Toxic or beneficial? Selenoneine in human selenium metabolism

Selenium supplementation is widely recommended for a myriad of health benefits, even though seleniums therapeutic window is narrow and it is unclear which selenium compounds are responsible for the beneficial effects. Mixed outcomes from supplementation trials underline the importance of knowing the form of selenium ingested in order to fully understand its metabolism and any ensuing beneficial or toxic effects. Selenoneine, the selenium analogue of the sulfur-containing compound ergothioneine, a ubiquitous cellular compound with putative antioxidant properties, has recently been discovered as a major selenium compound in marine fish. This joint project between the Institute of Chemistry at the University of Graz and the Institute of Nutritional Science at the University of Potsdam aims to investigate whether selenoneine and its human metabolite Se-methylselenoneine are important compounds in human selenium metabolism, with a significant role in seleniums beneficial health effects. Since neither compound is commercially available, they will be chemically synthesized according to procedures modified from those used to prepare their sulfur analogues. Chemical synthesis will establish a convenient source of the pure compounds in the quantities necessary to delineate their role in human selenium metabolism. The synthesized pure compounds will be used to develop a robust high performance liquid chromatography inductively coupled plasma mass spectrometry (HPLC/ICPMS) method for their quantitative determination in complex matrices such as human blood and urine. The HPLC separation must also be compatible with electrospray mass spectrometry to unequivocally prove the presence of selenoneine and Se-methylselenoneine in various matrices. Since selenoneine is easily oxidized, special emphasis will be placed on developing a fast and quantitative reduction/derivatization sample preparation procedure. The synthesized compounds will also be used to elucidate the bioavailability and metabolism as well as the toxicity and potential protective properties of selenoneine and Se-methylselenoneine by performing in vitro studies with mammalian cell cultures (human Caco-2 intestinal cells, human HEPG2 liver cells, LUHMES human neuronal precursor cells, porcine brain capillary endothelial cells and Plexus-Choroideus cells) and first in vivo studies with the nematode C. elegans. At the end of the cell tests, total selenium and selenium metabolites will be determined in cells, C. elegans and cell culture media implementing the developed HPLC/ICPMS method. Results from this project will provide important insights into the potential protective properties, toxicity and human metabolism of these novel forms of the essential trace element selenium, and help to shed light on their contribution to seleniums ascribed health benefits.

The essential trace element selenium and its species are highly health-relevant and the investigation of small, non-protein bound selenium species that occur naturally in many foods regarding their action and metabolism in the mammalian body is, therefore, of great importance. In the focus of this project was selenoneine, the selenium analogue of the naturally occurring sulfur species ergothioneine, which has been reported to act as an anti-oxidant. As selenoneine was quickly ascribed similar properties, research into its health aspects is necessary, but has been hampered by the non-availability of the pure species necessary for such investigations. Based on these aspects, the project aimed to synthesize quantities of pure selenoneine, to develop robust quantitative analytical methods for the determination of the species and its metabolites in complex biological matrices and to elucidate its metabolism, toxicity, and potential protective properties by in vitro studies in mammalian cell cultures and the in vivo model Caenorhabditis elegans. As chemical synthesis did not prove viable within the first year of the project, isolation from genetically modified fission yeast was investigated an alternative approach. Isolation of selenoneine was successfully achieved and provided for the first time selenoneine in a quantity and with a purity, which was sufficient to investigate its potential toxicological and protective effects. To assess the concentration of selenoneine as well as its metabolites in comparison with other small selenium species and ergothioneine in the toxicological tests, and to determine these potentially health-relevant species in further complex biological matrices like human body fluids, robust analytical methods were developed. Special emphasis was put on the development of a novel method, which allows the simultaneous determination of selenium and sulfur species in complex sample matrices like human blood cells. In vitro and in vivo experiments were performed to characterize selenoneine and to compare it to several other small selenium species and ergothioneine in terms of biological effects as well as their transport in models of the intestinal barrier and blood brain barrier. These tests provided new insights into potential protective effects and transport mechanisms of selenoneine and clearly demonstrated methylation of selenoneine to Se-methylselenoneine during its transport via the intestinal barrier in an in vitro model. Furthermore, novel metabolites of the reference selenium species Se-methylselenocysteine and selenomethionine were identified in human liver cells and C. elegans, respectively. The isolation of selenoneine from fission yeast has clearly advanced the toxicological, protective and metabolic characterization of this potentially health relevant selenium species. The analytical methods developed are expected to be used to address scientific questions far beyond the scope of this project in the future.