Mechanisms of Selenium-dependent antioxidant properties in milk
Special Symposium - Innovations in Food Technology (LMC Congress)
Innovations in Food Technology - Poster Session (LMC/Food - P1)
Keywords: selenium, oxidation
Selenium is an essential trace element for humans and animals since it is necessary for a narrow range of critical enzymes and thus plays a significant role in the immune system.
Se is specifically incorporated as selenocysteine into glutathione peroxidase (a major antioxidant enzyme of cells) and other Se-enzymes. Selenomethionine (Se-Met), in contrast, is incorporated non-specifically into proteins in place of its sulphur analogue, methionine.
Thus Se-Met may contribute distinct redox chemistry to proteins, since it is more easily oxidized compared to its sulphur analogue, and this may be important for the oxidative stability of milk.
Preliminary studies have indicated that oxidation is altered in milk from selenium-supplemented cows. We have therefore initiated a supplementation experiment, where cows are supplemented selenomethionine (as Se-rich yeast), and our research focuses on describing the Se-related antioxidant mechanisms in milk.
It is well known that milk proteins exert antioxidant properties, and we want to investigate the effects of the non-specific incorporation of selenomethionine into proteins on their antioxidant properties. This requires fractionation of milk into individual components and therefore we are combining size exclusion chromatography and other chromatographic separations with different functional antioxidant assays.
Addition of peroxides to milk accelerates oxidation of milk proteins, due to endogenous milk lactoperoxidase that catalyses the reduction of peroxide using available electron donors, such as protein bound tyrosine. Oxidized tyrosine, in the form of tyrosyl radicals form the stable oxidation product dityrosine which can be measured by amino acid analysis and fluorescence detection.
We hypothesize that incorporation of selenomethionine improve redox-properties of milk and lower dityrosine formation, either through a simple antioxidant mechanism or catalytically through recycling with low molecular weight antioxidants (ascorbate, vitamin E, thiol compounds, urate).
In order to pursue this we are currently conducting kinetic studies of the lactoperoxidase catalyzed formation of dityrosine and the effects of selenomethionine on this reaction. In addition we want to evaluate the ability of milk proteins from both selenium and control milk to inhibit dityrosine formation.
Metals in trace amounts (especially Cu and Fe) and exposure to UV-radiation result in the formation of reactive oxygen species in milk. This induces oxidation of un-saturated lipids and proteins, leading to lipid- and protein-derived off-flavours and protein cross-linking. The effects of selenium supplementation on these reactions are evaluated using head space solid phase micro extraction gas chromatography (SPME-GC MS). Both lipid and protein oxidation products are quantified in the same run, allowing us to follow the time course of oxidation in the two milk types and simultaneously identify the volatiles by mass spectrometry.
This project is funded by LMC FOOD Graduate School, University of Copenhagen and Alltech Biotechnology Inc, Nicholasville, KY
Presented Wednesday 19, 13:30 to 15:00, in session Innovations in Food Technology - Poster Session (LMC/Food - P1).
