Читать книгу Vitamin D in Clinical Medicine - Группа авторов - Страница 27

Nearly all, if not all, cells express the vitamin D receptor (VDR) at some stage in their development or activation, and many of these cells are also able to convert vitamin D to its active metabolites. As the appreciation that vitamin D affects numerous physiologic processes other than bone and mineral metabolism, and that these physiologic processes may have different optimal levels of vitamin D [1], interest in the measurement of the levels of vitamin D and its metabolites has soared. Moreover, disorders of vitamin D metabolism can be diagnosed by accurate measurement of these metabolites, and potential differences in ratios of vitamin D metabolites even in otherwise normal individuals can be predictive of differences in responses to dietary intakes of vitamin D in food and/or in supplements due to differences in metabolism [2]. However, the measurement of vitamin D metabolites is not trivial. These are lipophilic materials circulating in low concentrations tightly bound to proteins, vitamin D binding protein (DBP) and albumin in particular, making their measurements difficult. If one considers that only the free (i.e., non-protein bound) metabolite enters cells and is the biologically important concentration to consider [3], the requirements for sensitivity of measurement increase by several orders of magnitude. Moreover, distinguishing between the different metabolites that may differ only modestly chemically but with substantial differences biologically and quantitatively likewise contributes to the difficulties developers of assays have in providing a fast throughput assay at reasonable cost to meet the increasing demand for these measurements. In this review, the most common assays on the market today are described reviewing their advantages and limitations with a discussion of newer technologies including the development of assays intended to measure the free concentrations.