Cell specificity and properties of the C-3 epimerization of Vitamin D3 metabolites

Epimeric vitamin D and cardiovascular structure and function in advanced CKD and after kidney transplantation

BACKGROUND

25-hydroxyvitamin D can undergo C-3 epimerization to produce 3-epi-25(OH)D3. 3-epi-25(OH)D3 levels decline in chronic kidney disease (CKD), but its role in regulating the cardiovascular system is unknown. Herein, we examined the relationship between 3-epi-25(OH)D3, and cardiovascular functional and structural endpoints in patients with CKD.

METHODS

We examined n = 165 patients with advanced CKD from the Cardiopulmonary Exercise Testing in Renal Failure and After Kidney Transplantation (CAPER) study cohort, including those who underwent kidney transplant (KTR, n = 76) and waitlisted patients who did not (NTWC, n = 89). All patients underwent cardiopulmonary exercise testing and echocardiography at baseline, 2 months and 12 months. Serum 3-epi-25(OH)D3 was analyzed by liquid chromatography-tandem mass spectrometry.

RESULTS

Patients were stratified into quartiles of baseline 3-epi-25(OH)D3 (Q1: <0.4 ng/mL, n = 51; Q2: 0.4 ng/mL, n = 26; Q3: 0.5-0.7 ng/mL, n = 47; Q4: ≥0.8 ng/mL, n = 41). Patients in Q1 exhibited lower peak oxygen uptake [VO2Peak = 18.4 (16.2-20.8) mL/min/kg] compared with Q4 [20.8 (18.6-23.2) mL/min/kg; P = .009]. Linear mixed regression model showed that 3-epi-25(OH)D3 levels increased in KTR [from 0.47 (0.30) ng/mL to 0.90 (0.45) ng/mL] and declined in NTWC [from 0.61 (0.32) ng/mL to 0.45 (0.29) ng/mL; P < .001]. Serum 3-epi-25(OH)D3 was associated with VO2Peak longitudinally in both groups [KTR: β (standard error) = 2.53 (0.56), P < .001; NTWC: 2.73 (0.70), P < .001], but was not with left ventricular mass or arterial stiffness. Non-epimeric 25(OH)D3, 24,25(OH)2D3 and the 25(OH)D3:24,25(OH)2D3 ratio were not associated with any cardiovascular outcome (all P > .05).

CONCLUSIONS

Changes in 3-epi-25(OH)D3 levels may regulate cardiovascular functional capacity in patients with advanced CKD.

C3-epi-25(OH)D3 percentage, not level, may be a potential biomarker to reflect its pathological increase in multiple diseases: a cross-sectional case-control study

National surveys in developed countries have examined the presence of C3-epimer of 25-hydroxyvitamin D3 [C3-epi-25(OH)D3]. However, controversy remains regarding its association with disease occurrence due to its high correlation with 25-hydroxyvitamin D3 [25(OH)D3]. This study aims to investigate whether %C3-epi-25(OH)D3 can serve as an indicator for this relationship with various diseases. A total of 3086 healthy participants and 4120 patients were included in this study. We investigated the association between C3-epi-25(OH)D3 and %C3-epi-25(OH)D3 levels with gender, age, and season; compared the performance of C3-epi-25(OH)D3 and %C3-epi-25(OH)D3 across different disease conditions; and explored the correlation between %C3-epi-25(OH)D3 and various diseases. Results indicated that C3-epi-25(OH)D3 varied significantly by gender, age, and season (z/χ2 = 3.765, 10.163, and 150.975, all P < 0.01), while only season for %C3-epi-25(OH)D3 (χ2 = 233.098, P < 0.001). In contrast to the significant decrease in C3-epi-25(OH)D3, %C3-epi-25(OH)D3 showed a significant increase in 8 out of 11 disease categories (z = 3.464 ~ 11.543, all Padj < 0.05). Similar opposite changes were also observed in most of the investigated 32 specific diseases. Moreover, an elevation in %C3-epi-25(OH)D3 was found to be significantly associated with 29 specific diseases both in univariate analysis (OR = 1.16 ~ 2.10, all P < 0.05) and after adjusting for gender, age, and season (OR = 1.15 ~ 1.50, all P < 0.05). However, after further adjustment for 25(OH)D3 levels, the association remained significant only for 15 specific diseases (OR = 1.11 ~ 1.50, all P < 0.05). Seasonal stratification analysis further supports the consistent association of %C3-epi-25(OH)D3 with disease across all or nearly all four seasons. In conclusion, %C3-epi-25(OH)D3 may better reflect the production of C3-epi-25(OH)D3 in disease conditions, thereby offering a more applicable approach to investigate its association with diseases. However, the interpretation of this relationship may be confounded by 25(OH)D3 as a potential covariate.

