biological importance of microcystins in freshwater environments. J Toxicol Env Heal B 8 1-37... 

These natural toxins are heptapeptides produced by cyanobacteria, which are associated with algal blooms. These substances are a hazard to wild and farm animals and sometimes humans who come in contact with contaminated water. There are a number of these toxins, some of which such as microcystin LR are hepatotoxic, causing damage to both hepatocytes and endothelial cells. The toxins have some unusual structural features, incorporating three D-amino acids and two very unusual ones, namely, methyldehydro alanine (Mdha) and amino-methoxy-trimethyl-phenyl-decadi-enoic acid (Adda) (Fig. 7.26). 

Figure 7.26 The structure of the hepatotoxic cyclic heptapeptide microcystin LR. L-Arginine and L-leucine are variable amino acids. The reactive unsaturated group is indicated by the star. Abbreviations Adda, amino-methoxy-trimethyl-phenyl-decadienoic acid Mdha, methyldehydro-alanine Masp, methyl D-/so-aspartate D-Glu, D-/soglutamate D-Ala, D-alanine.

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. 

These natural toxins are heptapeptides produced by cyanobacteria and have unusual structural features, incorporating three D-amino acids. Microcystin LR is hepatotoxic, as a result of inactivation of protein phosphatases, which leads to breakdown of the cytoskeleton and cell death. 

Campbell, D.L., Lawton, L.A., Beattie, K.A. and Codd, G.A. (1994) Comparative assessment of the specificity of the brine shrimp and Microtox assays to hepatotoxic (microcystin-LR-containing) cyanobacteria, Environmental Toxicology and Water Quality 9 (1), 71-77. 

The C20 amino acid (2.V,3.S, 8.S, 9.S, 4/ , 6 )-3-amino-9-methoxy-2,6,8-tri-methyl-10-phenyldeca-4,6-dienoic acid (Adda 19) is a molecule of interest to biologists and organic chemists as a component of the hepatotoxic cyclic peptides called microcystins. Kim and Toogood used Ireland-Claisen rearrangement in their successful synthesis of Adda8 (Scheme 1.3h). The ester 20 underwent highly diastereoselective Ireland-Claisen rearrangement to provide the acid 21. Conversion of this acid to the phosphonium bromide 22 was achieved in nine... 

Park, H.-D, Kim, B., Kim, E., and Okino, T. 1998. Hepatotoxic microcystins and neurotoxic anatoxin-a in cyanobacterial blooms from Korean lakes. Environ Toxicol Water Qual 13, 225-234. 

This chapter focuses on hepatotoxic cyanobateria, including microcystins, nodularins, and cylin-drospermopsins. General aspects of these toxins as well as the analytical methods for their detection will be discussed, taking into account the increased interest on the development of biochemical approaches. 

One of the invariant amino acids is a unique P-amino acid called Adda (2S,3S,8S,9S)-3 amino-9 methoxy-2,6,8-trimethyl-10-phei5fldeca-4,6-dienoic acid is the most unusual structure in this group of cyanobacterial cyclic peptide toxins). A two-letter suffix (XY) is ascribed to each individual toxin to denote the two variant amino acids (Carmichael 1988). X is commonly leucine, arginine, or tyrosine. Y is arginine, alanine, or methionine. Variants of all the invarianf amino acids have now been reported, e.g., desmethyl amino acids and/or replacement of the 9-methoxy group of Adda by an acetyl moiety. Currently there are in excess of 60 variants of microcystin that have been characterized (Rinehart 1994 Sivonen and Jones 1999). Of these 60 compounds, microcystin-LR would appear to be the microcystin most commonly found in cyanobacteria. It is also common for more than one microcystin to be found in a particular strain of cyanobacterium (Namikoshi 1992 Lawton 1995). The microcystin variants may also differ in toxicity (Carmichael 1993). The literature indicates that hepatotoxic blooms ofM aeruginosa containing microcystins occur commonly worldwide. 

Nagata, S., Soutome, H., Tsutsumi, T, Hasegawa, A., Sekijima, M., Sugamata, M., Harada, K. L, Suganuma, M., andUeno,Y. 1995. Novel monoclonal antibodies against microcystin and their protective activity for hepatotoxicity. Nat Toxins 3,78-86. 

The microcystins are hepatotoxic produets of freshwater blooms of cyanobacteria of Microcystis, Anabena, and Oscillatoria species (Chen et al, 1993 Luu et al, 1993), with Microcystis aeruginosa being the most common. 

The most important of the peptidic phosphatase inhibitors are the microcystins and nodularin. Mycrocystins are heptapeptides characterised by the sequence cyclo(D-Ala-X-D-e/7t/i/ o-P-methylisoAsp-Y-Adda-D-isoGlu-iV-methyldehydroAla), where X and Y are different L-aminoacids, and Adda is the abbreviation of the P-aminoacid [25,35,85,95]-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4(E),6(E)-dienoic acid. In the most common microcystin, namely microcystin-LR, X is Leu and Y is Arg. This kind of compounds was considered to be the highly hepatotoxic principle of the cyanobacteria genera Microcystis, Artabaena and Oscillatoria. Apart Jfrom the variations represented by X and Y, other differences arising jfrom the demethylation of aminoacids, lead to the existence to more than fifty microcystins. The rare acid Adda is also... 

