My doctoral research examined the behavior of Vitamin E in model bilayer systems. The project was motivated by the recent advisement that Vitamin E supplementation be halted until a better understanding of its basic role is achieved. Besides Vitamin E my research has also examined the behaviour of cholesterol and chlorhexidine in model lipid membranes to understand their in vivo role. Currently I am studying the coupling of leaflet structure in asymmetric lipid bilayers. The central aims of the project are (i) to establish asymmetric lipid vesicles as a robust platform for neutron and x-ray scattering experiments and (ii) to derive an in-depth understanding of the coupling mechanisms in asymmetric lipid-only membranes based on a characterization of their transverse and lateral structure.
Lipid Membrane Asymmetry
Despite the ubiquity of transbilayer asymmetry in natural cell membranes, the vast majority of existing research has utilized chemically well-defined symmetric liposomes (i.e. where the inner and outer bilayer leaflets have the same composition). Currently my efforts are focused on the development of protocols to generate stress-free asymmetric liposomes and the appropriate quantitative assays to characterize them. Project funded through: Natural Science and Engineering Research Council of Canada (NSERC): Discovery Program.
I present evidence of an antioxidant mechanism for vitamin E that correlates strongly with its physical location in a model lipid bilayer. These data address the overlooked problem of the physical distance between the vitamin's reducing hydrogen and lipid acyl chain radicals. Combined data from neutron diffraction, NMR and UV spectroscopy experiments, all suggest that reduction of reactive oxygen species and lipid radicals occurs specifically at the membrane's hydrophobic-hydrophilic interface. The latter is possible when the acyl chain adopts conformations in which they snorkel to the interface from the hydrocarbon matrix. Moreover, not all model lipids are equal in this regard, as indicated by the small differences in the vitamin's location. The present result is a clear example of the importance of lipid diversity in controlling the dynamic structural properties of biological membranes. Importantly, these results suggest that measurements of alpha-tocopherol oxidation kinetics, and its products, should be revisited by taking into consideration the physical properties of the membrane in which the vitamin resides.
Poly Unsaturated Fatty Acids
Phospholipids that contain polyunsaturated fatty acids (PUFAs) are a special species of phospholipids. PUFAs constitute a biologically influential group of molecules essential for normal growth and development.* Recently, phospholipids containing polyunsaturated fatty acids have received increased attention since they are essential for a large number of cellular functions, specifically for proper neuronal cell function,* playing both a structural and functional role within membranes.+ Besides the PUFA chains' high mobility they are also susceptible to reactive oxygen species. PUFAs vulnerability to oxidation creates unique biological problems, both structurally and in terms of biological availability. Structurally, PUFA oxidation products can grossly alter the physical properties of a bilayer. The alteration of a bilayer can ultimately lead to the malfunction of integral proteins. Since PUFAs can only be obtained through dietary consumption, there is no in vivo pathway for their synthesis, thus limiting their availability.* Therefore there must be a mechanism for PUFA preservation. * Kučerka, N., et al.. Biochemistry 49, 7485-7493 (2010). +Catalá, A. Biochimie 94(1), 101-109 (2012).
Catalent is one of the largest suppliers of Vitamin A palmitate capsules to Nutrition International (previously Micronutrient Initiative), which is an organization aimed at eliminating malnutrition throughout the world. Currently, the Vitamin A capsules are stabilized by the addition of vitamin E acetate. However, as this product is often distributed in harsh climates with elevated temperatures and humidity, the shelf-life is reduced compared to storage in North America. It would be a great advantage to both patients and providers if a more stable lipid-based system with a longer shelf life could be developed. This project proposes to develop and implement a novel antioxidant (preservative) mixture to extend the shelf life of Catalent's current vitamin A soft gel supplement sold to Nutrition International. Due to regulations, there is added level of complexity to the project requiring that no synthetic preservatives be used. The Marquardt Group is working to better understand the stability and degradation of the vitamin A soft gel supplement, including the effect of various antioxidants on the degradation. The knowledge gained through this will lead to novel formulations designed to improve the stability of the vitamin A system and thereby improving the lives of people. Project funded through: Natural Science and Engineering Research Council of Canada (NSERC): Engage Program.