Effects of Vitamin D2 and 25-Hydroxyvitamin D2 Supplementation on Plasma Vitamin D Epimeric Metabolites in Adult Cats

Feline vitamin D status is based on dietary consumption but metabolism of this essential nutrient and the efficacy of supplementation forms are poorly described in cats. The aim of this study was to further elucidate the metabolites of vitamin D2 in cats and to compare the effectiveness of vitamin D2 and 25(OH)D2 for increasing feline vitamin D status. Eight adult male castrated domestic shorthair cats received vitamin D2 or 25(OH)D2 in a single crossover design. Vitamin D2 was dosed daily in a molar equivalent dosage to vitamin D3 ingested in the diet while 25(OH)D2 was provided at a daily dose of 20% molar equivalent intake of dietary vitamin D3 based on its expected higher potency. Plasma concentrations of 25-hydroxyvitamin D epimers were evaluated at baseline then every 2 weeks for a total of 10 weeks. Analysis of multiple vitamin D metabolite concentrations was completed at the end of each supplementation period, followed by a washout period preceding the second phase of the crossover trial. Results showed that supplementation with 25(OH)D2 more effectively and rapidly raised circulating 25(OH)D2 levels in cat plasma compared to vitamin D2. Formation of C-3 epimers of 25(OH)D3, 25(OH)D2, and 24,25R(OH)2D3, but not 24,25(OH)2D2, were observed in feline plasma. The abundant concentrations of epimeric forms of vitamin D metabolites found in circulation suggest that these metabolites should be considered during vitamin D analyses in cats. Further studies using 25(OH)D and vitamin D2 forms are needed to conclude safety and efficacy of these vitamers for supplementation in this species.

Overestimation of 3α- over 3β-25-Hydroxyvitamin D3 Levels in Serum: A Mechanistic Rationale for the Different Mass Spectral Properties of the Vitamin D Epimers

The metabolism of vitamin D₃ includes a parallel C-3 epimerization pathway-in addition to the standard metabolic processes for vitamin D₃-reversing the stereochemical configuration of the -OH group at carbon-3 (β→α). While the biological function of the 3α epimer has not been elucidated yet, the additional species cannot be neglected in the analytical determination of vitamin D₃, as it has the potential to introduce analytical errors if not properly accounted for. Recently, some inconsistent mass spectral behavior was seen for the 25-hydroxyvitamin D₃ (25(OH)D₃) epimers during quantification using electrospray LC-MS/MS. The present work extends that of Flynn et al. ( Ann. Clin. Biochem. 2014, 51, 352-559) and van den Ouweland et al. ( J. Chromatogr. B 2014, 967, 195-202), who reported larger electrospray ionization response factors for the 3α epimer of 25(OH)D₃ in human serum samples as compared to the regular 3β variant. The present work was concerned with the mechanistic reasons for these differences. We used a combination of electrospray ionization, atmospheric pressure chemical ionization, and density functional theory calculations to uncover structural dissimilarities between the epimers. A plausible mechanism is described based on intramolecular hydrogen bonding in the gas phase, which creates a small difference of proton affinities between the epimers. More importantly, this mechanism allows the explanation of the different ionization efficiencies of the epimers based on kinetic control of the ionization process, where ionization initially takes place at the hydroxyl group with subsequent proton transfer to a basic carbon atom. The barrier for this transfer differs between the epimers and is in direct competition with H₂O elimination from the protonated hydroxyl group. The "hidden" site of high gas phase basicity was revealed through computational calculations and appears to be inaccessible via direct protonation.

Epimers of Vitamin D: A Review

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.

Association between Vitamin D Levels and Nonalcoholic Fatty Liver Disease: Potential Confounding Variables

Nonalcoholic fatty liver disease (NAFLD), historically considered to be the hepatic component of the metabolic syndrome, is a spectrum of fat-associated liver conditions, in the absence of secondary causes, that may progress to nonalcoholic steatohepatitis (NASH), fibrosis, and cirrhosis. Disease progression is closely associated with body weight or fatness, dyslipidemia, insulin resistance, oxidative stress, and inflammation. Recently, vitamin D deficiency has been linked to the pathogenesis and severity of NAFLD because of vitamin D "pleiotropic" functions, with roles in immune modulation, cell differentiation and proliferation, and regulation of inflammation. Indeed, several studies have reported an association between vitamin D and NAFLD/NASH. However, other studies have failed to find an association. Therefore, we sought to critically review the current evidence on the association between vitamin D deficiency and NAFLD/NASH, and to analyze and discuss some key variables that may interfere with this evaluation, such as host-, environment-, and heritability-related factors regulating vitamin D synthesis and metabolism; definitions of deficient or optimal vitamin D status with respect to skeletal and nonskeletal outcomes including NAFLD/NASH; methods of measuring 25(OH)D; and methods of diagnosing NAFLD as well as quantifying adiposity, the cardinal link between vitamin D deficiency and NAFLD.

