Characterization of new conjugated metabolites in bile of rats administered 24,25-dihydroxyvitamin D(3) and 25-hydroxyvitamin D(3)

Analysis of vitamin D metabolites in biological samples using a nanoluc-based vitamin D receptor ligand sensing system: NLucVDR

Many assays are currently being developed to measure the levels of vitamin D metabolites in various samples (such as blood, urine, and saliva). This study focused on the measurement of vitamin D metabolites in serum and urine using the NLucVDR assay system, which consists of a split-type nanoluciferase and ligand-binding domain (LBD) of the human vitamin D receptor. Blood and urine samples were collected from 23 participants to validate the NLucVDR assay. The 25(OH)D3 levels in the serum and urine determined by the NLucVDR assay showed good correlations with those determined by standard analytical methods (ECLIA for serum and LC-MS/MS for urine), with correlation coefficients of 0.923 and 0.844 for serum and urine samples, respectively. In the case of serum samples, 25(OH)D3 levels determined by the NLucVDR assay were in good agreement with those determined by ECLIA. Therefore, the NLucVDR assay is a useful tool for measuring serum 25(OH)D3 levels. The contribution of each vitamin D metabolite to the luminescence intensity obtained during the NLucVDR assay depends on its concentration and affinity for NLucVDR. Thus, the contribution of 25(OH)D3 in serum appears to be much higher than that of the other metabolites. In contrast, the 25(OH)D3 levels in the urine determined by the NLucVDR assay were more than 20-fold higher than those determined by a standard analytical method (LC-MS/MS), suggesting that some vitamin D metabolite(s) in the urine remarkably increased the luminescence intensity of the NLucVDR assay. Notably, the 25(OH)D3 concentration in the urine determined by the NLucVDR assay and the serum 25(OH)D3 concentration determined by standard analytical methods showed a significant positive correlation (r = 0.568). These results suggest that the analysis of a small amount of urine using the NLucVDR assay may be useful for predicting the serum 25(OH)D3 levels.

23,25-Dihydroxyvitamin D<sub>3</sub> is liberated as a major vitamin D<sub>3</sub> metabolite in human urine after treatment with β-glucuronidase: Quantitative comparison with 24,25-dihydroxyvitamin D<sub>3</sub> by LC/MS/MS

The complete understanding of the excretion of surplus 25-hydroxyvitamin D<sub>3</sub> [25(OH)D<sub>3</sub>] in humans remains to be accomplished. In our previous study, 24,25-dihydroxyvitamin D<sub>3</sub> [24,25(OH)<sub>2</sub>D<sub>3</sub>] 24-glucuronide was identified as a major urinary vitamin D<sub>3</sub> metabolite, while the glucuronide of 23,25-dihydroxyvitamin D<sub>3</sub> [23,25(OH)<sub>2</sub>D<sub>3</sub>] is another metabolite of interest but has not been sufficiently evaluated. Although the quantitative analysis of 24,25(OH)<sub>2</sub>D<sub>3</sub> liberated in urine by the treatment with β-glucuronidase (GUS) has been conducted, no information was provided about the amount of the glucuronidated 23,25(OH)<sub>2</sub>D<sub>3</sub> in the urine. In this study, we first developed and validated a liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS)-based method for the simultaneous quantification of 23,25(OH)<sub>2</sub>D<sub>3</sub> and 24,25(OH)<sub>2</sub>D<sub>3</sub> liberated in urine by GUS. The analysis of the urine samples revealed that the amount of 23,25(OH)<sub>2</sub>D<sub>3</sub> was almost as much as that of 24,25(OH)<sub>2</sub>D<sub>3</sub>, in contrast to the fact that the plasma concentration of 23,25(OH)<sub>2</sub>D<sub>3</sub> was much lower than that of 24,25(OH)<sub>2</sub>D<sub>3</sub>. These results strongly suggested that 23,25(OH)<sub>2</sub>D<sub>3</sub> is more susceptible to glucuronidation and more promptly excreted into urine than 24,25(OH)<sub>2</sub>D<sub>3</sub>. Furthermore, the amount ratios of 23,25(OH)<sub>2</sub>D<sub>3</sub> to 24,25(OH)<sub>2</sub>D<sub>3</sub> in the urine samples did not markedly vary during the day (morning/evening) and even by a week-long vitamin D<sub>3</sub> supplementation (1000 IU/body/day). We concluded that the C-23 hydroxylation plays a crucial role in the urinary excretion of surplus 25(OH)D<sub>3</sub>.

