Catalyzed thermal isomerization between previtamin D3 and vitamin D3 via beta-cyclodextrin complexation

Unraveling the Catalytic Mechanism of β-Cyclodextrin in the Vitamin D Formation

Previous experimental studies have shown that the isomerization reaction of previtamin D3 (PreD3) to vitamin D3 (VitD3) is accelerated 40-fold when it takes place within a β-cyclodextrin dimer, in comparison to the reaction occurring in conventional isotropic solutions. In this study, we employ quantum mechanics-based molecular dynamics (MD) simulations and statistical multistructural variational transition state theory to unveil the origin of this acceleration. We find that the conformational landscape in the PreD3 isomerization is highly dependent on whether the system is encapsulated. In isotropic media, the triene moiety of the PreD3 exhibits a rich torsional flexibility. However, when encapsulated, such a flexibility is limited to a more confined conformational space. In both scenarios, our calculated rate constants are in close agreement with experimental results and allow us to identify the PreD3 flexibility restriction as the primary catalytic factor. These findings enhance our understanding of VitD3 isomerization and underscore the significance of MD and environmental factors in biochemical modeling.

Highly selective whole-cell 25-hydroxyvitamin D3 synthesis using molybdenum-dependent C25-steroid dehydrogenase and cyclodextrin recycling

BACKGROUND

The global prevalence of vitamin D (VitD) deficiency associated with numerous acute and chronic diseases has led to strategies to improve the VitD status through dietary intake of VitD-fortified foods and VitD supplementation. In this context, the circulating form of VitD3 (cholecalciferol) in the human body, 25-hydroxy-VitD3 (calcifediol, 25OHVitD3), has a much higher efficacy in improving the VitD status, which has motivated researchers to develop methods for its effective and sustainable synthesis. Conventional monooxygenase-/peroxygenase-based biocatalytic platforms for the conversion of VitD3 to value-added 25OHVitD3 are generally limited by a low selectivity and yield, costly reliance on cyclodextrins and electron donor systems, or by the use of toxic co-substrates.

RESULTS

In this study, we used a whole-cell approach for biocatalytic 25OHVitD3 synthesis, in which a molybdenum-dependent steroid C25 dehydrogenase was produced in the denitrifying bacterium Thauera aromatica under semi-aerobic conditions, where the activity of the enzyme remained stable. This enzyme uses water as a highly selective VitD3 hydroxylating agent and is independent of an electron donor system. High density suspensions of resting cells producing steroid C25 dehydrogenase catalysed the conversion of VitD3 to 25OHVitD3 using either O2 via the endogenous respiratory chain or externally added ferricyanide as low cost electron acceptor. The maximum 25OHVitD3 titer achieved was 1.85 g L-1 within 50 h with a yield of 99%, which is 2.2 times higher than the highest reported value obtained with previous biocatalytic systems. In addition, we developed a simple method for the recycling of the costly VitD3 solubiliser cyclodextrin, which could be reused for 10 reaction cycles without a significant loss of quality or quantity.

CONCLUSIONS

The established steroid C25 dehydrogenase-based whole-cell system for the value-adding conversion of VitD3 to 25OHVitD3 offers a number of advantages in comparison to conventional oxygenase-/peroxygenase-based systems including its high selectivity, independence from an electron donor system, and the higher product titer and yield. Together with the established cyclodextrin recycling procedure, the established system provides an attractive platform for large-scale 25OHVitD3 synthesis.

Vitamin D and depression in older adults: lessons learned from observational and clinical studies

Depression is a mental disorder triggered by the interaction of social, psychological and biological factors that have an important impact on an individual's life. Despite being a well-studied disease with several established forms of treatment, its prevalence is increasing, especially among older adults. New forms of treatment and prevention are encouraged, and some researchers have been discussing the effects of vitamin D (VitD) on depression; however, the exact mechanism by which VitD exerts its effects is not yet conclusive. In this study, we aimed to discuss the possible mechanisms underlying the association between VitD and depression in older adults. Therefore, we conducted a systematic search of databases for indexed articles published until 30 April 2021. The primary focus was on both observational studies documenting the association between VitD and depression/depressive symptoms, and clinical trials documenting the effects of VitD supplementation on depression/depressive symptoms, especially in older adults. Based on pre-clinical, clinical and observational studies, it is suggested that the maintenance of adequate VitD concentrations is an important issue, especially in older adults, which are a risk population for both VitD deficiency and depression. Nevertheless, it is necessary to carry out more studies using longitudinal approaches in low- and middle-income countries to develop a strong source of evidence to formulate guidelines and interventions.