Rapid Quantification of 25-Hydroxyvitamin D3 in Human Serum by Matrix-Assisted Laser Desorption/Ionization Mass Spectrometry

LC-MS/MS is widely utilized today for quantification of vitamin D in biological fluids. Mass spectrometric assays for vitamin D require very careful method optimization for precise and interference-free, accurate analyses however. Here, we explore chemical derivatization and matrix-assisted laser desorption/ionization (MALDI) as a rapid alternative for quantitative measurement of 25-hydroxyvitamin D₃ in human serum, and compare it to results from LC-MS/MS. The method implemented an automated imaging step of each MALDI spot, to locate areas of high intensity, avoid sweet spot phenomena, and thus improve precision. There was no statistically significant difference in vitamin D quantification between the MALDI-MS/MS and LC-MS/MS: mean ± standard deviation for MALDI-MS-29.4 ± 10.3 ng/mL-versus LC-MS/MS-30.3 ± 11.2 ng/mL (P = 0.128)-for the sum of the 25-hydroxyvitamin D epimers. The MALDI-based assay avoided time-consuming chromatographic separation steps and was thus much faster than the LC-MS/MS assay. It also consumed less sample, required no organic solvents, and was readily automated. In this proof-of-concept study, MALDI-MS readily demonstrated its potential for mass spectrometric quantification of vitamin D compounds in biological fluids. Graphical Abstract ᅟ.

The genetic determinants of circulating C3-epimers of 25-hydroxyvitamin D

BACKGROUND

The complexity of vitamin D metabolites especially the contribution of C3-epimers of 25-hydroxyvitamin D (C3-epimers) in human sera remains unclear. We hypothesized that genetic polymorphisms in the vitamin D-related gene pathway contribute to variation in C3-epimer levels. Therefore, we investigated candidate single nucleotide polymorphisms (SNPs) concerning C3-epimer levels.

METHODS

The candidate SNPs, including DHCR7/NADSYN1 (rs12785878), CYP2R1 (rs2060793) and GC (rs2282679), were genotyped in 1727 members of the third project of the Electricity Generating Authority of Thailand 3/1 cohort investigation. Each SNP was tested under three genetic effects (dominant, recessive and additive models) concerning the levels of total serum 25(OH)D [the sum of 25(OH)D2+3 and 3-epi-25(OH)D2+3], non-C3-epimers [25(OH)D2+3] and C3-epimers [3-epi-25(OH)D2+3], using linear regression analysis.

RESULTS

Among the participants, the median (range) levels of non-C3-epimers and C3-epimers were 22.7 (6.4-49.2) ng/mL and 1.3 (0.01-14.2) ng/mL, respectively. In regression analysis, we found the genetic variation of two SNPs, the DHCR7/NADSYN1 (rs12785878; G > T) and GC (rs2282679; T > G) under additive genetic models, explained the variation of C3-epimer levels about 1.5% (p = 1.66 × 10-7) and 1.1% (p = 1.10 × 10-5), respectively. Interestingly, participants carrying the minor T-allele of rs12785878 exhibited a trend to increase C3-epimer levels, while those carrying the minor G-allele of rs2282679 exhibited a trend to decrease levels of both non-C3-epimers and C3-epimers. In addition, CYP2R1 (rs2060793; G > A) was clearly associated only with non-C3-epimer levels (p = 2.46 × 10-8). In multivariate analyses, sex, age and BMI were predictors for variation in C3-epimer concentration; sex and age for variation in non-C3-epimers.

CONCLUSION

To the best of our knowledge, this is the first study to demonstrate genetic models concerning the variation in C3-epimer levels. Our results emphasize that genetic determinants and the potential factors of C3-epimers differ from non-C3-epimers. This study contributes fundamental knowledge of the endogenous vitamin D pathway.

25-Hydroxyvitamin D3 and its C-3 epimer are elevated in the skin and serum of Skh-1 mice supplemented with dietary vitamin D3

SCOPE

UV exposure is a risk factor for keratinocyte carcinoma (KC) while critical for endogenous vitamin D production. We investigated dietary modulation of skin and serum 25-hydroxyvitamin D₃ (25OHD₃ ) and its C-3 epimer (C3epi) in a mouse model of KC. C3epi is an under-investigated metabolite of vitamin D with respect to its biological implications.

