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Home ›› About Speciation ›› Speciation News

Discovery of Selenoneine in Boletus edulis: A Potential Terrestrial Source of a Potent Antioxidant

(05.10.2024)

Researchers from the University of Graz found high amounts of the health-relevant antioxidant selenoneine in the edible mushroom Boletus edulis.

Background:

Selenium is essential for humans and other living organisms, making the identification of dietary sources crucial. Selenium has a very narrow range between dietary deficiency (<40μg/day) and toxic levels (>400μg/day) (WHO 1996), necessitating careful control of human intake. Among the various selenium species in food, organic species like selenoamino acids are particularly noteworthy due to their antioxidant properties. Selenoneine, the selenium analogue of the sulphur antioxidant ergothioneine, stands out for its potent antioxidant properties. It interacts with reactive oxygen species and enhances the activity of Glutathione peroxidase 1 (GPx1). Additionally, selenoneine detoxifies mercury by forming a mercury tetraselenoate complex. These health benefits make selenoneine a potential food supplement or chemopreventive agent. Currently, selenoneine is predominantly found in certain types of marine fish.

Figure 1: Structure of selenoneine

Selenoneine, similar to ergothioneine, is a thiol/thione antioxidant synthesized by some fungi, mycobacteria, and cyanobacteria. Due to their chemical similarity, selenoneine and ergothioneine share biosynthetic pathways and are likely to occur in the same biological matrices.

The New Study:

Photo: Boletus edulis
CC BY-SA 3.0

Among various foods, mushrooms exhibit the highest ergothioneine content, with Boletus edulis reported containing substantial amounts as a secondary metabolite. Researchers from the University of Graz hypothesized that B. edulis also contains significant levels of selenoneine. Using HPLC coupled with elemental and molecular mass spectrometry, the researchers investigated the selenium speciation in mushroom samples collected from a forest in Styria, Austria.

Mushroom samples were thoroughly cleaned, cut into pieces, and homogenized with a blender. Total selenium in these samples, determined after microwave-assisted acid digestion, was high (1.06 ± 0.03 mg/kg wet mass). Given that the maximum tolerable daily intake recommended by the European Food Safety Authority (EFSA) is 255 µg Se/day, consuming 250 g of the mushroom would reach this dose. However, since mushrooms are typically consumed in small quantities and not daily, they are not expected to pose a health risk but could serve as a natural selenium source in case of deficiency.

Selenium species were extracted from 1 g of homogenized mushrooms with 10 ml of pure water. Three 5-minute extraction cycles were performed in an ultrasonic bath with intermediate vortexing. The extraction solutions were centrifuged for 10 minutes, and the supernatants were filtered through 0.2 µm filters before HPLC-ICP-MS analysis.

Separation of selenium species was achieved on a reversed-phase HPLC column using 20 mM aqueous ammonium formate adjusted to pH 3.0 with formic acid as the mobile phase. Detection by ICP-MS was performed with the collision/reaction cell operated in H₂ mode for reducing interferences and with 1% (v/v) CO₂ in Ar as the optional plasma gas for enhanced sensitivity.

The presence of selenoneine in the mushroom extract was confirmed by HPLC-ESI-MS. Selenium speciation analysis by HPLC-ICP-MS showed a single dominant peak with a retention time matching that of selenoneine, and a very minor peak eluted at the retention time of Se-methylselenoneine. The sum of these selenium species nearly quantitatively matched the total Se content, indicating that the total selenium content was extracted with water.

The authors concluded that over 80% of the selenium in B. edulis was in the form of selenoneine, making this mushroom a significant terrestrial source of highly beneficial selenoneine. Consuming only 80 g of the mushroom would be sufficient to meet the adequate intake of 70 µg Se/day. However, the authors acknowledge that their sample size was too small to draw general conclusions and recommend further research, including samples from different collection sites.

Comment:

As the authors admit, further studies should be performed related to this mushroom to support the findings of the authors. Prior speciation analysis of the same species by Wilburn et al. (2004) indicated totally different selenium species. Both groups have analysed very few samples ( Peer and Kuehnelt: 2; Wilburn et al. : 1)

Michael Sperling

The original publication

Franziska Peer, Doris Kuehnelt, High levels of the health-relevant antioxidant selenoneine identified in the edible mushroom Boletus edulis, J. Trace Elem. Med. Biol., 86 (2024) 127536. DOI: 10.1016/j.jtemb.2024.127536

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