The vitamin D metabolome: An update on analysis and function

Current understanding of vitamin D tends to be focussed on the measurement of the major circulating form 25-hydroxyvitamin D3 (25OHD3) and its conversion to the active hormonal form, 1α,25-dihydroxyvitamin D3 (1α,25(OH)₂ D3) via the enzyme 25-hydroxyvitamin D-1α-hydroxylase (CYP27B1). However, whilst these metabolites form the endocrine backbone of vitamin D physiology, it is important to recognise that there are other metabolic and catabolic pathways that are now recognised as being crucially important to vitamin D function. These pathways include C3-epimerization, CYP24A1 hydroxylase, CYP11A1 alternative metabolism of vitamin D3, and phase II metabolism. Endogenous metabolites beyond 25OHD3 are usually present at low endogenous levels and may only be functional in specific target tissues rather than in the general circulation. However, the technologies available to measure these metabolites have also improved, so that measurement of alternative vitamin D metabolic pathways may become more routine in the near future. The aim of this review is to provide a comprehensive overview of the various pathways of vitamin D metabolism, as well as describe the analytical techniques currently available to measure these vitamin D metabolites.

Simultaneous quantification of 25-hydroxyvitamin D3-3-sulfate and 25-hydroxyvitamin D3-3-glucuronide in human serum and plasma using liquid chromatography-tandem mass spectrometry coupled with DAPTAD-derivatization

25-hydroxyvitamin D₃-3-sulfate (25-OHD₃-S) and 25-hydroxyvitamin D₃-3-glucuronide (25-OHD₃-G) are major conjugative metabolites of vitamin D₃ found in the systemic circulation and potentially important reservoirs for 25-hydroxyvitamin D₃. Simultaneous and accurate quantification of these metabolites could advance assessment of the impact of vitamin D₃ on health and disease. In this study, a highly sensitive and accurate liquid chromatography-tandem mass spectrometry (LC-MS/MS) method was developed and validated for simultaneous quantification of 25-OHD₃-S and 25-OHD₃-G in human serum or plasma. Following protein precipitation, the analytes of interest were partially purified by solid-phase extraction and subjected to derivatization with 4-(4'-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD). Quantification of the analytes was based on multiple reaction monitoring (MRM) operated in the positive ion mode, and deuterated internal standards were used for each conjugative metabolite. Applying this method to the analysis of 25-OHD₃-S and 25-OHD₃-G concentrations in human serum or plasma samples achieved satisfactory reproducibility, accuracy and sensitivity. We subsequently used this method to simultaneously determine serum concentrations of the two metabolites in archived samples from a rifampin treatment study. Drug treatment had no effect on metabolite concentrations, but significantly increased the 25-OHD₃-S/25-OHD₃ concentration ratio (p=0.01). The availability of this new method should improve sample throughput and our ability to quantify and monitor circulating 25-OHD₃-S and 25-OHD₃-G concentrations.

A Novel Facile Synthesis of Cholesta-5,7-Diene-3β,25-Diol, the Precursor of 25-Hydroxyvitamin D3

An efficient four-step procedure is described for the synthesis of cholesta-5,7-diene-3β,25-diol from 7-dehydrodesmosterol in an overall yield of 72.4%. The 3-hydroxy group of 7-dehydrodesmosterol is protected as the acetate and the 5,7-diene system as a hetero Diels–Alder adduct with 4-phenyl-1,2,4-triazoline-3,5-dione. The key step in this synthesis is a very mild method for the hydroxybromination of the Diels–Alder adduct with N-bromosuccinimide in water, followed by reduction and deprotection with LiAlH4. This method gives cholesta-5,7-diene-3β,25-diol in a better yield than those reported previously and the reagents that are used are all relatively cheap, non-toxic and stable.

Interplay between vitamin D and the drug metabolizing enzyme CYP3A4

Cytochrome P450 3A4 (CYP3A4) is a multifunctional enzyme involved in both xenobiotic and endobiotic metabolism. This review focuses on two aspects: regulation of CYP3A4 expression by vitamin D and metabolism of vitamin D by CYP3A4. Enterohepatic circulation of vitamin D metabolites and their conjugates will be also discussed. The interplay between vitamin D and CYP3A4 provides new insights into our understanding of how enzyme induction can contribute to vitamin D deficiency. This article is part of a Special Issue entitled 'Vitamin D Workshop'.

