Effect of selenium (Se(IV)) on the protein profile and ethanol production in Saccharomyces cerevisiae cultures – a proteomic study using 15N metabolic labeling

The species-dependent benefic effects of selenium have been demonstrated in a variety of organisms, and its role in human health and disease has been studied extensively. S. cerevisiae, biofortified with Se is the most common and effective form of this element [1, 2]. Although this microorganism is not strictly accepted as a probiotic, it is used to make functional foods, specifically to produce Se-enriched dairy products. To understand the mechanisms for the health-promoting effects of selenized yeast and for those involved in the production of selenized food, it is necessary to investigate the impact of Se on the growth and metabolism of yeasts. In this regard, the selenium pathway is well documented [3] and the metabolic response under selenium exposure has been studied [4-6]. However, less effort has been devoted to unraveling the effects of Se at the proteome level.

One promising strategy in comparative proteomics relies on the metabolic labeling of target microorganisms with heavy nitrogen during their growth. Thanks to the isotopic capability of mass spectrometry, samples from two different treatments and labeled differentially can be combined and analyzed in a single run, thereby increasing the reliability, and facilitating comparative assessment. In practice, inverse metabolic labeling is often performed by preparing two sets of biological samples; in one of them 15N is applied to the control culture whereas in another set, 15N is introduced into the exposed organisms. In this way, the interpretation of mass spectrometry data becomes less error-prone [7-10].

This work aimed to perform 15N - metabolic labeling of baker´s yeasts to ensure a high-fidelity assessment of Se(IV) impact at the proteome level. For this purpose, nitrogen-containing salt was added to the growth medium as a sole nitrogen source, using 15N salt for yeast exposed to Se and a salt with natural 14N isotope composition for non-exposed controls (and inversely). Protein extracts from differentially labeled Se-exposed and non-exposed control cultures, were combined, and prepared for peptide analysis by capillary liquid chromatography-electrospray ionization - mass spectrometry with quadrupole - time of flight mass analyzer (HPLC-ESI-QTOF-MS). The incorporation of selenium into three proteins participating in glycolytic processes and the increased abundance of these proteins in the presence of Se were found. The results obtained suggested that selenized yeasts may have better fermentation capacity compared to non-exposed organisms. The increase in ethanol production by selenized yeasts was confirmed by analyzing the respective spent media using gas chromatography with flame ionization detection.