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Analysis of Vitamin D Metabolites by Mass Spectrometry

Authors: Dietrich A. Volmer, Caroline S. Stokes,

Publish Date: 2016
Volume: , Issue:, Pages: 1-20
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This chapter provides an overview of the state-of-the-art of mass spectral analysis of vitamin D and its metabolites. Most activities in the vitamin D analytical field today center around clinical measurements, and emphasis in the following sections is therefore placed on vitamin D in human samples, mostly from serum or plasma. Virtually all modern mass spectrometry analyses of vitamin D are performed using hyphenated liquid chromatography-tandem mass spectrometry (LC-MS/MS) techniques. Measurements using gas chromatography (GC)-MS are now rarely performed, mainly because of problems with compound degradations during analysis. For those readers interested in GC-MS of vitamin D compounds, the present authors refer to an excellent review by Vouros and coworkers (Gathungu et al. 2013). Food analysis is also not addressed in this chapter, as requirements are quite different, usually focusing only on fortified or unfortified levels of vitamin D2 and D3 in food components (Thomson and Cressey 2014), whereas clinical analyses are mostly concerned with concentrations of vitamin D metabolites.The chapter begins with a short introduction to vitamin D photosynthesis and metabolism in humans and abundances of vitamin D metabolites in various compartments of the body, followed by a discussion of sample matrices for clinical measurements and appropriate sample preparation procedures. Next, liquid chromatography of vitamin D is briefly summarized, leading to mass spectrometry ionization and analyzer techniques. The mass spectrometry section concludes with a discussion on selectivity and accuracy issues, and certified reference materials. Finally, selected clinical applications of vitamin D metabolite analysis are highlighted at the end of the chapter.The 25(OH)D3 metabolite is present in abundant concentrations and can be easily measured in blood samples. Moreover, the 25(OH)D3 metabolite has the longest half-life of all readily measureable metabolites, thus representing the status marker of vitamin D (Holick 2009). 25(OH)D3 is, however, not the most active of vitamin D metabolites, which is produced under renal conversion so as to form 1,25-dihydroxyvitamin D (1,25(OH)2D3), when 25(OH)D3 reaches the kidneys (Fig. 1). This active metabolite exerts its actions via the vitamin D receptor (VDR); however, it is present in much smaller concentrations and has a comparably shorter half-life than 25(OH)D3 (several hours as compared to 3 weeks) (Jones 2008; Nagpal et al. 2005). The smaller concentrations of 1,25(OH)2D3 in blood also pose a challenge in its analytical determination (Volmer et al. 2015).Both 25(OH)D3 and 1,25(OH)2D3 conversion occurs via the vitamin D catabolites, 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) and 1,24,25-trihydroxyvitamin D3 (1,24,25(OH)3D3), respectively. Both catabolites are formed through what is known as the C-24-hydroxylation process and are excreted in bile (Nagpal et al. 2005). The 24,25(OH)2D3 compound can be analytically determined and is present in higher concentrations than 1,25(OH)2D3. Currently, these vitamin D metabolites/catabolites are quantified in blood samples. There are, however, indications that other body tissues, for example, adipose tissue, which also contains VDR (Clemente-Postigo et al. 2015), may prove interesting targets for vitamin D quantification. Adipose tissue is known to sequester and thus store vitamin D (Wortsman et al. 2000), yet recently, the active 1,25(OH)2D3 metabolite has been implicated as having significant effects on adipogenesis and inflammation on a cellular level in adipose tissue (Mutt et al. 2014). Therefore, the extension of vitamin D metabolite quantification beyond that in serum and plasma samples may prove to be biologically significant in the near future.Virtually all published assays for 25(OH)D3 and other vitamin D metabolites measure from serum or plasma, as metabolites circulate in blood. The various mass spectrometric analyses of vitamin D metabolites, which are discussed in the following sections of this chapter, are based on serum or plasma samples.Variations of sample matrix have included dried blood spots (DBS), in particular for samples of infants (Eyles et al. 2009, 2010; Heath et al. 2014; Higashi et al. 2011; Hoeller et al. 2015; Newman et al. 2009). As sampling for DBS on filter paper is minimally invasive, the technique has also been successfully used for vitamin D status screening of adult cohorts, in particular outside hospital environments (“unsupervised sampling”) (Hoeller et al. 2015). In addition, DBS can usually be transported and stored at room temperature over extended periods of time, thus offering significant advantages over venous blood taking and low temperature transport and storage.An interesting alternative as sample matrix is saliva, which was shown to contain sufficient levels of 25(OH)D3 for successful quantification by LC-MS/MS after chemical derivatization (Higashi et al. 2008). Other liquid matrices that were used were urine (Ogawa et al. 2014) – where 24,25(OH)2D3 dominated over 25(OH)D3, a reverse situation to serum – and cerebrospinal fluid (CSF) (Holmøy et al. 2009).TOF-SIMS images of visceral adipose tissue biopsies (size 258 × 258 μm2) from one obese individual (lateral resolution ≈1 μm). (a–d) Distribution of secondary ions of sodium [Na]+ (a) and potassium [K]+ (b), phosphatidylcholine headgroup [C5H15PNO4]+ m/z 184.2 (c), and diacylglycerol (DAG) at m/z 551.2 (d). (e) Vitamin D3 [M-OH]+ at m/z 367.1, (f) 25(OH)D3 [M-OH]+ at m/z 383.2, (g) 1,25(OH)2D3 [M-OH]+ at m/z 399.8, (h) overlay images of vitamin D3 [M-OH]+ (red), 25(OH)D3 (green), and 1,25(OH)2D3 (blue). Adipocytes are indicated by arrows in panel (d). The color bar ranges from black (representing 0 intensity) to white (indicating highest signal intensity) (Reprinted with permission from Malmberg et al. (2014))LC-MS/MS assays for vitamin D from serum or plasma predominantly apply liquid/liquid extraction (LLE), solid-phase extraction (SPE), or combinations of the two techniques to remove the hydrophobic vitamin D compounds from the sample matrix (van den Ouweland et al. 2013; Volmer et al. 2015). Often protein precipitation is performed, solely or as part of further extraction steps (Ding et al. 2010; Herrmann et al. 2010; Kushnir et al. 2010). Furthermore, techniques such as ballistic turbulent flow chromatography (TFC) (Bunch et al. 2009; Singh et al. 2006), supported liquid extraction (SLE) (Geib et al. 2015; Jenkinson et al. 2016), or immunoenrichment (Laha et al. 2012) have been applied to vitamin D sample preparation.



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