Sivonen, K. Carmichael, W.W. Namikoshi, M. Rinehart, M.L. Dahlem, A.M. Niemela, S.I. Isolation and characterization of hepatotoxic microcystin homologues from the filamentous freshwater cyanobacterium Nostoc sp. strain 152. Appl. Environ. Microbiol. 1991, 56, 2650- 2657. 

Freshwater cyanobacteria Microcystis, Oscillatoria, Ana-baena, and Nostoc produce several types of toxins, among which the most commonly detected are the hepatotoxic peptides microcystins. The general structure of the microcystins is cyclo-(D-Ala -X -D-MeAsp -Z -Adda -D-Glu -Mdha ), in which X and Z represent variable L-amino acids, D-MeAsp is D-eryf/iro-p-methylaspartic acid, Mdha is A-methyldehydroalanine, and Adda is the unusual C20 amino acid, (25,35,85,95)-3-amino-9-meth-oxy-2,6,8-trimethyl-10-phenyldeca-4( ),6( )-dienoic acid (Fig. 1). The structural differences in the microcystins mainly depend on the variability of the two L-amino acids (denoted X and Z), and secondarily on the methylation or demethylation of D-MeAsp and/or Mdha. More than 60 microcystins have been isolated from bloom samples and isolated strains of cyanobacteria. 

Microcystins have caused the poisoning of wild and domestic animals worldwide, and in 1996, they caused the death of 76 people in Caruaru, Brazil, which was attributed to the use of microcystin-contaminated hemodialysis water. Microcystins, like the well-documented tumor promoter, okadaic acid, strongly and specifically inhibit the protein phosphatases 1 and 2A and have a tumor-promoting activity in the rat liver. In addition to acute hepatotoxicity, microcystins pose problems to human health—which could result from low-level, chronic exposure to microcystins in drinking water, as suggested by the high incidence of primary liver cancer in the... 

Among the few known examples of secondary metabolites synthesized by the phycobiont are the nostoclides (e.g., 90), " hepatotoxic microcystins (e.g., [ADMAdda ]microcystin-LR, 91), and the cytotoxic cryptophy-cins " from cyanobacteria. The cryptophycin analogue cryptophycin 52 (92) reached clinical trials for cancer chemotherapy but was later abandoned. " ... 

Cyanobacteria known to produce hepatotoxins include species of Microcystis (32-36), Anabaena (37-40), Nostoc (41,42), Oscillatoria (43,44) and the brackish water Nodularia spumigena (45,46). These cyanobacteria produce a wide range of toxins including neurotoxic alkaloids, lipopolysaccharides, phenolic compounds and most importantly, the cyclic hepatotoxic peptides microcystins and nodularins. Since both microcystins and nodularins were recently discovered even in shellfish (47) and tropical fish species (48), and a nodularin variant was isolated in Papua New Guinea also from the tropical marine sponge Theonella swinhoei (49), marine prokaryotes are apparently producing identical toxins as the cyanobacteria. However, since this nodularin variant was isolated from a marine sponge it may well be produced by a microbial symbiont. 

The identical hepatotoxicity and binding affinity for protein phosphatases of microcystins and nodularins clearly show that the dipeptide D-Ala -L-Leu(X)2 is unimportant although the D-Ala unit is almost invariant. In fact, only one mutant with D-Ser in this position has been identified so far (40). However, as mentioned above, the cyclic forms of the penta- and hexapeptides are essential. 

On closer examination, microcystins have been reported to cause hepatotoxic effects in fanned catfish (Zimba et al., 2001) and farmed sahnon (Andersen et al., 1993). Microcystins also produce pathological effects in tilapia (Preito et al., 2006), silver carp (Li et al., 2007), common carp (Fischer and Dietrich, 2000), and loach (Liu et al., 2002). The neurotoxin anatoxin-a, is produced by several genera of cyanobacteria and it has been reported to cause mortalities in juvenile common carp under experimental conditions (Oswald et al., 2006). Exposure to 0.768 pg/g dry weight of freeze-dried Anabaena spp. caused mortalities with 28 h. 

Zhang, X., Xie, R, Hao, L., Guo, N., Gong, Y, Hu, X., Chen, J. and Liang, G. Effects of the phytoplanktivo-rous silver carp (Hypophthalmichthys molitrixon) on plankton and the hepatotoxic microcystins in an enclosure experiment in a eutrophic lake. Lake Shichahai in Beijing, Aquaculture, 257, 173, 2006. 

Rouhiainen, L. et al.. Genes coding for hepatotoxic heptapeptides (microcystins) in the cyanobacterium Anabaena strain 90, Appl Environ. Microbiol, 70, 686, 2004. 

---