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

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.

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.

Catalytic properties of 25-hydroxyvitamin D3 3-epimerase in rat and human liver microsomes

25-Hydroxyvitamin D3 3-epimerase catalyzes the 3β → 3α epimerization of 25-hydroxyvitamin D3 (25(OH)D3) producing 3-epi-25-hydroxyvitamin D3 (3-epi-25(OH)D3). 3-Epi-25(OH)D3 is one of the most abundant forms of vitamin D present in the serum. It can be converted to 3-epi-1α,25-dihydroxyvitamin D3 by CYP27B1 which generally displays lower biological activity than 1α,25-dihydroxyvitamin D3 (1,25(OH)₂D3). The 25(OH)D3 3-epimerase has been poorly characterized to date and the gene encoding it has not been identified. The 3-epimerase has been reported to be present in the microsomal fraction of cells, including liver cells, and to use NADPH as cofactor. It can also act on 1,25(OH)₂D3 and 24,25(OH)₂D3 forming the 3α-epimers. In this study we have characterized the activity of the 25(OH)D3 3-epimerase in rat and human liver microsomes, using 25(OH)D3 as substrate and HPLC to analyze product formation. For both rat and human liver microsomes the preferred cofactor was NADH, with the rat enzyme displaying a 6-fold greater catalytic efficiency (V_max/K_m) for NADH over that for NADPH. No activity was observed with oxidized cofactor, either NAD⁺ or NADP⁺. This was unexpected since the initial step in the epimerization, predicted to be the oxidation of the 3β-OH to a ketone, would require oxidized cofactor. The rat 3-epimerase in microsomes gave a K_m for 25(OH)D3 of 14 μM. The reverse reaction, conversion of 3-epi-25(OH)D3 to 25(OH)D3, was catalyzed by both rat and human liver microsomes but at lower rates than the forward reaction. In conclusion, both rat and human 25-hydroxyvitamin D3 3-epimerase catalyze the reversible interconversion of 25(OH)D3 and 3-epi-25(OH)D3, and use NADH as the preferred cofactor. The lack of requirement for exogenous NAD⁺ suggests that the enzyme has a tightly bound NAD⁺ in its active site that is released only upon its reduction.

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.

Mass spectrometric profiling of vitamin D metabolites beyond 25-hydroxyvitamin D

BACKGROUND

The frequency of measurements of vitamin D in the human population has significantly increased over the last decade because vitamin D has now been linked to many diseases, in addition to its established role in bone health. Usually, serum 25-hydroxyvitamin D concentrations are measured to assess the vitamin D status of individuals. Unfortunately, many studies investigating links between vitamin D and disease also use only this single metabolite. Intricate correlations with other vitamin D metabolites or dynamic effects of downstream metabolites may therefore be overlooked. Fortunately, powerful LC-MS/MS approaches have recently become available that can simultaneously quantify the concentrations of multiple vitamin D metabolites. These approaches are challenging, however, because of inherent instrumental problems with detection of vitamin D compounds and the low concentrations of the metabolites in biological fluids.

CONTENT

This review summarizes recent mass spectrometry assays for the quantitative measurement of multiple vitamin D metabolites and their application in clinical research, with a particular focus on the low-abundance downstream metabolic species generated after the initial hydroxylation to 25-hydroxyvitamin D.

SUMMARY

To study the pathobiological effects and function of vitamin D metabolites in disease, in particular in low-abundance species beyond 25-hydroxyvitamin D, we need to know their concentrations. Although detection of these vitamin D species is challenging, a number of recent mass spectrometry assays have successfully demonstrated that LC-MS/MS methods can quantify multiple vitamin D compounds over a wide dynamic range individually or as part of multimetabolite assays.

Revisiting the roles of progesterone and allopregnanolone in the nervous system: resurgence of the progesterone receptors

Progesterone is commonly considered as a female reproductive hormone and is well-known for its role in pregnancy. It is less well appreciated that progesterone and its metabolite allopregnanolone are also male hormones, as they are produced in both sexes by the adrenal glands. In addition, they are synthesized within the nervous system. Progesterone and allopregnanolone are associated with adaptation to stress, and increased production of progesterone within the brain may be part of the response of neural cells to injury. Progesterone receptors (PR) are widely distributed throughout the brain, but their study has been mainly limited to the hypothalamus and reproductive functions, and the extra-hypothalamic receptors have been neglected. This lack of information about brain functions of PR is unexpected, as the protective and trophic effects of progesterone are much investigated, and as the therapeutic potential of progesterone as a neuroprotective and promyelinating agent is currently being assessed in clinical trials. The little attention devoted to the brain functions of PR may relate to the widely accepted assumption that non-reproductive actions of progesterone may be mainly mediated by allopregnanolone, which does not bind to PR, but acts as a potent positive modulator of γ-aminobutyric acid type A (GABA(A) receptors. The aim of this review is to critically discuss effects of progesterone on the nervous system via PR, and of allopregnanolone via its modulation of GABA(A) receptors, with main focus on the brain.

