Toxic Fungi of Western North America

by Thomas J. Duffy, MD

Toxins of Amanita phalloides, other amatoxic Amanita and amatoxic species in the genera Lepiota, Galerina and Conocybe

Of the toxins in Amanita phalloides and Amanita ocreata, only the amanitins are effectively toxic in humans. The amanitins—alpha, beta, gamma and epsilon (α, β, γ, ε)—are bicyclic octopeptides (8 amino acid chains) with an unusual sulfur bridge and are lethal in the alpha form at about 0.1 mg per kilogram of body weight. (32) ε-amanitin is present only in minute amounts and does not contribute significantly to the toxicity. Destroying the sulfur bridge renders the compounds inactive. The amount of amatoxin in a single Amanita phalloides cap varies considerably, but may be enough to cause death. The amino acid group near the sulfur bridge is tryptophan and this hydroxylated amino acid is the portion of the molecule assayed in thin layer chromatography. (33) The phallotoxins have one less amino acid and these heptapeptides apparently play no role in human poisoning. The cause of the early GI symptoms starting between 8-14 hours is unknown.

Figure 1. The Amanitins

At low concentrations, amanitins bind specifically to the enzyme RNA polymerase II (or B) in cell nuclei. With reduced amounts of polymerase II to catalyze the transcription (“reading”) of messenger RNA from DNA, there is less protein synthesis in the body. (34) At very high doses RNA I (or A) and RNA III (or C) are also affected. (35) Phallotoxins & virotoxins are present in Amanita phalloides, but do not produce symptoms in humans. (36)

In 1966, Tyler in Washington found that the amount of beta-amanitin was generally higher in our specimens than in those from central Europe where the amounts of α- and β-amanitins are about equal. (37) This geographical difference was confirmed in 1974 by Dr. Udo Boerner studying over 100 specimens of Amanita phalloides collected by the MSSF. (38) Similar chemical differences have been noted for many geographical “strains” of fungi and it is not clear how much is directly environmental and how much is a genetic adaptation to different conditions including available mycorrhizal hosts. Some fruiting bodies, even from what is presumably the same mycelial mass, contain no or little toxin.

The amount of amanitins in western species averaged 2.3 mg/gram dry weight for Amanita phalloides. Amanita ocreata is thought to contain about the same amount of amatoxins as eastern amatoxic species, such as Amanita verna containing 1.5 mg/gram or less. (32,37,39) Amatoxin amounts for Galerina autumnalis averaged 1.5 mg/gram and Galerina marginata 0.45 mg/gram. (32,40) These two species are now considered conspecific and the correct name is Galerina marginata. Since the toxin was given per dry weight, it is surprising that there was a difference in amatoxin amount for the two presumed species. Possibilities include chemical races and age differences between the specimens.