Background

Mushrooms in the genus Agaricus have a worldwide distribution, with up to 90 species recorded in Europe. The genus includes the most economically important and commercially cultivated mushroom in the world, A. bisporus (button mushroom) as well as many other edible species [1]. Some Agaricus species are inedible, including A. placomyces and A. pseudopratensis Bohus, which are similar in appearance to certain edible species, yet are known to possess unpleasant odours and result in gastrointestinal problems if consumed [2, 3]. To the best of our knowledge, there is no information available in the literature concerning the chemical composition of these two species.

In this article, we report the results of the GC-MS analyses of volatile and polar compounds, in addition to the sterol fraction obtained from the fruiting bodies of A. placomyces and A. pseudopratensis. The results can help characterise the species investigated, indicate the presence of some biologically active compounds and shed light upon their reported mild toxicity.

Results and discussion

The volatile fractions were obtained from the fresh mushrooms' fruiting bodies and analysed as described in the experimental section. The results obtained are outlined in Table 1. The most abundant constituent of the volatiles in both species was phenol: over 40% in A. placomyces and over 80% in A. pseudopratensis. Besides phenol, the main groups of compounds in the volatile fractions were fatty acids and their esters. Because the extraction was performed using ethanol, it is possible that the ethyl esters are artefacts. Our findings offer the first analytical proof of the presence of phenol in these two species.

Table 1 Volatile constituents of A. placomyces and A. pseudopratensis (% of the total ion current, GC-MS)*
Full size table

The phenol levels detected are not hugely surprising, given the numerous reports of the mushrooms' phenol-like odour, in addition to their belonging to the Agaricus section, Xanthodermatei, which is characterised by this typical unpleasant odour. It has been suggested that the production of phenol originates from an evolutionary ancestral biochemical shift, also demonstrated by the Agaricus species of the section Xanthodermatei in defence mechanisms [4]. According to Del Signore et al. [5] phenolic substances in mushrooms may be involved in a chemical defence mechanism against insects and micro-organisms, very similar to those described for certain plants (e.g. Salicaceae). Recently, it has been suggested that species with higher evolutionary positions in the section Xanthodermatei demonstrate higher phenol contents [4]. For this reason it could therefore be surmised that A. pseudopratensis is a more recently differentiated species than A. placomyces. However, this is only a preliminary hypothesis that will require further more detailed study.

The polar fractions obtained from the fungal species investigated were analysed by GC-MS after silylation. The derivatisation was necessary in order to increase the volatility of the polar fraction, thereby enabling its analysis by gas chromatography. The results obtained are outlined in Table 2. The presence of phenolics is clearly evident, as is the higher amount of phenol observed in A. pseudopratensis. This finding supports the suggestion of the defensive role of phenolics in section Xanthodermatei [6]. Another characteristic was the presence of the aromatic amino acids tryptophan and tyrosin, but only in A. pseudopratensis.

Table 2 Constituents of the polar fraction of A. placomyces and A. pseudopratensis (% of the total ion current, GC-MS)
Full size table

Sugar alcohols are amongst the major soluble carbohydrates found in fungi [7]. In the butanol fractions of both species we detected a sugar content of around 30%: glucitol in A. placomyces and a mixture of glucitol and mannitol in A. pseudopratensis. The most abundant constituent of the polar extracts was urea. This finding once again confirms that higher fungi of the family Agaricaceae accumulate substantial amounts of urea in their fruiting bodies [8]. It has been suggested that urea acts as an osmotically favourable nitrogen reserve for fungi [9].

We also investigated the sterol composition of the two Agaricus species. As expected the predominant sterol in A. placomyces, was ergosterol, although two others were detected, each possessing a Δ7 double bond. In A. pseudopratensis only the latter two sterols were present in substantial quantities, with ergosterol detected in trace amounts.