Importance of apical membrane delivery of 1,25-dihydroxyvitamin D3 to vitamin D-responsive gene expression in the colon

Synthetic conjugation of a glucuronide to 1,25-dihydroxyvitamin D3 (1,25D3) to produce β-25-monoglucuronide-1,25D3 (βGluc-1,25D3) renders the hormone biologically inactive and resistant to mammalian digestive enzymes. However, β-glucuronidase produced by bacteria in the lower intestinal tract can cleave off the glucuronide, releasing the active hormone. In mice given a single oral dose of 1,25D3, 24-hydroxylase (Cyp24a1) gene expression was strongly enhanced in the duodenum, but not in the colon, despite circulating concentrations of 1,25D3 that peaked at ∼3.0 nmol/l. In contrast, in mice treated with an equimolar dose of βGluc-1,25D3, Cyp24a1 gene expression increased 700-fold in the colon but was significantly weaker in the duodenum compared with mice treated with 1,25D3. Similar results were observed with another vitamin D-dependent gene. When administered subcutaneously, 1,25D3 weakly stimulated colon Cyp24a1 gene expression while βGluc-1,25D3 again resulted in strong enhancement. Surgical ligation to block passage of ingesta beyond the upper intestinal tract abolished upregulation of colon Cyp24a1 gene expression by orally and subcutaneously administered βGluc-1,25D3. Feeding βGluc-1,25D3 for 5 days revealed a linear, dose-dependent increase in colon Cyp24a1 gene expression but did not significantly increase plasma 1,25D3 or calcium concentrations. This study indicates that the colon is relatively insensitive to circulating concentrations of 1,25D3 and that the strongest gene enhancement occurs when the hormone reaches the colon via the lumen of the intestinal tract. These findings have broad implications for the use of vitamin D compounds in colon disorders and set the stage for future therapeutic studies utilizing βGluc-1,25D3 in their treatment.

Identification of human UDP-glucuronosyltransferases catalyzing hepatic 1alpha,25-dihydroxyvitamin D3 conjugation

The biological effects of 1alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) are terminated primarily by P450-dependent hydroxylation reactions. However, the hormone is also conjugated in the liver and a metabolite, presumably a glucuronide, undergoes enterohepatic cycling. In this study, the identity of human enzymes capable of catalyzing the 1,25(OH)2D3 glucuronidation reaction was investigated in order to better understand environmental and endogenous factors affecting the disposition and biological effects of vitamin D3. Among 12 different UGT isozymes tested, only UGT1A4 >> 2B4 and 2B7 supported the reaction. Two different 1,25(OH)2D3 monoglucuronide metabolites were generated by recombinant UGT1A4 and human liver microsomes. The most abundant product was identified by mass spectral and NMR analyses as the 25-O-glucuronide isomer. The formation of 25-O-glucuronide by UGT1A4 Supersomes and human liver microsomes followed simple hyperbolic kinetics, yielding respective Km and Vmax values of 7.3 and 11.2 microM and 33.7 +/- 1.4 and 32.9 +/- 1.9 pmol/min/mg protein. The calculated intrinsic 25-O-glucuronide M1 formation clearance for UGT1A4 was 14-fold higher than the next best isozyme, UGT2B7. There was only limited (four-fold) inter-liver variability in the 25-O-glucuronidation rate, but it was highly correlated with the relative rate of formation of the second, minor metabolite. In addition, formation of both metabolites was inhibited >80% by the selective UGT1A4 inhibitor, hecogenin. If enterohepatic recycling of 1,25(OH)2D3 represents a significant component of intestinal and systemic 1,25(OH)2D3 disposition, formation of monoglucuronides by hepatic UGT1A4 constitutes an important initial step.

Development of a HPLC/tandem-MS method for the analysis of the larvicides methoprene, hydroprene, and kinoprene at trace levels using Diels-Alder derivatization

The invasion and subsequent spread of the mosquito-borne West Nile virus in the United States has resulted in increased use of methoprene. With the increased need for sensitive detection and monitoring of methoprene in the environment, an analytical LC/ESI-MS/MS method has been developed for the analysis of methoprene and two analogues, kinoprene and hydroprene, in water. To improve the ionization efficiency of the nonpolar analytes, a derivatization step with the Cookson-type reagent 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) was used. Derivatization improved the limit of detection 100-fold. For tandem MS analyses, limits of detection in environmental water samples (S/N = 3) are about 6 pg/mL for methoprene and 20 pg/mL for kinoprene and hydroprene, resulting in limits of quantification (S/N = 10) of 20 pg/mL for methoprene and 60 pg/mL for hydroprene and kinoprene extracted from 10 mL of water. This method was applied to measure methoprene concentrations in water samples from a treated site.

