Peptides hepatotoxic

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. 

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. 

In 1878 George Francis published the first written report of animal poisoning by a Cyanobacterium (blue-green alga). However, only in the last 30 years have significant amounts of information appeared on the structure and function of neurotoxin alkaloids and hepatotoxic peptides. Only the latter are discussed here. The peptide nature of the isolated fast death factor was recognized in 1959 by C.J. Bishop et al. [57] the structure of cyanoginosin-LA was revealed only in 1984 by Botes et al. (Fig. 25) [58]. 

Blue-green algal hepatotoxic peptides such as nodularin, microcystin-LR, -RR, and -YR 26,36,37 analyzed under MALDI-TOF-MS conditions using ACHCA as the matrix. 

Extracts of toxic isolates of M. aeruginosa or a purified peptide toxin injected into the bloodstream or the peritoneal cavity of mice elicit hepatotoxic effects (5-7,8). The fresh weight of the liver is greatly increased, and sinusoidal congestion, hemorrhage, and necrosis of the liver are observed. A lethal dose of purified toxin also induces multiple thrombi in the lungs and causes a marked reduction in the number of circulating blood platelets (6,7). 

The introduction of cyclosporin, a peptide derived from a fungus, revolutionised immunosuppressive therapy, and was one of the major influences in the improvement of early graft survival in transplant surgery when introduced in the 1980s. The main action is a relatively selective inhibition of IL-2 production and consequently a decreased proliferation of T cells. A major advantage of cyclosporin is that it does not cause myelosuppression in therapeutic doses. The major side effect is nephrotoxicity, which occurs in about 20% of patients, and about 50% of patients develop moderate hypertension. The other major side effect is hepatotoxicity with cholestasis and hyperbilirubinaemia. 

W.P. Brooks and G.A. Codd, Immunoassay of hepatotoxic cultures and water blooms of cyanobacteria using Microcystis areuginosa peptide toxin polyclonal antibodies, Environ. Technol. Lett., 9 (1988) 1343-1348. 

Bosentan, an orally active competitive inhibitor of endothelin (see Chapter 17 Vasoactive Peptides), has been shown to have some benefits in experimental animal models of heart failure, but results in human trials have not been impressive. This drug is approved for use in pulmonary hypertension (see Chapter 11 Antihypertensive Agents). It has significant teratogenic and hepatotoxic effects. 

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... 

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. 

The number of useful biomarkers to predict neurotoxicity, hepatotoxicity or cardiovascular toxicity is still rather limited and they are not yet well established as tools in pharmaceutical laboratories. Even though some decent correlations between biomarkers and toxicological events have been demonstrated, " a significant amount of validation work still has to be performed. Eor example, while natriuretic peptides and troponin can be clinical markers for cardiovascular toxicity, preclinical use in... 

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... 

Rhizopus microsporus strains have also been described as a source of food mycotoxins. Rhizonins, such as 93 (Scheme 11), which were the first-described toxins from zygomycetes, are strongly hepatotoxic nonribosomal peptides isolated from moldy peanuts in Mozambique. Examination for the presence of bacteria again showed that a Burkholderia sp. endosymhiont is the true biosynthetic source.As with the rhizoxin (28) producer, the establishment of a pure symbiont culture was successful. 

Penicillium islandicum is a chlorine-containing peptide whose structure was determined later. This organism also produced cyclochlorotine (92), which is an infectant of yellowed rice [133], The fungus Metarhizium anisopliae produces the chlorohydrin cyclic peptide (93) [134], Cyclochlorotine, a hepatotoxic mycotoxin, was also isolated from Penicillium islandicum. 

Though there are few differences between the peptide sequences of the astins and those of cyclochlorotine and islanditoxin, astins exhibited antitumor activity and only hepatotoxicity was shown by cyclochlorotine and islanditoxin [116]. The fact that minor structural changes cause such a noticeable change in biological activity in both the astins and the toxins is of interest. 

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