Using column and preparative thin-layer chromatography, a crystalline compound was isolated from the extracts of both species and identified as 5α,8α-epidioxi-24(ξ)-methylcholesta-6,22-diene-3β-ol through comparison of its spectral data (MS, 1H- and 13C-NMR) with those reported in the literature [10]. The compound was first isolated from these species and might be regarded as an artefact produced from the oxidation of the corresponding Δ5,7 sterol. It is noteworthy that this type of compound has recently been shown to express inhibitory behaviour against the HTLV-1 virus, in addition to demonstrating cytotoxic activity against human breast cancer cell line (MCF7WT) [10].

Conclusion

The results obtained add to the knowledge on the chemical composition of mushrooms belonging to the genus Agaricus, and help provide further explanation for their reported mild toxicity, which we attribute to their high phenol content similar to that of other Xanthodermatei species [11].

Experimental

Collection of the samples

The macromycetes (age 3 – 5 days), were collected near Plovdiv, Bulgaria, in October 2005.

Extraction

The fresh fruiting bodies of macromycetes (A. pseudopratensis – 105 g; A. placomyces – 245 g) were cut into small pieces and consecutively extracted with ethanol, ethanol-chloroform (1:1) and chloroform. The extracts were combined, water was added and the chloroform layers were removed and evaporated in vacuo to yield 1.88 g of dry residue for A. placomyces and 0.6 g for A. pseudopratensis.

Isolation and analysis of volatile compounds

A portion of the chloroform extract (about 300 mg) was subjected to a four-hour distillation-extraction on Lickens-Nickerson apparatus [12]. The volatile compounds were extracted from the distillate with diethyl ether (yields of volatiles in Table 1), and analysed by GC/MS on a GC Hewlett Packard 6890 + MS 5973 (Hewlett Packard, Palo Alto, California, USA), with a HP5-MS capillary column (30 m × 0.25 mm, 0.25 μm film thickness, Agilent Technologies, Wilmington, Delaware, USA). The ion source was set at 250°C and the ionisation voltage at 70 eV. The temperature was programmed from 40 – 280°C at a rate of 6°C min-1, and a helium carrier gas was used used.

Analysis of polar compounds

A portion of the n-butanol extract (5 mg) was dissolved in 50 μL of dry pyridine, before the addition of 75 μL of bis-(trimethylsilyl)-trifluoroacetamide (BSTFA). The mixture was heated at 80°C for 30 min and analyzed by GC/MS. The silylated extract was investigated by GC/MS using the same instrument described above, with a capillary column HP-5 (23 m × 0.2 mm, 0.5 μL film thickness, Agilent Technologies, Wilmington, Delaware, USA). A helium carrier gas was used. The temperature programmed at 100 – 315°C at a rate of 5°C min-1, with a 10 min hold at 315°C.

Identification of compounds

Identification was carried out by searching commercial library databases. Some components remained unidentified, however, owing to both a lack of authentic samples and library spectra of the corresponding compounds.

Isolation and identification of sterols

The chloroform extracts was subjected to column chromatography on silica gel with an n-hexane – acetone gradient (30:1 – 5:1) to produce several fractions. The third set of fractions, after further purification by preparative TLC (silica gel G, n-hexane – acetone 10:1), yielded sterol mixtures: 23 mg from A. placomyes and 7.9 mg from A. pseudopratensis, which were analysed by GC-MS. A gas chromatograph (Hewlett Packard 5890) linked to a mass spectrometer (Hewlett Packard 5972) with a capillary column SPB-50 (30 m × 0.32 mm, 0.25 μm film thickness) was employed. A helium carrier gas was used with a temperature programme set at 270°C – 290°C at a rate of 4°C min-1 with a 20 min hold. The ion source was set at 250°C with the ionisation voltage at 70 eV.

From the fourth set of preparative TLC fractions (silica gel, n-hexane – methyl ethyl ketone 10:1) obtained from both species 5α,8α-epidioxi-24(ξ)-methylcholesta-6,22-diene-3β-ol was isolated (10.4 mg from A. pseudopratensis and 81 mg from A. placomyces) and characterised through comparison of its EIMS, 1H- and 13C-NMR spectra with literature data [10].