An investigation into protein modification by short-chain lipid oxidation products:development of mass spectrometry-based approaches for their detection and assessment of the effect on cellular proteins structure and activity

Abstract

The oxidative modification of lipids containing polyunsaturated fatty acids results in a wide diversity of reactive products, including short-chain aldehydes, which can covalently modify proteins, a process called lipoxidation, and affect their function. The detection of lipoxidation adducts is extremely challenging due to their low abundance, so there is a need for new detection methods. Using the model proteins lysozyme and human serum albumin, five reporter ions for acrolein modification, eight for 4-hydroxyhexenal and one for 2-chlorohexadecanal were found by LC-MS/MS-based label-free method. Subsequently, a targeted multiple reaction monitoring method was developed as a potential tool for the identification and characterization of these modifications of human serum albumin in biological samples. Comparison between three different chromatographic methods for the separation of intact proteins unmodified or modified by short-chain aldehydes demonstrated that anion exchange chromatography was the best method for protein isoform separation while reverse phase chromatography was the best for the separation of lipoxidized proteins. To understand the cellular effects of lipoxidation by small aldehydes, two cellular proteins were studied. The Cys328-mediated effect of short-chain aldehydes on vimentin organization was evaluated by confocal microscopy, which showed that aldehydes caused vimentin aggregation around the nucleus, and Cys328 was important in both assembly of the filament network and as a target for lipoxidation. The effect of short-chain aldehydes on the glycolytic enzyme pyruvate kinase was also studied, and this protein was found to be highly susceptible to modification; under pathophysiological concentrations acrolein, malondialdehyde and 4-hydroxyhexenal were able to inhibit its activity in a time- and dose-dependent manner. Similar inhibition was observed in breast cancer cells, altering cellular metabolism and suggesting that these aldehydes may contribute to mechanisms of tumorigenesis. Overall, this work has improved the analytical tools for detecting protein lipoxidation, as well as understanding of its functional effects.