Progesterone synthesis in the nervous system: implications for myelination and myelin repair

Progesterone is well known as a female reproductive hormone and in particular for its role in uterine receptivity, implantation, and the maintenance of pregnancy. However, neuroendocrine research over the past decades has established that progesterone has multiple functions beyond reproduction. Within the nervous system, its neuromodulatory and neuroprotective effects are much studied. Although progesterone has been shown to also promote myelin repair, its influence and that of other steroids on myelination and remyelination is relatively neglected. Reasons for this are that hormonal influences are still not considered as a central problem by most myelin biologists, and that neuroendocrinologists are not sufficiently concerned with the importance of myelin in neuron functions and viability. The effects of progesterone in the nervous system involve a variety of signaling mechanisms. The identification of the classical intracellular progesterone receptors as therapeutic targets for myelin repair suggests new health benefits for synthetic progestins, specifically designed for contraceptive use and hormone replacement therapies. There are also major advantages to use natural progesterone in neuroprotective and myelin repair strategies, because progesterone is converted to biologically active metabolites in nervous tissues and interacts with multiple target proteins. The delivery of progesterone however represents a challenge because of its first-pass metabolism in digestive tract and liver. Recently, the intranasal route of progesterone administration has received attention for easy and efficient targeting of the brain. Progesterone in the brain is derived from the steroidogenic endocrine glands or from local synthesis by neural cells. Stimulating the formation of endogenous progesterone is currently explored as an alternative strategy for neuroprotection, axonal regeneration, and myelin repair.

Role of microsomal retinol/sterol dehydrogenase-like short-chain dehydrogenases/reductases in the oxidation and epimerization of 3alpha-hydroxysteroids in human tissues

Allopregnanolone (ALLO) and androsterone (ADT) are naturally occurring 3alpha-hydroxysteroids that act as positive allosteric regulators of gamma-aminobutyric acid type A receptors. In addition, ADT activates nuclear farnesoid X receptor and ALLO activates pregnane X receptor. At least with respect to gamma-aminobutyric acid type A receptors, the biological activity of ALLO and ADT depends on the 3alpha-hydroxyl group and is lost upon its conversion to either 3-ketosteroid or 3beta-hydroxyl epimer. Such strict structure-activity relationships suggest that the oxidation or epimerization of 3alpha-hydroxysteroids may serve as physiologically relevant mechanisms for the control of the local concentrations of bioactive 3alpha-hydroxysteroids. The exact enzymes responsible for the oxidation and epimerization of 3alpha-hydroxysteroids in vivo have not yet been identified, but our previous studies showed that microsomal nicotinamide adenine dinucleotide-dependent short-chain dehydrogenases/reductases (SDRs) with dual retinol/sterol dehydrogenase substrate specificity (RoDH-like group of SDRs) can oxidize and epimerize 3alpha-hydroxysteroids in vitro. Here, we present the first evidence that microsomal nicotinamide adenine dinucleotide-dependent 3alpha-hydroxysteroid dehydrogenase/epimerase activities are widely distributed in human tissues with the highest activity levels found in liver and testis and lower levels in lung, spleen, brain, kidney, and ovary. We demonstrate that RoDH-like SDRs contribute to the oxidation and epimerization of ALLO and ADT in living cells, and show that RoDH enzymes are expressed in tissues that have microsomal 3alpha-hydroxysteroid dehydrogenase/epimerase activities. Together, these results provide further support for the role of RoDH-like SDRs in human metabolism of 3alpha-hydroxysteroids and offer a new insight into the enzymology of ALLO and ADT inactivation.

Measurement and characterization of C-3 epimerization activity toward vitamin D3

Recently, epimerization of the hydroxyl group at C-3 has been identified as a unique metabolic pathway of vitamin D compounds. We measured C-3 epimerization activity in subcellular fractions prepared from cultured cells and investigated the basic properties of the enzyme responsible for the epimerization. C-3 epimerization activity was detected using a NADPH-generating system containing glucose-6-phosphate, NADP, glucose-6-phosphate dehydrogenase, and Mg(2+). The highest level of activity was observed in a microsomal fraction prepared from rat osteoblastic UMR-106 cells but activity was also observed in microsomal fractions prepared from MG-63, Caco-2, Hep G2, and HUH-7 cells. In terms of maximum velocity (V(max)) and the Michaelis constant (K(m)), 25-hydroxyvitamin D(3) [25(OH)D(3)] exhibited the highest specificity for the epimerization at C-3 among 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)], 25(OH)D(3), 24,25-dihydroxyvitamin D(3) [24,25(OH)(2)D(3)], and 22-oxacalcitriol (OCT). The epimerization activity was not inhibited by various cytochrome P450 inhibitors and antiserum against NADPH cytochrome P450 reductase. Neither CYP24, CYP27A1, CYP27B1 nor 3(alpha-->beta)hydroxysteroid epimerase (HSE) catalyzed the epimerization in vitro. Based on these results, the enzyme(s) responsible for the epimerization of vitamin D(3) at C-3 are thought to be located in microsomes and different from cytochrome P450 and HSE.