This two-part column explains the evolution of lipids analysis, current approaches such as targeted and untargeted lipidomics, and the variety of techniques involved, including sample preparation, separations, and, of course, mass spectrometry.
The study of lipid biology is undergoing a remarkable, technology-driven transformation that most notably involves mass spectrometry (MS) and its ancillary techniques, such as liquid chromatography (LC) and ionization sources. Greater investment inflows and a surge of activity, especially in the field of drug and biomarker discovery, is fueling the growth of global lipid analysis — lipidomics — making it a standard research tool in academic, pharmaceutical, and biotechnology sectors. Additional areas of interest in lipidomics include plant, microbial, and nutritional research.
Why Lipids?Lipids are absolutely essential for life, playing diverse and important roles in nutrition and health. Alterations in lipid metabolites are associated with various human diseases including obesity, heart disease, and diabetes mellitus (1,2). The ability to profile the lipid composition of biological samples is important in disease diagnosis and drug discovery, attracting strong social and economic interests. For example, the discovery that cholesterol and triglycerides are linked to heart disease affected clinical testing, as well as drug, food, and lifestyle enterprises. Indeed, for the last 50 years, we experienced a lipid phobia, the emblematic manifestations of which are readily apparent in the many food labels advertising "low fat" and "low cholesterol." Cholesterol-lowering agents, such as statins like atorvastatin (Lipitor, from Pfizer, New York, New York), are among the most successful drug classes, accounting for $21.5 billion a year market in the United States. Other commonplace drugs, such as nonsteroidal anti-inflammatory drugs including acetylsalicylic acid (aspirin) and celecoxib (Celebrex, Pfizer), are also directed against lipid-metabolizing enzymes. However, not all lipids are bad for us. Actually, some lipids promote health, such as omega-3 fatty acids and vitamins A, D, and E, and those benefits explain their exponential growth as food supplements and nutraceuticals.
New research in lipid biology suggests that, especially in the areas of drug discovery and disease diagnostics, we may have formerly adopted a myopic view of lipid metabolism. For years, biomedical research focused on analyzing only a handful of lipids. From the extrapolated results of those narrowly focused analyses, we sought to understand more than we could reasonably expect to know: the multifaceted mechanisms of action and the effects of drugs, the causes of complex diseases, and the intricate biological alterations associated with those diseases.
Types of Lipids
Lipids constitute one of the largest classes of biological macromolecules. Together with nucleic acids, proteins, and carbohydrates, they are present in living organisms that span the spectrum of biological complexity: animals, plants, fungi, protists, bacteria, archaea, and viruses. Chemically, lipids are hydrophobic or amphipathic small molecules (<1500 Da) of biosynthetic origin, which can be counted on the order of tens of thousands. Lipids have enormously diverse chemical structures (Figure 1) and are classified into eight main categories (3):
Each lipid heads its own subclassification hierarchy, according to the classification system proposed by the Lipid Metabolites and Pathways Strategy consortium (LIPID MAPS, La Jolla, California; http://www.lipidmaps.org/).
Several websites provide useful overviews of lipid structure and function, as well as analytical procedures for lipid analysis: Lipid Library (lipidlibrary.co.uk/), Lipid Bank (lipidbank.jp), and the Cyberlipid Center ( http://www.cyberlipid.org/).
Parallel to the development of new technologies is how our understanding of the biological role of lipids has changed with time. Lipids were known to serve as the structural backbone of cell membranes and as storage for metabolic energy. Recently, the biomedical community learned that lipids play pivotal roles in regulating a wide variety of cellular processes in all organisms. Within each cell exist thousands of types of lipids whose composition, or lipid profile, changes in response to chemical signals from the cell's environment. Studying lipid profiles can provide insight to certain health and disease processes.
Comprehensive analysis of a wide array of lipids in biological samples is a challenge primarily for analytical chemistry. These complex mixtures of lipids have a large variety of chemical structures and a large, dynamic range of concentrations. Consequently, interest in adapting novel technologies for lipids analysis continues undiminished.