3.8.1 Mushrooms Producing Muscarine
Muscarine is found in mushrooms of the genera Inocybe and Clitocybe (I. patouillardi, I. fastigiata, I. geophylla, and C. dealbata), in which it accounts for 1 to 3 mg/g of dry weight. Both genera occur commonly and have a worldwide distribution. While Inocybe mushrooms are mycorrhizal on conifers or broad-leafed trees, Clitocybe mushrooms are saprophytic and grow on forest litter or grassland humus. Confusion with edible mushrooms most commonly occurs with the species growing on grassland.
Muscarine is found in tiny amounts, usually under 20 mg/g, in other agaric genera, too, such as Amanita, Boletus, Hygrocybe, Lactarius, Mycena, and Russula. It was by the small amounts of muscarine present in A. muscaria, 90 mg/g, that the toxin was detected and identified (and got its name); but muscarine in A. muscaria usually does not account for the symptoms that occur after ingestion of this fungus, which are produced by muscimol and its precursor ibotenic acid.
Muscarine is a tetrahydrofuran derivative with the structure shown in Figure 3.7a. Because of the three chirality centers present in the molecule, muscarine exists in eight isomers, of which only one, L-(+)-muscarine, is active. The remaining isomers also have been detected in toxic fungi, but because of their low biological activity and low concentration they do not contribute to toxicity.
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