Abstract
Recent developments in MALDI have enabled direct detection of lipids as intact molecular species present within cellular membranes. Abundant lipid-related ions are produced from the direct analysis of thin tissue slices when sequential spectra are acquired across a tissue surface that has been coated with a MALDI matrix. The lipid-derived ions can often be distinguished from other biomolecules because of the significant mass defect that these ions present due to the large number of covalently bound hydrogen atoms in hydrophobic molecules such as lipids. Collisional activation of the molecular ions can be used to determine the lipid family and often structurally define the molecular species. Specific examples in the detection of phospholipids, sphingolipids, and glycerolipids are presented with images of mouse brain and kidney tissue slices. Regional distribution of many different lipid molecular species and Na+ and K+ attachment ions often define anatomical regions within the tissues.
Highlights
Recent developments in MALDI have enabled direct detection of lipids as intact molecular species present within cellular membranes
While there are powerful techniques based on mass spectrometry that can identify and characterize lipids, mass spectrometric imaging (MSI) is an emerging technology that has surprising power to fill the missing gap in finding the location of specific lipids in animal and plant tissues (Fig. 1)
We found that depositing the matrix onto the tissue by direct sublimation of an organic matrix, such a 2,5-dihydroxybenzoic acid [9], is advantageous over applying the matrix from an organic solution because no solvent, which could dissolve target lipids in the tissue, is employed and the crystal size of the deposited matrix is extremely small
Summary
A critical feature of MSI has been the instruments employed, where the most frequently used mass analyzer has been the time-of-flight (TOF) mass spectrometer. A specific example is the image generated for the ion m/z 796.6, identified as the K1 adduct of 16:0/18:2-PC (Fig. 3B) In this case, it appears that the abundance of this specific ion is relatively higher in the cortex and pelvic regions of the mouse kidney when compared with the medulla. Phospholipid ionization and ion formation are somewhat different in the MALDI-MSI experiment in that abundant alkali metal adduct ions are often observed when compared with the [M1H]1 ions observed in the MALDI of pure lipids This is no doubt due to the local intracellular concentration of Na1 and K1 in the tissue slices. These TAG molecular species appeared to be outside the kidney itself and present as perirenal fat tissue
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