METHODS AND RESULTS

Male and female Skh-1 mice were supplemented with 25, 150 or 1000 IU/kg diet vitamin D₃ for 25 weeks, with some exposed to UV light. Skin and serum vitamin D metabolites were quantitated using HPLC-MS/MS (n = 3 per dose/sex/UV treatment). Serum and skin 25OHD₃ and C3epi significantly increased with dose (P<0.0001), but with different response patterns. UV exposure significantly attenuated serum, but not skin, levels of both metabolites (P<0.001, P = 0.0287), while up-regulating expression of renal Cyp24a1 (P < 0.01). A dose by sex interaction trended toward significance with serum and skin levels of C3epi, wherein male mice attained higher levels of C3epi with higher dietary vitamin D₃ . This reflected a similar, but non-significant pattern in average tumor size.

CONCLUSION

The complex relationship between vitamin D and KC requires further investigation. This study provides insight into modulation of local and systemic vitamin D status with dietary supplementation.

Metabolism of selective 20-epi-vitamin D3 analogs in rat osteosarcoma UMR-106 cells: Isolation and identification of four novel C-1 fatty acid esters of 1α,25-dihydroxy-16-ene-20-epi-vitamin D3

Analogs of 1α,25-dihydroxyvitamin D₃ (S1) with 20-epi modification (20-epi analogs) possess unique biological properties. We previously reported that 1α,25-dihydroxy-20-epi-vitamin D₃ (S2), the basic 20-epi analog is metabolized into less polar metabolites (LPMs) in rat osteosarcoma cells (UMR-106) but not in a perfused rat kidney. Furthermore, we also noted that only selective 20-epi analogs are metabolized into LPMs. For example, 1α,25-dihydroxy-16-ene-20-epi-vitamin D₃ (S4), but not 1α,25-dihydroxy-16-ene-23-yne-20-epi-vitamin D₃ (S5) is metabolized into LPMs. In spite of these novel findings, the unequivocal identification of LPMs has not been achieved to date. We report here on a thorough investigation of the metabolism of S4 in UMR-106 cells and isolated two major LPMs produced directly from the substrate S4 itself and two minor LPMs produced from 3-epi-S4, a metabolite of S4 produced through C-3 epimerization pathway. Using GC/MS, ESI-MS and ¹H NMR analysis, we identified all the four LPMs of S4 as 25-hydroxy-16-ene-20-epi-vitamin D₃-1-stearate and 25-hydroxy-16-ene-20-epi-vitamin D₃-1-oleate and their respective C-3 epimers. We report here for the first time the elucidation of a novel pathway of metabolism in UMR-106 cells in which both 1α,25(OH)₂-16-ene-20-epi-D₃ and 1α,25(OH)₂-16-ene-20-epi-3-epi-D₃ undergo C-1 esterification into stearic and oleic acid esters.

Vitamin D metabolite concentrations in umbilical cord blood serum and associations with clinical characteristics in a large prospective mother-infant cohort in Ireland

Vitamin D deficiency is widespread among mothers and neonates and quality clinical and analytical data are lacking. We used a CDC-accredited LC-MS/MS method to analyze vitamin D metabolites in cord sera from 1050 maternal-infant dyads in the prospective SCOPE Ireland Pregnancy and BASELINE Birth cohort studies, based in Cork, Ireland. The mean±SD total 25(OH)D was 34.9±18.1nmol/L; 35% of cords (50% during winter) had 25(OH)D <25nmol/L, 46% were <30nmol/L and 80% were <50nmol/L. In this predominantly white cohort, the main predictor of cord 25(OH)D [adj. mean difference in nmol/L (95% CI)] was summer delivery [19.2 (17.4, 20.9), P<0.0001]. Maternal smoking during pregnancy (9% prevalence) was negatively associated (P<0.002) with cord 25(OH)D [-4.83 (-7.9, -1.5) nmol/L]. There were no associations between cord 25(OH)D and birth weight or any anthropometric measures at birth. Despite the high prevalence of vitamin D deficiency at birth, there were no documented musculoskeletal complications during infancy, which was likely due to widespread supplementation with vitamin D. The mean±SD concentration of 3-epi-25(OH)D₃, detectable in 99.4% of cord samples, was 3.3±1.9nmol/L. The proportion of 25(OH)D as 3-epi-25(OH)D₃ was 11.2%. Cord 3-epi-25(OH)D₃ concentrations were positively predicted by cord 25(OH)D₃ [0.101 (0.099, 0.103) nmol/L, P<0.0001] and negatively by gestational age [-0.104 (-0.131, -0.076) nmol/L, P<0.0001] and maternal age [-0.010 (-0.019, -0.001) nmol/L, P<0.05]. 25(OH)D₂ was detected in 98% of cord sera (mean±SD; 2.2±1.9nmol/L) despite low antenatal consumption of vitamin D₂ supplements. In conclusion, these first CDC-accredited data of vitamin D metabolites in umbilical cord blood emphasise the high risk of very low vitamin D status in infants born to un-supplemented mothers. Experimental data to define maternal vitamin D requirements for prevention of neonatal deficiency at high latitude are required.

