Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 34

Although theoretically 24,25(OH)₂D could be measured by CPBA using DBP because of its equivalent affinity for DBP compared to 25(OH)D or immunoassay using a 1,25(OH)₂D antibody that cross reacts 24,25(OH)₂D [69,] given its relatively high concentrations in the blood (0.7–24 nM) [70], modern assays for this metabolite use LC-MS exclusively. Frequently this is done as part of a multimetabolite profile. Like that for 25(OH)D and 1,25(OH)₂D the C3-beta epimer of 24,25(OH)₂D has been identified [71]. Moreover, 24,25(OH)₂D exists as both the 24R,25(OH)₂D and 24S,25(OH)₂D epimer, but only the R epimer is biologically active [72]. Therefore, careful separation of these epimers as well as other dihydroxylated vitamin D metabolites prior to MS is required to obtain accurate results. As noted, 24,25(OH)₂D measurement is often part of a multimetabolite profile. The approaches to such assays have recently been reviewed [40]. Most of these assays used ESI for ionization, triplequadrupole instruments for MS, and nonspecific water loss transitions for monitoring. Derivitazation is often employed to increase sensitivity for the less abundant metabolites in the profile, but PTAD derivatization of 25(OH)D was found to interfere with the separation of C3-beta epi-25(OH)D from 25(OH)D [73]. Other methods of derivitazation have been developed that may circumvent this problem [74], and derivitazation may not be required if larger samples are used [75].