Analysis of C-3 epimerization in (24R)-24,25-dihydroxyvitamin D3 catalyzed by hydroxysteroid dehydrogenase

Studies on the C-3 epimerization in (24R)-24,25-dihydroxyvitamin D(3) [24R,25(OH)(2)D(3)] were performed using hydroxysteroid dehydrogenases (HSDs). 3-Epi-24R,25(OH)(2)D(3) was formed from 24R,25(OH)(2)D(3) by the catalysis of 3alpha- or beta-HSD. These HSDs also catalyzed the C-3 epimerization in 3-epi-24R,25(OH)(2)D(3) to form 24R,25(OH)(2)D(3). 24R,25(OH)(2)D(3) and its C-3 epimer were separated by inclusion high-performance liquid chromatography using gamma-cyclodextrin (gamma-CD) as the mobile phase additive or a gamma-CD bonded chiral column. The product derived from the intermediate during the C-3 epimerization was isolated from the incubation specimens and identified as (7Z)-(24R)-24,25-dihydroxy-9,10-secocholesta-4,7,10(19)-trien-3-one by several instrumental analyses including (1)H-nuclear magnetic resonance spectrometry. The occurrence of this compound strongly proves that the formation of the C-3 epimer by HSD involves a dehydrogenation process. The present study suggests that HSDs may catalyze the C-3 epimerization of vitamin D compounds and modulate their concentrations and biological activities in animals and humans.

Recent progress in derivatization methods for LC and CE analysis

The derivatization procedure with a suitable fluorescence or chemiluminescence reagent is performed for the purpose of increasing the detection sensitivity and selectivity, in high-performance liquid chromatography (HPLC) and/or capillary electrophoresis (CE). In this article, recent derivatization methods and their applications to biosamples are described. In HPLC, femto mol order of mass detection limits are obtained by derivatization. Regarding the fluorescence reagents, the use of water-soluble reagents has been effective to avoid an undesired adsorption in the process of determination of peptides. In CE, the advantages of having extremely low mass detection limits (ranging from atto to yocto mol level) and requiring only a very short analysis time (less than a few minutes) are made possible by using laser-induced fluorescence or near infra-red detections.

Characterization of urinary metabolites of vitamin D(3) in man under physiological conditions using liquid chromatography-tandem mass spectrometry

The characterization of the urinary metabolites of vitamin D(3) in man under physiological conditions was performed using liquid chromatography-tandem mass spectrometry (LC-MS-MS). The urine specimens obtained from healthy volunteers were treated with beta-glucuronidase, purified with disposal solid-phase extraction cartridges, derivatized with a Cookson-type reagent, 4-[2-(6,7-dimethoxy-4-methyl-3-oxo-3,4-dihydroquinoxalyl)ethyl]-1,2,4-triazoline-3,5-dione, and subjected to LC-MS-MS. The derivatization was employed to increase the ionization efficiencies of the vitamin D(3) metabolites, which enabled detection of the metabolites in the picogram range. The identification of the genin parts of the metabolites was done by comparison with authentic samples based on their LC-MS-MS data. The glucuronides of 23S,25-dihydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) were obtained as the main metabolites from the urine in almost equal amounts. In contrast to the fact that the plasma/serum concentration of the former is much lower than that of the latter, the hydroxylation at the C-23 position was considered to be the important side-chain modification of 25(OH)D(3) to excrete the excess vitamin D(3) in man. In addition, 23S,25-dihydroxy-24-oxovitamin D(3) occurred as its glucuronide in most of the urine, which suggested that this metabolite also plays a part in the excretion of vitamin D(3) in man.

Gas chromatography and high-performance liquid chromatography of natural steroids

This review article underlines the importance of gas chromatography (GC), high-performance liquid chromatography (HPLC) and their hyphenated techniques using mass spectrometry (MS) for the determination of natural steroids, especially in human biological fluids. Steroids are divided into eight categories based on their structures and functions, and recent references using the above methodologies for the analysis of these steroids are cited. GC and GC-MS are commonly used for the determination of volatile steroids. Although HPLC is a widely used analytical method for the determination of steroids including the conjugated type in biological fluids, LC-MS is considered to be the most promising one for this purpose because of its sensitivity, specificity and versatility.