The generic structure of a microcystin. Variations occur primarily at positions 1 and 2. For example, Microcystin-LR contains the amino acids leucine (L) and arginine (R) at positions 1 and 2 respectively. Microcystin-RR has arginine at both positions. Nodularins are similar, with the five amino acids Adda-γGlu-Mdhb-βMeAsp-Arg making up the core ring system (Harada, 1996).
Globally the most frequently found cyanobacterial toxins in blooms from fresh and brackish waters are the cyclic peptide toxins of the microcystin and nodularin family. They pose a major challenge for the production of safe drinking water from surface waters containing cyanobacteria with these toxins. In mouse bioassays, which traditionally have been used to screen toxicity of field and laboratory samples, cyanobacterial hepatotoxins (liver toxins) cause death by liver haemorrhage within a few hours of the acute doses. Microcystins have been characterised from planktonic Anabaena, Microcystis, Oscillatoria (Planktothrix), Nostoc, and Anabaenopsis species, and from terrestrial Hapalosiphon genera. Nodularin has been characterised only from Nodularia spumigena.
The cyclic peptides are comparatively large natural products, molecular weight (MW) apprx. 800-1,100, although small compared with many other cell oligopeptides and polypeptides (proteins) (MW > 10,000). They contain either five (nodularins) or seven (microcystins) amino acids, with the two terminal amino acids of the linear peptide being condensed (joined) to form a cyclic compound. They are water soluble and, except perhaps for a few somewhat more hydrophobic microcystins, are unable to penetrate directly the lipid membranes of animal, plant and bacterial cells. Therefore, to elicit their toxic effect, uptake into cells occurs through membrane transporters which otherwise carry essential biochemicals or nutrients. As will be outlined in section 4.2, this restricts the target organ range in mammals largely to the liver. In aquatic environments, these toxins usually remain contained within the cyanobacterial cells and are only released in substantial amounts on cell lysis. Along with their high chemical stability and their water solubility, this containment has important implications for their environmental persistence and exposure to humans in surface water bodies.
The first chemical structures of cyanobacterial cyclic peptide toxins were identified in the
early 1980s and the number of fully characterised toxin variants has greatly increased
during the 1990s. The first such compounds found in freshwater cyanobacteria were
cyclic heptapeptides (that is they contain seven peptide-linked amino acids) with the
general structure of:
Z4-Adda5-D-glutamate6-Mdha7) in which X and Z are variable L amino acids, D-MeAsp3 is D-erythro-ß-methylaspartic acid, and Mdha is N-methyldehydroalanine. The amino acid Adda, (2S,3S,8S,9S)-3-amino-
9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid, is the most unusual structure in this group of cyanobacterial cyclic peptide toxins. These compounds were first isolated from the cyanobacterium Microcystis aeruginosa and therefore the toxins were named microcystins (Carmichael et al., 1988). Structural variations have been reported in all seven amino acids, but most frequently with substitution of L-amino acids at positions 2 and 4, and demethylation of amino acids at positions 3 and/or 7. About 60 structural variants of microcystins have been characterised so far from bloom samples and isolated strains of cyanobacteria.
In one species of brackish water cyanobacterium, an identically acting and structurally very similar, cyclic pentapeptide occurs. It has been named as nodularin after its producer, Nodularia spumigena. The chemical structure of nodularin is cyclo-(D-MeAsp1- L-arginine2-Adda3-D-glutamate4-Mdhb5), in which Mdhb is 2-(methylamino)-2- dehydrobutyric acid. A few naturally occurring variations of nodularins have been found: two demethylated variants, one with D-Asp1 instead of D-MeAsp1, the other with DMAdda3 instead of Adda3; and the non-toxic nodularin which has the 6Zstereoisomer of Adda3 (Namikoshi et al., 1994).
|N. spumigena||Baltic Sea|
|N. spumigena||New Zealand|
The equivalent 6Z-Adda3 stereoisomer of microcystins is also non-toxic. In the marine sponge, Theonella swinhoei, a nodularin analogue called motuporin has been found. It differs from nodularin only by one amino acid, having hydrophobic L-Val in place of the polar L-Arg in nodularin (de Silva et al., 1992). The toxin might be cyanobacterial in origin because the sponge is known to harbour cyanobacterial symbionts.
Cylindrospermopsin is a toxin produced by cyanobacteria, or blue-green algae, that has severe effects on the liver and other organs.
The Guidelines for Drinking-water Quality Final Task Force meeting (Geneva, 2003) recommended that a background document on cylindrospermopsin be prepared. Toxicity data for cylindrospermopsin are now available, but their adequacy for the preparation of a background document is questionable.
|Anabaena planctonica bloom||Italy|
|Oscillatoria sp. benthic||Scotland|
|Oscillatoria sp. ?||Ireland|
Anatoxin-a is a low molecular weight alkaloid (MW = 165), a secondary amine, 2-acetyl- 9-azabicyclo(4-2-1)non-2-ene (Devlin et al., 1977). Anatoxin-a is produced by Anabaena flos-aquae, Anabaena spp. (flos-aquae-lemmermannii group), Anabaena planktonica, Oscillatoria, Aphanizomenon and Cylindrospermum. Homoanatoxin-a (MW = 179) is an anatoxin-a homologue isolated from an Oscillatoria formosa (Phormidium formosum) strain. It has a propionyl group at C-2 instead of the acetyl group in anatoxin-a (Skulberg et al., 1992). The LD50 (lethal dose resulting in 50 per cent deaths) of anatoxin-a and homoanatoxin-a are 200 - 250 µg kg-1 bw (Devlin et al., 1977; Carmichael et al., 1990; Skulberg et al., 1992).
Anatoxin-a(S) is a unique phosphate ester of a cyclic N-hydroxyguanine (MW = 252) produced by Anabaena flos-aquae strain NRC 525-17 (Matsunaga et al., 1989). It has more recently been identified in blooms and isolated strains of Anabaena lemmermannii (Henriksen et al., 1997; Onodera et al., 1997a). The LD50 of anatoxin-a(S) is 20 µg kg-1 bw (i.p. mouse) (Carmichael et al., 1990). Structural variants of anatoxina( S) have not been detected.
Saxitoxins are a group of carbamate alkaloid neurotoxins which are either non-sulphated (saxitoxins - STX), singly sulphated (gonyautoxins - GTX) or doubly sulphated (C-toxins) . In addition, decarbamoyl variants and several new toxins have been identified in some species.