This journal is c The Royal Society of Chemistry 2012 Mol. BioSyst., 2012, 8, 2559–2565 2559 Cite this: Mol. BioSyst., 2012, 8, 2559–2565 Metabolomic analysis of polar metabolites in lipoprotein fractions identifies lipoprotein-specific metabolic profiles and their association with insulin resistancewz Tuulia Hyo¨tyla¨inen,* a Ismo Mattila, a Susanne K. Wiedmer, b Artturi Koivuniemi, a Marja-Riitta Taskinen, c Hannele Yki-Ja¨rvineny cd and Matej Oresˇicˇ y a Received 23rd March 2012, Accepted 11th May 2012 DOI: 10.1039/c2mb25115a While the molecular lipid composition of lipoproteins has been investigated in detail, little is known about associations of small polar metabolites with specific lipoproteins. The aim of the present study was to investigate the profiles of polar metabolites in different lipoprotein fractions, i.e., very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), low-density lipoprotein (LDL) and two sub-fractions of the high-density lipoprotein (HDL). The VLDL, IDL, LDL, HDL 2 , and HDL 3 fractions were isolated from serum of sixteen individuals having a broad range of insulin sensitivity and characterized using comprehensive two-dimensional gas chromatography combined with time-of-flight mass spectrometry (GCÂGC-TOFMS). The lipoprotein fractions had clearly different metabolite profiles, which correlated with the particle size and surface charge. Lipoprotein-specific associations of individual metabolites with insulin resistance were identified, particularly in VLDL and IDL fractions, even in the absence of such associations in serum. The results indicate that the polar molecules are strongly attached to the surface of the lipoproteins. Furthermore, strong lipoprotein-specific associations of metabolites with insulin resistance, as compared to their serum profiles, indicate that lipoproteins may be a rich source of tissue-specific metabolic biomarkers. Introduction Lipoproteins are of fundamental importance for the transport of water-insoluble lipids in biological fluids. 1 The function and metabolism of lipoproteins is closely linked to the biophysical properties of their chemical composition and to their size. Structurally, lipoproteins are complex aggregates of lipids and proteins that render the lipids compatible with the aqueous environment of body fluids and enable their transport throughout the body. The surface of the lipoproteins is composed of a monolayer of phospholipids, free cholesterol, and specific proteins, while the core of the particles mainly consists of neutral lipids including triglycerides and cholesterol esters. The lipoprotein surface is polar and negatively charged, mainly due to the presence of negatively charged phosphatidylinositol (PI) and phosphatidylethanolamine (PE) molecules on the surface of lipoproteins and by the conformation of the resident apolipoproteins, which is influenced by the presence of neutral lipids and also by the particle shape. 2,3 Most studies aimed at molecular characterization of lipo- proteins have been focused on the major lipid classes such as cholesterol, cholesteryl esters, triglycerides (TG) as well as the main glycerophospholipids such as phosphatidylcholines (PCs) and sphingolipids. 4,5 These studies have shown that TGs, PCs, lysophosphatidylcholines (lysoPC), and ether-linked PCs are present in all lipoproteins but at varying ratios and concentrations. 5 Some lipids, e.g. ceramides, have been identified only in VLDL and LDL, whereas ethanolamine plasmalogens have predominantly been found in LDL and HDL 2 . In the aqueous environment polar compounds can be attached by non-covalent interactions to the negative surface of the lipo- proteins. It is likely that this interaction is reciprocal, so that metabolites also modify the surface of the particles. The associa- tions of metabolites with lipoproteins may be physiologically a VTT Technical Research Centre of Finland, Tietotie 2, P.O. Box 1000, FIN-02044 VTT, Espoo, Finland. E-mail: tuulia.hyotylainen@vtt.fi; Fax: +358-20-722-7071; Tel: +358-20-722-6248 b Laboratory of Analytical Chemistry, Department of Chemistry, University of Helsinki, A.I. Virtasen aukio 1, P.O. Box 55, FIN-00014 Helsinki, Finland c Department of Medicine, University of Helsinki, Tukholmankatu 8B, P.O. Box 20, FIN-00014 Helsinki, Finland d Minerva Foundation Institute for Medical Research, Tukholmankatu 8, FIN-00290, Helsinki, Finland w Published as part of a themed issue dedicated to Emerging Investigators. z Electronic supplementary information (ESI) available. See DOI: 10.1039/c2mb25115a y Equally contributed. Molecular BioSystems Dynamic Article Links www.rsc.org/molecularbiosystems PAPER Downloaded by University of Helsinki on 29 November 2012 Published on 11 May 2012 on http://pubs.rsc.org | doi:10.1039/C2MB25115A View Article Online / Journal Homepage / Table of Contents for this issue