Cell membranes are composed of phospholipids containing polyunsaturated fatty acids. Due to the presence of double bonds, these are highly susceptible to lipid peroxidation, which occurs through radical formation. The consequence of lipid peroxidation is lipid oxidation and subsequent damage to the cell membrane. Lipid oxidation can be effectively slowed down or even prevented by adding an appropriate antioxidant. Cell membranes are structurally very similar to liposomes, making liposomes a good model for evaluating the antioxidant protection of lipids with selected antioxidants. Bilirubin and its precursor biliverdin are endogenous antioxidants that are formed during the breakdown of heme. Bilirubin plays an important role in protecting the kidneys, heart, and brain from oxidative damage in the body, but its presence in excessively high concentrations can lead to various disease states, a symptom of which is the yellowish discoloration of the skin and mucous membranes—jaundice.
As part of the experimental part of the master's thesis, we manufactured liposomes from the selected phospholipid 1,2-didocosahexaenoyl-sn-glycero-3-phosphocholine using the film method, with varying proportions of bilirubin and biliverdin added or without any antioxidant. We exposed the liposomes to oxygen and evaluated the concentration of remaining lipid at various time points. Quantitative analysis was performed using ultra-high performance liquid chromatography with a charged aerosol detector. We demonstrated that the lipid, without the presence of an antioxidant, oxidizes very quickly in an oxygen atmosphere. The results of our experiments, where we varied the proportions of bilirubin and biliverdin in liposome samples, confirmed that the effectiveness of antioxidant protection depends on the identity and concentration of the antioxidant: biliverdin demonstrated the ability to protect lipids from oxidation at lower concentrations compared to bilirubin.
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