The Fungus and Its Secondary Metabolites
The fruiting bodies of the brown rot fungus Gloeophyllum odovatum (Wulf. ex Fr.) Imaz. syn. Trametes, odorata (Wulf. ex Fr.) Osmoporus odoratus (Wulf. ex Fr.) (Aphyllophorales, Basidiomycetes) are found in coniferous forests, chiefly in northern and rocky mountains in central Europe, in Asia, and occasionally in North America. In Fennoscandia, the fungus grows mostly on old stumps of the Norway spruce [Picea abies (L.) Karst.], very rarely on pine (Pinus sylvestris L.). The perennial brown fruit bodies are knotty, wedge- or plate-like medium-sized or large. The young parts are ochraceous to light brown in color, later becoming dark brown to almost black or blackish gray. G. odovatum is not very common. The other known Gloeophyllum species are G. protactum, G. sepiarium, G. abietinum and G. trabeum. Only the fresh fruit bodies of G. odoratum produce a strong scent of aniseed, when it grows on spruce.
The sporophore of the fungus is primarily interesting because of its volatiles; however, they also contain steroids. The principal volatiles from the fruiting body grown on spruce have been identified as aromatics, i.e., methyl p-methoxyphenylacetate (33.5%) accompanied by ethyl p-ethoxyphenylacetate, a mushroom alcohol l-octen-3-ol, and a biscyclofarnesol drimenol. Linalool, the common monoterpene alcohol, in the essential oil of higher plants, occurs as a minor compound. The amount of fatty acids is as high as 30.1% of the total oil constituents. Drimenol has also been identified from other wood-rotting fungi, i.e., Lentinus lepideus and milk caps, Lactarius uvidus. Generally, drimane-type skeleton sesquiterpenes are characteristic of bryophytes, but they have also been identified in several plants, viz. from the bark of Drimys winteri and from the genera of Polygonum and Warburgia ().
The triterpene acids in the lanosterol group are characteristic metabolites in a certain group of wood-rotting fungi that causes brown rot, with some exceptions. Trametenolic acid has been identified in the following genera in Basidiomycetes, i.e., Trametes, Inonotus, Lenzites, Polyporus, Heterobasidion, Dadalea, Fomex, and Phellinus (). Of these genera, Phellinus and Inonotus are white rot fungi. Eburicoic acid, C-24 methylene derivative of trametenolic acid, has been previously identified also in some Gloeophyllum species apart from G. odoratum and from a number of other genera, e.g., Polyporus, Lenzites and Fomes (). G. odoratum also contains mycosterols; ergosterol, ergosta-7,22-diene, fungisterol, and ergosterol peroxide. Ergosterol is thought to be the most common sterol in Basidiomycetes, but in wood-rotting fungi, ergosta-7,22-dien-3β-ol has often proved to be the principal sterol, accompanying ergosterol and fungisterol.
Many metabolites, sterols, triterpenes, sesquiterpenes, monoterpenes, and poly-saccharides in Basidiomycetes have been found to possess antibiotic, cyto-static, or immunostimulatory activity, etc.. Of these, trametenolic acid had proved to be only slightly active in the antitumor tests against MCF-7 adenocarcinoma and Walker-256 carcinosarcoma in vitro, but the activity of trametenolic acid was increased after methylation of the C-21 COOH group, killing as many as 95% of the cells after the growth period.
Ergosterol peroxide is active against various cancer cells (HTC, ZHC, 3T3), MCF-7 adenocarcinoma, and Walker-256 carcinosarcoma, in vitro. The active fractions of ergosterol and ergosterol peroxide from the mushroom Hypsizigus marmoreus inhibited TPA-induced inflammatory ear odema. Also, the fractions which contained these sterols showed inhibitory activity against tumor promotion by TPA. In addition, ergosterol peroxide has also inhibited certain immunological reactions and in vitro the influenza A- and B-viruses, and is assumed to be also responsible for the antiallergic effect of Tricholoma populinum ().
Several metabolites other than sterols, viz. the hydrodistilled volatiles of G. odoratum, may also possess cytotoxic activity. According to tests on Artemia salina and Agrobacterium tumefaciens, some fungal constituents may be responsible for the biological activity in this oil. The tests have shown that the activity of the total volatile oil was of the same range as the activity of certain pure components which were used in the tests. Pure drimenol was not tested, but according to Fukuyama et al., several related drimane-type sesquiterpenes have shown potent antifeedant, antimicrobial, plant growth-inhibitory, cytotoxic, and piscisidal activities. The folk medicinal herb, Polygonum hydropiper, which is rich in drimane-type sesquiterpenoids, is used against cancer. G. odoratum is not known as a folk medicine, whereas another Gloeophyllum species, G. separium, has been reported to possess antifungal activity. G. separium contains an antifungistatic antibiotic, oospolacton.
Conclusions and Future Prospects
It was expected that the strain (1-89) of Gloeophyllum odoratum isolated from the fruiting body grown under cold climate conditions would produce different volatiles than mycelia grown in different laboratories. The results in this study have shown no remarkable differences in oil composition within the same strain. However, some additional substances and different culture media affected the production of metabolites at least quantitatively. Phenylalanine, especially, proved to be a remarkable elicitor of the production of different aromatic volatiles. The appearance of citronellol, drimenol, and aromatic benzoyl derivatives was most sensitive to elicitor treatment, especially in the case of chitosan, chitin, and phenylalanine media. In contrast, the amount of linalool, geraniol, and 3,7-dimethyl-3-hydroxy-6-octenicacid methylester proved to be quite stable. Linalool was the characteristic major volatile in the strain of (1-89). These results may also have some chemotaxonomical value.
The significance of the fungal oil from G. odoratum in medicine, phytotherapy, or in flavoring compounds is not known. In perfumery, for example, a fungal metabolite, citronellol, with its rosaceous odor, is widely used. It is produced by the catalytic hydrogenation of citronellol oil, converting cironellal into citronellol. Optically active citronellols are also of interest for their further chemical reactions. Photooxidation, reduction, and cyclization result in ( + )- or ( — )-rose oxides, which are highly priced perfumery specialities, and their odor quality is superior to that of the racemic and less expensive material. Geraniol, linalool, and citronellol and other closely related compounds are also major components, e.g., in the medicinal plant Melissa officinalis, which have many pharmacological effects. The individual components geraniol, linalool, and citronellol have been reported to possess antibacterial, antiviral, and antispasmodic effects. Geraniol also possesses expec-torical and mycolytic effects. Geraniol, citronellol, and linalool were minor compounds in the wild G. odoratum but under in vitro conditions, these compounds could be transformed into main components.
It would be worthwhile to produce the mycosterols and lanosterol-type triterpenes in large amounts in vitro in order to investigate more exactly their structures and biological activities. The biological activity of some mycosterols has been tested, but fungal triterpenes are also interesting.
Selections from the book: “Medicinal and Aromatic Plants IX” (1996).