The C-3α Epimer of 25-Hydroxycholecalciferol from Endogenous and Exogenous Sources Supports Normal Growth and Bone Mineral Density in Weanling Rats

BACKGROUND

The C-3α epimer of 25-hydroxycholecalciferol [3-epi-25(OH)D₃] is elevated in infants.

OBJECTIVES

We tested whether increasing cholecalciferol intake results in a dose-response in plasma 3-epi-25(OH)D₃ We also examined bone and mineral metabolism in response to 3-epi-25(OH)D₃ treatment.

METHODS

Sprague Dawley rats (4 wk old) were randomly assigned (n = 6/group of each sex) to AIN-93G diets with cholecalciferol at 1 (control), 2, or 4 IU/g diet for objective 1 and to diets with 3-epi-25(OH)D₃ at 0.5 or 1 IU/g diet or 25-hydroxycholecalciferol [25(OH)D₃] at 0.5 IU/g diet for objective 2 for 8 wk. Measurements at weeks 0, 4, and 8 included body weight and length, plasma vitamin D metabolites, bone biomarkers, and bone mineral density determined by using dual-energy X-ray absorptiometry. Lumbar vertebra 3 (L3) geometry and volumetric bone mineral density (vBMD) were measured using microcomputed tomography. Differences between groups were identified for males and females separately.

RESULTS

Weight and food intake were not different between groups. Elevated plasma 3-epi-25(OH)D₃ was observed only in females in the 4 IU cholecalciferol/g diet group (mean ± SD: 24.7 ± 17.1 ng/mL), compared with the control group (5.3 ± 1.4 ng/mL; P = 0.001). By week 8, both male and female rats in the 3-epi-25(OH)D₃ groups had >87% greater plasma 3-epi-25(OH)D₃ concentrations relative to the 25(OH)D₃ reference group (P < 0.0001). At week 8 in males only, parathyroid hormone was significantly lower (P = 0.019) in both 3-epi-25(OH)D₃ groups than in the 25(OH)D₃ group, and L3 total vBMD was higher (P = 0.004) in the 0.5 IU 3-epi-25(OH)D₃ group than in the 25(OH)D₃ group.

CONCLUSIONS

Endogenously generated 3-epi-25(OH)D₃ is more prominent in female than in male rats. Exogenous 3-epi-25(OH)D₃ was as effective as 25(OH)D₃ in supporting bone mineral accretion in both sexes. It thus appears that 3-epi-25(OH)D₃ has biological activity and should be further explored.

Vitamin D status is associated with uteroplacental dysfunction indicated by pre-eclampsia and small-for-gestational-age birth in a large prospective pregnancy cohort in Ireland with low vitamin D status

BACKGROUND

Associations between vitamin D and pregnancy outcomes have been inconsistent.

OBJECTIVES

We described the distribution of 25-hydroxyvitamin D3 [25(OH)D3], 3-epi-25(OH)D3, and 25(OH)D2 in early pregnancy and investigated associations with pre-eclampsia and small-for-gestational-age (SGA) birth, which are indicative of uteroplacental dysfunction.

DESIGN

The SCOPE (Screening for Pregnancy Endpoints) Ireland prospective pregnancy cohort study included 1768 well-characterized low-risk, nulliparous women resident at 52°N. Serum 25(OH)D3, 3-epi-25(OH)D3, and 25(OH)D2 were quantified at 15 wk of gestation with the use of a CDC-accredited liquid chromatography-tandem mass spectrometry method.

RESULTS

The mean ± SD total 25(OH)D concentration was 56.7 ± 25.9 nmol/L, and 17% and 44% of women had 25(OH)D concentrations <30 and <50 nmol/L, respectively. The prevalence of pre-eclampsia was 3.8%, and 10.7% of infants were SGA. There was a lower risk of pre-eclampsia plus SGA combined (13.6%) at 25(OH)D concentrations >75 nmol/L (adjusted OR: 0.64; 95% CI: 0.43, 0.96). The main predictors of 25(OH)D were the use of vitamin D-containing supplements (adjusted mean difference: 20.1 nmol/L; 95% CI: 18.5, 22.7 nmol/L) and summer sampling (adjusted mean difference: 15.5 nmol/L; 95% CI: 13.4, 17.6 nmol/L). Non-Caucasian ethnicity (adjusted mean difference: -19.3 nmol/L; 95% CI: -25.4, -13.2 nmol/L) and smoking (adjusted mean difference: -7.0 nmol/L; 95% CI: -10.5, -3.6 nmol/L) were negative predictors of 25(OH)D. The mean ± SD concentration of 3-epi-25(OH)D3, which was detectable in 99.9% of samples, was 2.6 ± 1.6 nmol/L. Determinants of 3-epi-25(OH)D3 were 25(OH)D3 (adjusted mean difference: 0.052 nmol/L; 95% CI: 0.050, 0.053 nmol/L) and maternal age (adjusted mean difference: -0.018 nmol/L; 95% CI: -0.026, -0.009 nmol/L). The mean ± SD concentration of 25(OH)D2 was 3.1 ± 2.7 nmol/L, which was present in all samples. No adverse effects of 25(OH)D concentrations >125 nmol/L were observed.

