Int. J. Mol. Sci. 2020, 21(2), 470; https://doi.org/10.3390/ijms21020470
by Bashar Al-Zohily 1,Asma Al-Menhali 2,*,Salah Gariballa 3,Afrozul Haq 4 andI ltaf Shah 1,
1 Department of Chemistry, College of Science, United Arab Emirates University, Al Ain 15551, UAE
2 Department of Biology, College of Science, United Arab Emirates University, Al Ain 15551, UAE
3 Internal Medicine, Faculty of Medicine & Health Sciences, United Arab Emirates University, Al Ain 15551, UAE
4 Department of Food Technology, School of Interdisciplinary Sciences and Technology, Jamia Hamdard University, New Delhi-110062, India
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Vitamin D (A) and Epimer (B) both use Binding Protein and Receptor
Figure 3. Comparison between 1α,25(OH)2D3 and 3-epi-1α,25(OH)2D3 in terms of gene regulation.
(A) When 1α,25(OH)2D3 is released from DBP, it will cross the cell membrane and enter the target
cell to bind with VDR. The 1α,25(OH)2D3–VDR complex undergoes translocation to the nucleus and
performs conformational changes in order to link with other transcriptional factors and heterodimerize
with RXR. After that, the 1α,25(OH)2D3–VDR–RXR complex binds to the vitamin D response element
(VDRE); then, the transcription of RNA begins for the specific genes that are then expressed as different
proteins responsible for vitamin D homeostasis.
(B) We assume that 3-epi-1α,25(OH)2D3 will carry out the same function but at a slower rate.
The 3-epi-1α,25(OH)2D3 complex could likely perform the same gene regulation as its non-epimeric forms but at a lower rate.
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In this review, we discuss the sources, formation, metabolism, function, biological activity, and potency of C3-epimers (epimers of vitamin D). We also determine the role of epimerase in vitamin D-binding protein (DBP) and vitamin D receptors (VDR) according to different subcellular localizations. The importance of C3 epimerization and the metabolic pathway of vitamin D at the hydroxyl group have recently been recognized. Here, the hydroxyl group at the C3 position is orientated differently from the alpha to beta orientation in space. However, the details of this epimerization pathway are not yet clearly understood. Even the gene encoding for the enzyme involved in epimerization has not yet been identified. Many published research articles have illustrated the biological activity of C3 epimeric metabolites using an in vitro model, but the studies on in vivo models are substantially inadequate. The metabolic stability of 3-epi-1α,25(OH)2D3 has been demonstrated to be higher than its primary metabolites. 3-epi-1 alpha, 25 dihydroxyvitamin D3 (3-epi-1α,25(OH)2D3) is thought to have fewer calcemic effects than non-epimeric forms of vitamin D.
Some researchers have observed a larger proportion of total vitamin D as C3-epimers in infants than in adults. Insufficient levels of vitamin D were found in mothers and their newborns when the epimers were not included in the measurement of vitamin D. Oral supplementation of vitamin D has also been found to potentially cause increased production of epimers in mice but not humans.
Moreover, routine vitamin D blood tests for healthy adults will not be significantly affected by epimeric interference using LC–MS/MS assays. Recent genetic models also show that the genetic determinants and the potential factors of C3-epimers differ from those of non-C3-epimers.
Most commercial immunoassays techniques can lead to inaccurate vitamin D results due to epimeric interference, especially in infants and pregnant women. It is also known that the LC–MS/MS technique can chromatographically separate epimeric and isobaric interference and detect vitamin D metabolites sensitively and accurately. Unfortunately, many labs around the world do not take into account the interference caused by epimers. In this review, various methods and techniques for the analysis of C3-epimers are also discussed. The authors believe that C3-epimers may have an important role to play in clinical research, and further research is warranted.