The Sunlight-Vitamin D Connection: Implications for Patient Outcomes in the Surgical Intensive Care Unit

This review delves into the intricate relationship between Vitamin D and patient outcomes in the Surgical Intensive Care Unit (SICU). Vitamin D, known for its multifaceted roles in immune modulation, inflammation regulation, and maintenance of calcium homeostasis, emerges as a pivotal factor in the care of critically ill patients. Our exploration reveals a high prevalence of Vitamin D deficiency in the SICU, primarily attributable to limited sunlight exposure, comorbidities, and medication use. Importantly, Vitamin D status impacts infection rates, mortality, and length of stay in the SICU, making it a clinically relevant consideration. Mechanistic insights into the immunomodulatory and anti-inflammatory effects of Vitamin D shed light on its potential benefits in critical care. However, challenges, including accurate assessment, individualised supplementation, and ethical considerations regarding sunlight exposure, are evident. The prospect of personalised Vitamin D supplementation strategies offers promise for optimising patient care. In conclusion, the Sunlight-Vitamin D Connection holds significant potential to improve outcomes in the SICU, emphasising the importance of further research and tailored approaches for the well-being of critically ill individuals.

Vitamin D and Vitamin D-Binding Protein in Health and Disease

Vitamin D is a fat-soluble secosteroid that exists in two forms: vitamin D₂ and vitamin D₃ [...].

Vitamin D3 Metabolic Enzymes in Plateau Zokor (Myospalax baileyi) and Plateau Pika (Ochotona curzoniae): Expression and Response to Hypoxia

Vitamin D3 (D3) is produced endogenously from 7-dehydrocholesterol by irradiation and is an important secosteroid for the absorption of calcium and phosphate. Lithocholic acid (LCA) increases intestinal paracellular calcium absorption in a vitamin D receptor-dependent manner in vitamin D-deficient rats. The plateau zokor (Myospalax baileyi), a strictly subterranean species, and plateau pika are endemic to the Qinghai-Tibet Plateau. To verify whether the zokors were deficient in D3 and reveal the effects of hypoxia on D3 metabolism in the zokors and pikas, we measured the levels of 25(OH)D3, calcium, and LCA, and quantified the expression levels of D3 metabolism-related genes. The results showed an undetectable serum level of 25(OH)D3 and a significantly higher concentration of LCA in the serum of plateau zokor, but its calcium concentration was within the normal range compared with that of plateau pika and Sprague-Dawley rats. With increasing altitude, the serum 25(OH)D3 levels in plateau pika decreased significantly, and the mRNA and protein levels of CYP2R1 (in the liver) and CYP27B1 (in the kidney) in plateau pika decreased significantly. Our results indicate that plateau zokors were deficient in D3 and abundant in LCA, which might be a substitution of D3 in the zokor. Furthermore, hypoxia suppresses the metabolism of D3 by down-regulating the expression of CYP2R1 and CYP27B1 in plateau pika.

Quantitative NMR (qNMR) spectroscopy based investigation of the absolute content, stability and isomerization of 25-hydroxyvitamin D2/D3 and 24(R),25-dihydroxyvitamin D2 in solution phase

Vitamin D is an important parameter, in serum/plasma based diagnostic analysis, for the determination of optimal regulation of calcium and phosphate homeostases in the human body, vital for the monitoring/progression of osteomalacia and rickets. Particularly, the quantification of 25-hydroxyvitamin D2, 25-hydroxyvitamin D3 and 24R,25-dihydroxyvitamin D in blood is an excellent indicator for the vitamin D status of a patient. For this purpose, LC–MS/MS methods, based on appropriate vitamin D reference standards, are considered to be ‘gold standard’ for such measurements. We have utilized quantitative NMR spectroscopy to determine the absolute content of these molecules, available as non-certified chemicals, and have determined the stability of these callibrators in borderline polar solvents at room temperature. We have observed significant isomerization of the analytes, which can play a big role in quantification of these analytes by hyphenated LC and GC analytical techniques. Appropriate explanations are given for the observation of new impurities with time in solution phase. The spin system selected for quantitation was determined using relevant 1D and 2D NMR pulse sequences. The advantage of the qNMR approach is that it is based on the quantification of atoms rather than molecular properties (e.g., quantitation by LC/UV, GC, etc.). Since the signals in an NMR spectrum are different nuclear spin-systems dispersed precisely in a magnetic environment, with the intensity being directly proportional to the amount of a particular type of nuclear spin, this technique delivers unparalleled information about the chemical structure and the absolute content.