CONCLUSIONS

In the first report to our knowledge of CDC-accredited 25(OH)D data and pregnancy outcomes from a large, clinically validated, prospective cohort study, we observed a protective association of a 25(OH)D concentration >75 nmol/L and a reduced risk of uteroplacental dysfunction as indicated by a composite outcome of SGA and pre-eclampsia. Well-designed, adequately powered randomized controlled trials are required to verify this observation. The SCOPE pregnancy cohort was registered at www.anzctr.org.au as ACTRN12607000551493.

Serum C3 epimer of 25-hydroxyvitamin D and its determinants in adults: a national health examination survey in Thais

A high percentage have detectable C3 epimer of 25-hydroxyvitamin D3 (3-epi-25(OH)D3) in the population of Thai National Health Examination Survey IV.

INTRODUCTION

C3 epimers of vitamin D have recently been shown to contribute significantly to 25-hydroxyvitamin D (25(OH)D) levels in an infant population. However, the findings in the general adult population are unclear. Therefore, the purpose of the present study is to determine the percentage of the C3 epimer of 25(OH)D (3-epi-25(OH)D) and its determinants in an adult population.

METHODS

A subsample of 1148 sera randomly selected from the Thai National Health Examination Survey IV (2009) samples were measured for serum 25(OH)D2, 25(OH)D3, 3-epi-25(OH)D2, and 3-epi-25(OH)D3 by LC-MS/MS method. The relative 3-epimer contribution (%) was used to express the amount of 3-epimer-25(OH)D3 as a percentage of total 25(OH)D3 (the sum of 25(OH)D3, and 3-epi-25(OH)D3).

RESULTS

A high proportion of subjects had detectable 3-epi-25(OH)D3 that was <10 % of the total 25(OH)D levels. Since the level of total 25(OH)D2 is low, only a minority of subjects had detectable 3-epi-25(OH)D2. Multivariate analysis suggested that age, male gender, and rural residence were independently related to the 3-epi-25(OH)D3/total 25(OH)D3 ratio.

CONCLUSIONS

A high percentage of Thai adults had detectable 3-epi-25(OH)D3 that was <10 % of the total 25(OH)D levels. Age, gender, and living in a rural area were associated with the relative amount of 3-epi-25(OH)D3 to total 25(OH)D3.

Vitamin D metabolism in the premature newborn: A randomized trial

BACKGROUND & AIMS

Vitamin D status during infancy has been associated with important pediatric health outcomes; however concentrations of many vitamin D metabolites in premature infants are not yet described. The objective of this study was to evaluate concentrations of 25(OH)D3, 24,25(OH)2D3, and 3-epi-25(OH)D3 in premature infants.

METHODS

32 infants <32 weeks gestation were randomized to receive 400 or 800IU/day of vitamin D3 orally. Vitamin D metabolites from serum obtained monthly were analyzed in triplicate using a novel, very sensitive Liquid Chromatography-Tandem Mass Spectrometry-based method. Statistical analysis was conducted using the Fisher's exact test, Wilcoxon Rank Sum test, and Spearman correlation coefficients. Measurements over time were fit with linear mixed effect models. A p-value of <0.05 was considered statistically significant.

RESULTS

Mean serum 25(OH)D3 concentrations in cord blood were 17.3 ng/mL; mean 3-epi-25(OH)D3 were 1.3 ng/mL, mean 24,25(OH)2D3 were 1.4 ng/mL. Both 25(OH)D3 and 3-epi-25(OH)D3 increased significantly over time, and the percent of total 25(OH)D3 concentration that was 3-epi-25(OH)D3 also increased significantly (7.2% vs. 29.7%, p < 0.0001 for cord blood vs. 8 weeks). Serum 25(OH)D3:24,25(OH)2D3 ratios at weeks 4 and 8 were higher than ratios reported in older children and adults.

CONCLUSION

Vitamin D metabolism in infants appears to have distinct differences from adults. Vitamin D supplementation was effective in raising 25(OH)D3 concentrations; however significant increases in 3-epi-25(OH)D3 also occurred. Increased 25(OH)D3: 24,25(OH)2D3 ratios in premature infants may be due to immature expression of CYP24A1. Further work is necessary to determine if there are developmental advantages to this unique vitamin D metabolism.

