Wood - Lipidomics

Swansea University Medical School, Singleton Park, Swansea, SA2 8PP, UK

William J. Griffiths

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Lipidomics can be defined as the quantitative identification of all lipids in an organism, tissue, fluid, or cell type. Lipids themselves are defined as hydrophobic or amphipathic small molecules that may originate entirely or in part by carbanion-based condensations of thioesters (e.g., fatty acyls) and/or by carbocation-based condensations of isoprene units (e.g., prenols, sterols) [].

MS gives molecular weight information via measurement of the mass-to-charge ratio ( m/z ) of ionized species and many modern MS instruments are capable of providing mass accuracy at a level of 0.0010.002 m/z . This degree of mass accuracy allows m/z searches against, e.g., the Lipid Maps database of compounds ].

MS not only provides information for lipid identification, it can also be used for lipid quantification. Ideally, quantification is performed with the use of isotope-labeled standards added during sample preparation, or if these are not available via structurally related analogues. Avanti Polar Lipids http://avantilipids.com/ provide a large number of high-quality authentic standards imperative for accurate quantification.

There are essentially two strategies in lipidomics analysis. There is the global shotgun approach developed by Han and colleagues in the USA [].

The term shotgun lipidomics was coined by Han and Gross and the methodology was excellently reviewed in their 2005 publication []. The scanning protocols used depend on the mass spectrometer available, but the goal is always to identify and quantify as many lipid molecular species as possible.

An alternative to the shotgun lipidomic approach is provided by targeted lipidomics, where each class of lipid is analyzed one by one, exploiting extraction and MS procedures specifically designed for the target class of analyte. This approach is the one adopted by the Lipid Maps consortium in the US ].

Modern mass spectrometry imaging (MSI) has been pioneered by Richard Caprioli and colleagues in Vanderbilt University [].

Surface analysis can also be achieved using ESI via the technique desorption electrospray ionization (DESI) developed by Graham Cooks and colleagues in Purdue []. DESI generally provides lower lateral resolution than MALDI-MSI but avoids the necessity for matrix application.

Lipidomics has progressed over the last decade to a mature technology []. There are still major challenges ahead, e.g., in the localization of lipids to defined spatial regions, the observation of their temporal changes, and the analysis of lipids that may be minor in most situations and only become abundant after appropriate stimulation. There are also dangers for lipid analysis using an omics approach. The availability of metabolite and lipid databases has led to a careless use of the term identification. To an analytical chemist a compound is only identified when its chemical and physical properties exactly match those of an authentic standard. When only a single property, e.g., exact mass, is matched to a structure in a database, the term annotate is more appropriate. Care must also be taken when performing quantification. The gold standard for quantification is isotope dilution MS where an isotope labeled standard is added during analyte extraction. This may not always be possible in which case quantification is best performed against structural analogues. Finally, with respect to quantification, the analyst should be aware of the precision of their method by analysis of repeated quality control samples before claiming changes in analyte concentrations are a consequence of a biological pertebation. With these caveats in mind the future is bright.