An Update on Vitamin D Metabolism

Vitamin D is a steroid hormone classically involved in the calcium metabolism and bone homeostasis. Recently, new and interesting aspects of vitamin D metabolism has been elucidated, namely the special role of the skin, the metabolic control of liver hydroxylase CYP2R1, the specificity of 1α-hydroxylase in different tissues and cell types and the genomic, non-genomic and epigenomic effects of vitamin D receptor, which will be addressed in the present review. Moreover, in the last decades, several extraskeletal effects which can be attributed to vitamin D have been shown. These beneficial effects will be here summarized, focusing on the immune system and cardiovascular system.

Differential Thermal Isomerization: Its Role in the Analysis of Vitamin D3 in Foods

Background

For nutritional purposes, the measurement of vitamin D3 (defined as the sum of vitamin D3 and previtamin D3) is required to obtain an accurate and reliable estimate of its content in foods. An often neglected aspect in the development of methods for the analysis of vitamin D3 is accounting for any potential analytical bias in the results associated with differential thermal isomerization between previtamin D and vitamin D.

Conclusions

For LC-UV methods using a vitamin D2 internal standard, cold saponification, or direct lipid extraction techniques should be avoided, unless chromatographic separation of vitamin D2, vitamin D3, and their previtamin forms is achieved so that UV absorbance corrections can be made. For both LC-UV and LC-MS methods using calciferol internal standards, the simplest solution to avoid analytical bias due to the presence of previtamin D is to utilize heating conditions (typically during saponification) such that previtamin D and vitamin D in the sample and the internal standard reach an equivalent equilibrium state prior to instrumental analysis. Only under such circumstances is the integration of previtamin D unnecessary to obtain accurate results for vitamin D3.

Highlights

A detailed discussion of the quantitation of vitamin D3 in food with concise recommendations for avoiding measurement bias as a consequence of differential thermal isomerization.

New aspects of vitamin D metabolism and action - addressing the skin as source and target

Vitamin D has a key role in stimulating calcium absorption from the gut and promoting skeletal health, as well as many other important physiological functions. Vitamin D is produced in the skin. It is subsequently metabolized to its hormonally active form, 1,25-dihydroxyvitamin D (1,25(OH)₂D), by the 1-hydroxylase and catabolized by the 24-hydroxylase. In this Review, we pay special attention to the effect of mutations in these enzymes and their clinical manifestations. We then discuss the role of vitamin D binding protein in transporting vitamin D and its metabolites from their source to their targets, the free hormone hypothesis for cell entry and HSP70 for intracellular transport. This is followed by discussion of the vitamin D receptor (VDR) that mediates the cellular actions of 1,25(OH)₂D. Cell-specific recruitment of co-regulatory complexes by liganded VDR leads to changes in gene expression that result in distinct physiological actions by 1,25(OH)₂D, which are disrupted by mutations in the VDR. We then discuss the epidermis and hair follicle, to provide a non-skeletal example of a tissue that expresses VDR that not only makes vitamin D but also can metabolize it to its hormonally active form. This enables vitamin D to regulate epidermal differentiation and hair follicle cycling and, in so doing, to promote barrier function, wound healing and hair growth, while limiting cancer development.

The role of UVR and vitamin D on T cells and inflammatory bowel disease

Vitamin D deficiency is associated with the development of inflammatory bowel disease (IBD). In experimental IBD the targets of vitamin D that result in protection from IBD include gut epithelial cells, innate immune cells, T cells, and the microbiota. Ultraviolet radiation (UVR) induces production of vitamin D in the skin and suppresses T cell responses in the host. There is limited data demonstrating an effect of UVR on experimental IBD but the mechanisms of UVR suppression in IBD have not been defined. There are several shared effects of vitamin D and UVR on T cells including inhibition of proliferation and suppression of IFN-γ and IL-17 producing T cells. Conversely UVR decreases and vitamin D increases IL-4 production from T cells. Together the data suggest that UVR suppression of T cells and potentially IBD are both vitamin D dependent and independent.

Four Molybdenum-Dependent Steroid C-25 Hydroxylases: Heterologous Overproduction, Role in Steroid Degradation, and Application for 25-Hydroxyvitamin D3 Synthesis