Candidate Reference Measurement Procedure for the Determination of (24R),25-Dihydroxyvitamin D3 in Human Serum Using Isotope-Dilution Liquid Chromatography-Tandem Mass Spectrometry

The two major forms of vitamin D, vitamin D3 and vitamin D2, are metabolized in the liver through hydroxylation to 25-hydroxyvitamin D species, and then further hydroxylated in the kidney to various dihydroxyvitamin D species. (24R),25-Dihydroxyvitamin D3 ((24R),25(OH)2D3) is a major catabolite of 25-hydroxyvitamin D metabolism and is an important vitamin D metabolite used as a catabolism marker and indicator of kidney disease. The National Institute of Standards and Technology has recently developed a reference measurement procedure for the determination of (24R),25(OH)2D3 in human serum using isotope-dilution LC-MS/MS. The (24R),25(OH)2D3 and added deuterated labeled internal standard (24R),25(OH)2D3-d6 were extracted from serum matrix using liquid-liquid extraction prior to LC-MS/MS analysis. Chromatographic separation was performed using a fused-core C18 column. Atmospheric pressure chemical ionization in the positive ion mode and multiple reaction monitoring were used for LC-MS/MS. The accuracy of the measurement of (24R),25(OH)2D3 was evaluated by recovery studies of measuring (24R),25(OH)2D3 in gravimetrically prepared spiked samples of human serum with known (24R),25(OH)2D3 levels. The recoveries of the added (24R),25(OH)2D3 averaged 99.0% (0.8% SD), and the extraction efficiencies averaged 95% (2% SD). Excellent repeatability was demonstrated with CVs of ∼1%. The limit of quantitation at a signal-to-noise ratio of ∼10 was 0.2 ng/g. Potential isomeric interferences from other endogenous species and from impurity components of the reference standard were investigated. LC baseline resolution of (24R),25(OH)2D3 from these isomers was achieved within 35 min. This method was used for value assignment of (24R),25(OH)2D3 in Standard Reference Materials of Vitamin D Metabolites in Human Serum, which can serve as an accuracy base for routine methods used in clinical laboratories.

Cytochrome P450 Vitamin D Hydroxylases in Inflammation and Cancer

Vitamin D insufficiency correlates with increased incidence of inflammatory disorders and cancer of the colon, breast, liver, and prostate. Preclinical studies demonstrated that the hormonally active form of vitamin D, 1,25(OH)2D3, has antiproliferative, proapoptotic, anti-inflammatory, and immunomodulatory effects. Tissue levels of 1,25(OH)2D3 are determined by expression and activity of specific vitamin D hydroxylases expressed at renal and extrarenal sites. In order to understand how perturbations in the vitamin D system affect human health, we need to understand the steps involved in the synthesis and catabolism of the active metabolite. This review provides an overview about recent findings on the altered vitamin D metabolism in inflammatory conditions and carcinogenesis. We will summarize existing data on the pathophysiological regulation of vitamin D hydroxylases and outline the role of adequate levels of 1,25(OH)2D3 on tissue homeostasis.

Analysis of vitamin D metabolic markers by mass spectrometry: current techniques, limitations of the "gold standard" method, and anticipated future directions

Vitamin D compounds belong to a group of secosteroids, which occur naturally as vitamin D3 in mammals and D2 in plants. Vitamin D is vital for bone health but recent studies have shown a much wider role in the pathologies of diseases such as diabetes, cancer, autoimmune, neurodegenerative, mental and cardiovascular diseases. Photosynthesis of vitamin D in the human skin and subsequent hepatic and renal metabolism generate a wide range of transformation products occurring over a large dynamic range spanning from picomolar to nanomolar levels. This necessitates selective and sensitive analytical methods to quantitatively capture these low concentration levels in relevant tissues such as blood. Ideally, vitamin D assessment would be performed using a universal and standardized analytical method available to clinical laboratories that provides reliable and accurate quantitative results for all relevant vitamin D metabolites with sufficiently high throughput. At present, LC-MS/MS assays are the most promising techniques for vitamin D analysis. The present review focuses on developments in mass spectrometry methodologies of the past 12 years. It will highlight detrimental influences of the biological matrix, epimer contributions, pitfalls of specific mass spectrometry data acquisition routines (in particular multiple reaction monitoring, MRM), influence of ionization source, derivatization reactions, inter-laboratory comparisons on precision, accuracy, and application range of vitamin D metabolites.

