The Amatoxins

An amatoxin present only in minimal amounts, amanin , was found to yield no characteristic color in the systems just described. It was shown to be 

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Poisoning and sometimes death from eating (unidentified) mushrooms is well known. In particular, Amanita sp. are particularly dangerous, with much emphasis on the death cap fungus , Amanita phalloides.24 The best known toxins are the amatoxins and phallotoxins, which are complex, bicyclic peptides. An unusual feature relates to sulfur a tryptophan (or substituted tryptophan) unit is linked to a cysteine sulfur at the carbon atom next to the NH group of the pyrrole ring, forming the unit, -CH2-S-C(NH)=C, e.g. in... 

The phallotoxins 106, e.g. phalloidin, and the amatoxins 107, e.g. a-amanitin, produced by Amanita phalloides or death cup, are among the best-known peptide poisons 233). 

The amatoxins are cyclic octapeptides composed only of L-amino acids and containing a sulfoxide group instead of the thioether bridge in phallotoxin. Over 90% of the fatal cases of mushroom poisoning can be traced back to the amatoxins. Wieland et al. 234) have shown that, in addition to the toxins, the death cup contains a low concentration of an antitoxic cyclic decapeptide antamanide 108. 

The amatoxins and phallotoxins are cyclic peptides (peptides are made... 

The most serious form ofmycetism is produced by Amanita phalloides, other Amanita species, Lepiota, and Galerina species. These species account for >90% of fatal cases. Ingestion of as little as 50 g of A. phalloides (deadly nightcap) can be fatal. The principal toxins are the amatoxins (a- and (3-amanitin), a group of cyclic octapeptides that inhibit RNA polymerase 11 and hence... 

Some of the naturally occurring amatoxins from the amanullin series do not contain a y-hydroxylated side chain of isoleucine and are nontoxic. Therefore, it has been concluded that, in contrast to the case for phallotoxins, the presence of such a group in the amatoxins is prerequisite for toxic activity. This type of hydroxyl group is located on the concave side of the 24-membered macrocycle in the three-dimensional molecular structure and is involved in binding to the RNA polymerase enzyme. The chemical effect of the amatoxin side chain and the contribution of every structural element to biological activity have been successfully investigated 107,114). 

Phalotoxins. Together with the amatoxins (see aman-itins) the P. are the toxic principles of the green and white death cups (Amanita phalloides and A. verna). The P. are responsible for the chronic components of death cup poisonings. They are amorphous, water-soluble solids. Seven naturally occurring P. are currently known. 

In mammals, phallotoxins, and certainly also virotoxins, are absorbed from the intestines into the blood only slowly, if at all (in contrast to the amatoxins). After intraperitoneal injection they rapidly reach the liver and exert their similar toxic effect there. They weaken the structure of the hepatocyte membrane, hence blood penetrates the Uver cells and the periphery is depleted of blood to such an extent that anemic shock occurs from internal bleeding. A cause of damage to the membranes is supposedly the very strong binding of phallotoxins and virotoxins to F-actin that under the lipid-bilayer stabilizes the membrane. At this point a brief discussion of actin is necessary. 

The amatoxins are cyclic octapeptides of L-amino acids the backbone is divided into two rings by a sulfur bridge analogously to the phallotoxins, but with an (jR)-sulfoxide. The amanitins (a-, j8-etc.) are derived from 6-hydroxy tryptophan, amanin lacks the phenolic OH-group. As in collagen, 4-frans-hydroxyproline is the imino acid the occurrence of y, -dihydroxyisoleucine is uncommon. 

Isolation and Characterization of the Peptide Toxins. - The Chemistry of the Amatoxins,... 

Two major types of cyclic peptides have been isolated from Amanita species. Phallotoxins, such as phalloin (6), have seven amino acids in the ring, whereas, amatoxins [e.g., a-amanitin (7)], have eight (Fig. 14.3). Certain portions of the molecules are crucial for toxicity for example, in the amatoxin amanullin (8), elimination of a single hydroxyl group from one amino acid eliminates activity. However, generalizations about the effects of structure changes on toxicity cannot yet be made reliably. 

Phallotoxins heterodetic, cyclic heptapeptides, present in Amanita phalloides. Together with the Amatoxins (see), P. are the main toxic components of this fungus. The chief P. are phalloidin, phalloin and phallacidin (Fig.). Phallacidin is similar to phalloidin, but the D-threonine-alanine grouping is replaced by valyl-D-erythro-P-hydroxyaspartic acid. P. are not as toxic as the amatoxins, but they act more quickly. The structural requirements for toxicity... 

Toxins are present in all parts of toe fia us and are stable to heat and drying. The amatoxins are relatively more toxic than the phallotoxins. 

Brodner, O. G., and Weiland, T., 1976, Identification of the amatoxin binding subunit of RNA polymerase B by affinity labeling experiments. Biochemistry 15 3480-3484. 

The green death cap Amanita phalloides produces two families of toxic peptides, the amatoxins and the phallotoxins. Amatoxins are by far the more dangerous toxins and account for all fatalities occurring in human mushroom poisoning. 

Interestingly, all three antibodies recognized the hydroxytryptophan residue of a-amanitin, while the RNA polymerase II did not. Particularly the indole NH (with AAG values up to -12 kJ/mol) and the OH in 6 position (with AAG values up to -8 kJ/mol) seem to be involved in hydrogen bonds with the antibodies. The fact that RNA polymerase II is unable to recognize the aromatic part of amanitin is in line with previous experiments, which showed that various alkylation reactions at the 6 hydroxy group, or azo-coupling in 7 posi-tion, did not seriously impair the inhibitory capacity of the amatoxin derivatives. We conclude that in the complex with the enzyme the hydroxytryptophan moiety is located juxtaposed to that part of amanitin that binds to RNA polymerases II. 

Among the homodetic, polycyclic, biologically-active tryptophan peptides, the toxins of the Amanita phalloides represent a special case [see reference 427) and 349) for a review]. In fact, the poisonous components of this deathcap, the phallotoxins and amatoxins (Fig. 9), both contain a cycloheptapeptide structure with an additional intramolecular covalent bond connecting the p-sulfur atom of a L-cysteine residue with the carbon atom in position 2 of the indole ring of a L-tryptophan residue. In the amatoxins the thioether bridge is oxidized to the sulfoxide (Fig. 9). 

WiELAND, Th., M. P. Jordan de Urries, H. Indest, H. Faulstich, A. Gieren, M. Sturm, and W. Hoppe Components of the Green Deathcap Toadstoal, Amanita phalloides, XLV. The Absolute Configurations of the Toxic and Nontoxic Phalloidin Sulfoxide and of the Amatoxins. Liebigs Ann. Chem. 1974, 1570-1579. 

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