Review Articlethe toxicology of microcystins
Introduction
Microcystins (also known as cyanoginosins) are a family of toxins produced by species of freshwater cyanobacteria, primarily Microcystis aeruginosa, but also other Microcystis species and other genera, namely Anabaena, Oscillatoria and Nostoc. The microcystins are monocyclic heptapeptides composed of d-alanine at position 1, two variable l-amino acids at positions 2 and 4, γ-linked d-glutamic acid at position 6, and 3 unusual amino acids: β-linked d-erythro-β-methylaspartic acid (MeAsp) at position 3; (2S,3S,8S,9S)-3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid (Adda) at position 5; and N-methyl dehydroalanine (MDha) at position 7. There are over 50 different microcystins which differ primarily in the two l-amino acids at positions 2 and 4 and methylation/demethylation on MeAsp and MDha. The unusual amino acid Adda is essential for expression of biological activity, and a different stereochemistry about the conjugated double bond, for example, results in abolition of toxicity (An and Carmichael, 1994; Luukainen et al., 1994; Trogen et al., 1996). The most common microcystin is microcystin-LR, where the variable l-amino acids are leucine (L) and arginine (R). Its structure is shown below. Another cyanobacterial toxin, nodularin, from Nodularia spumigena, has a similar toxicity and structure to microcystins, but is a cyclic pentapeptide rather than a heptapeptide (An and Carmichael, 1994).
There is some dispute about the 3-dimensional structure of microcystin-LR, as deduced by NMR spectroscopy and mathematical calculations. Rudolph-Böhner et al. (1994) proposed a compact boat-like ring structure with the Adda chain bent into a position above the ring, whereas Trogen et al. (1996)deduced a saddle-shaped ring with Adda directed away from the cyclic backbone.
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Analysis
Four sensitive methods or potential methods of analysis of microcystins have been published, based on reaction with a fluorescent probe, enzyme-linked immunosorbent assays (ELISAs), inhibition of protein phosphatase, and mass spectrometry, respectively.
Shimizu et al. (1995)synthesized a fluorogenic reagent to target conjugated dienes and called it DMEQ-TAD. This reagent reacted quantitatively with vitamin D metabolites and synthetic analogues, and the fluorescent products could be quantified
Stability
In laboratory experiments using low levels of microcystin-LR (10 μg/l) in reservoir water, Cousins et al. (1996)found that primary degradation of the toxin occurred in less than 1 week. It was stable over 27 days in deionised water, and over 12 days in sterilised reservoir water, indicating that the instability in normal reservoir water is due to biodegradation. The mechanism of inactivation is probably by modification of the Adda side chain (Cousins et al., 1996).
Purified microcystins are also
Toxicity
Poisoning of livestock from drinking water containing blue–green algae (cyanobacteria) was first observed at Lake Alexandrina in South Australia in 1878. In this case, the toxin was nodularin (Falconer, 1992). Since then there have been many reports of intoxications of birds, fish and other animals by cyanobacterial toxins, including microcystins. Growth of cyanobacterial blooms is encouraged by an increase of nutrients such as nitrates and phosphates in the water; these nutrients may be
Mechanism of action
It appears that microcystins mediate their toxicity by uptake into hepatocytes, via a carrier-mediated transport system, followed by inhibition of serine/threonine protein phosphatases 1 and 2A. The consequent protein phosphorylation imbalance causes disruption of the liver cytoskeleton, which leads to the massive hepatic haemorrhage which is the cause of death (Honkanen et al., 1990; Eriksson et al., 1990a; Falconer et al., 1981).
Protection against microcystin toxicity
Because of the rapid, irreversible and severe damage to the liver caused by microcystins, therapy is likely to have little or no value, and effective prophylaxis is critical.
The basis of the toxicity of the microcystins (inhibition of protein phosphatases) was not known until recently (1990; above). Prior to this, Adams et al. (1989)investigated chemicals known to affect macrophage function as potential prophylactic agents. This rationale was based on the known protective effect of
Future directions
Further research on microcystins could well focus on a characterisation of the transport mechanism of the toxin into hepatocytes (Runnegar et al., 1995a), and clarification of the specificity of microcystins as inhibitors of protein phosphatases (Honkanen et al., 1994). In searching for more effective medical countermeasures to the microcystins, primarily as prophylactic drugs, investigations into the role of glutathione as an antidote, and into potent inhibitors of the binding of microcystins
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