Analytical measurement and clinical relevance of vitamin D(3) C3-epimer

With an ever-increasing clinical interest in vitamin D insufficiency, numerous automated immunoassays, protein binding assays, and in-house LC-MS/MS methods are being developed for the quantification of 25-hydroxyvitamin D(3) (25(OH)D(3)). Recently, LC-MS/MS methods have identified an epimeric form of 25(OH)D(3) that has been shown to contribute significantly to 25(OH)D(3) concentration, particularly in infant populations. This review describes the metabolic pathway and physiological functions of 3-epi-vitamin D, compares the capability of various 25(OH)D(3) methods to detect the epimer, and highlights recent publications quantifying 3-epi-25(OH)D(3) in infant, pediatric, and adult populations. In total, this review summarizes the information necessary for clinicians and laboratorians to decide whether or not to report/consider the C3-epimer in the analysis and clinical assessment of vitamin D status.

Misleading measures in Vitamin D analysis: a novel LC-MS/MS assay to account for epimers and isobars

Background

Recently, the accuracies of many commercially available immunoassays for Vitamin D have been questioned. Liquid chromatography tandem mass spectrometry (LC- MS/MS) has been shown to facilitate accurate separation and quantification of the major circulating metabolite 25-hydroxyvitamin-D3 (25OHD3) and 25-hydroxyvitamin-D2 (25OHD2) collectively termed as 25OHD. However, among other interferents, this method may be compromised by overlapping peaks and identical masses of epimers and isobars, resulting in inaccuracies in circulating 25OHD measurements. The aim of this study was to develop a novel LC-MS/MS method that can accurately identify and quantitate 25OHD3 and 25OHD2 through chromatographic separation of 25OHD from its epimers and isobars.

Methods

A positive ion electrospray ionisation (ESI) LC-MS/MS method was used in the Multiple Reaction Monitoring (MRM) mode for quantification. It involved i) liquid-liquid extraction, ii) tandem columns (a high resolution ZORBAX C18 coupled to an ULTRON chiral, with guard column and inlet filter), iii) Stanozolol-D3 as internal standard, and iv) identification via ESI and monitoring of three fragmentation transitions. To demonstrate the practical usefulness of our method, blood samples were collected from 5 healthy male Caucasian volunteers; age range 22 to 37 years and 25OHD2, 25OHD3 along with co-eluting epimers and analogues were quantified.

Results

The new method allowed chromatographic separation and quantification of 25OHD2, 25OHD3, along with 25OHD3 epimer 3-epi-25OHD3 and isobars 1-α-hydroxyvitamin-D3 (1αOHD3), and 7-α-hydroxy-4-cholesten-3-one (7αC4). The new assay was capable of detecting 0.25 ng/mL of all analytes in serum.

Conclusions

To our knowledge, this is the first specific, reliable, reproducible and robust LC-MS/MS method developed for the accurate detection of 25OHD (Vitamin D). The method is capable of detecting low levels of 25OHD3 and 25OHD2 together with chromatographic separation from the co-eluting epimers and isobars and circumvents other instrumental/analytical interferences. This analytical method does not require time-consuming derivatisation and complex extraction techniques and could prove very useful in clinical studies.

A perspective on how the Vitamin D sterol/Vitamin D receptor (VDR) conformational ensemble model can potentially be used to understand the structure-function results of A-ring modified Vitamin D sterols

The steroid hormone 1alpha,25(OH)(2)-Vitamin D(3) (1,25D) activates both genomic and non-genomic intracellular signaling cascades. It is also well recognized that co-incubation of 1,25D with its C-1 epimer, 1beta,25D (HL), suppresses the efficiency of the non-genomic signal activated by 1,25D alone and that its C-3 epimer, 3alpha-1,25D (HJ) is nearly as potent as 1,25D in suppressing PTH secretion, believed to be propagated by 1,25D's genomic signaling. Both these sterols lack the hypercalcemic effect induced by pharmacological doses of 1,25D and have reduced VDR affinity compared to 1,25D, as measured in a steroid competition assay. Recent functional studies suggest that the VDR is required for both non-genomic and genomic signaling. Along these lines we have recently proposed a Vitamin D sterol/VDR conformational ensemble model that posits the VDR contains two distinct, yet overlapping ligand binding sites, and that the potential differential stabilities of 1,25D and HL in these two pockets can be used to explain their different non-genomic signaling properties. The overlapping region is predominantly occupied by the sterol's A-ring when it is bound to either the genomic ligand binding pocket (G-pocket), defined by X-ray crystallography, or the alternative ligand binding pocket (A-pocket), discovered using in silico techniques (directed docking). Therefore, to gain further insight into the potential application of this model we docked the other A-ring diastereomer [(1beta,3alpha)=HH] of 1,25D and its 1- and 3-deoxy forms (25D and CF, respectively) to the A- and G-pockets to assess their potential stabilities in the pockets, relative to 1,25D. The models were then used to provide putative mechanistic arguments for their known structure-function experimental results. This model may provide new insights into how Vitamin D sterols that uncouple the unwanted hypercalcemic effect from attractive growth inhibitory/differentiation properties can do so by differentially stabilizing different subpopulations of VDR conformational ensemble members.