Side chain-containing steroids are ubiquitous constituents of biological membranes that are persistent to biodegradation. Aerobic, steroid-degrading bacteria employ oxygenases for isoprenoid side chain and tetracyclic steran ring cleavage. In contrast, a Mo-containing steroid C-25 dehydrogenase (S25DH) of the dimethyl sulfoxide (DMSO) reductase family catalyzes the oxygen-independent hydroxylation of tertiary C-25 in the anaerobic, cholesterol-degrading bacterium Sterolibacterium denitrificans Its genome contains eight paralogous genes encoding active site α-subunits of putative S25DH-like proteins. The difficult enrichment of labile, oxygen-sensitive S25DH from the wild-type bacteria and the inability of its active heterologous production have largely hampered the study of S25DH-like gene products. Here we established a heterologous expression platform for the three structural genes of S25DH subunits together with an essential chaperone in the denitrifying betaproteobacterium Thauera aromatica K172. Using this system, S25DH₁ and three isoenzymes (S25DH₂, S25DH₃, and S25DH₄) were overproduced in a soluble, active form allowing a straightforward purification of nontagged αβγ complexes. All S25DHs contained molybdenum, four [4Fe-4S] clusters, one [3Fe-4S] cluster, and heme B and catalyzed the specific, water-dependent C-25 hydroxylations of various 4-en-3-one forms of phytosterols and zoosterols. Crude extracts from T. aromatica expressing genes encoding S25DH₁ catalyzed the hydroxylation of vitamin D₃ (VD₃) to the clinically relevant 25-OH-VD₃ with >95% yield at a rate 6.5-fold higher than that of wild-type bacterial extracts; the specific activity of recombinant S25DH₁ was twofold higher than that of wild-type enzyme. These results demonstrate the potential application of the established expression platform for 25-OH-VD₃ synthesis and pave the way for the characterization of previously genetically inaccessible S25DH-like Mo enzymes of the DMSO reductase family.IMPORTANCE Steroids are ubiquitous bioactive compounds, some of which are considered an emerging class of micropollutants. Their degradation by microorganisms is the major process of steroid elimination from the environment. While oxygenase-dependent steroid degradation in aerobes has been studied for more than 40 years, initial insights into the anoxic steroid degradation have only recently been obtained. Molybdenum-dependent steroid C₂₅ dehydrogenases (S25DHs) have been proposed to catalyze oxygen-independent side chain hydroxylations of globally abundant zoo-, phyto-, and mycosterols; however, so far, their lability has allowed only the initial characterization of a single S25DH. Here we report on a heterologous gene expression platform that allowed for easy isolation and characterization of four highly active S25DH isoenzymes. The results obtained demonstrate the key role of S25DHs during anoxic degradation of various steroids. Moreover, the platform is valuable for the efficient enzymatic hydroxylation of vitamin D₃ to its clinically relevant C-25-OH form.

Electrocatalytic Hydroxylation of Sterols by Steroid C25 Dehydrogenase from Sterolibacterium denitrificans

The electrochemically driven catalysis of the complex molybdoenzyme steroid C25 dehydrogenase (S25DH) from the β-Proteobacterium Sterolibacterium denitrificans is reported. S25DH catalyses the oxygen-independent regioselective hydroxylation of the tertiary C25 atom of sterols and also their derivatives. Cholest-4-en-3-one is a native substrate for S25DH, which produces 25-hydroxycholest-4-en-3-one as a product of catalytic turnover. Cholecalciferol (vitD₃ ) is also a substrate. S25DH was immobilised on a modified gold working electrode with the co-adsorbent chitosan. The complexes ferricyanide ([Fe(CN)₆ ]^3- ) and ferrocenium methanol (FM⁺ ) are effective artificial electron acceptors from S25DH and act as mediators of electron transfer between the electrode and the enzyme. 2-Hydroxypropyl-β-cyclodextrin (HPCD) was employed as a sterol solubiliser, in addition to 2-methoxyethanol. The catalytic activity varied, depending upon the concentration of solubiliser in the reaction mixture. Parallel studies with [Fe(CN)₆ ]^3- as a chemical (as opposed to electrochemical) oxidant coupled to HPLC analysis show that S25DH is capable of oxidising both vitD₃ and its less stable isomer, pre-vitD₃ , and that the former substrate is stabilised by HPCD.

Endocrine actions of vitamin D in skin: Relevance for photocarcinogenesis of non-melanoma skin cancer, and beyond

The skin represents a pivotal organ for the human body's vitamin D endocrine system, being both the site of ultraviolet (UV)-B-induced vitamin D synthesis and a target tissue for the pluripotent effects of 1,25(OH)₂D₃ and other biologically active vitamin D metabolites. As many other steroid hormones, 1,25(OH)₂D₃ exerts its effects via two independent signal transduction pathways: the classical genomic and the non-genomic pathway. While non-genomic effects of 1,25(OH)₂D₃ are in part exerted via effects on intracellular calcium, genomic effects are mediated by the vitamin D receptor (VDR). Recent findings convincingly support the concept of a new function of the VDR as a tumor suppressor in skin, with key components of the vitamin D endocrine system, including VDR, CYP24A1, CYP27A1, and CYP27B1 being strongly expressed in non-melanoma skin cancer (NMSC). It has now been shown that anti-tumor effects of VDR, that include some of its ligand-induced growth-regulatory effects, are at least in part mediated by interacting in a highly coordinated manner with the p53 family (p53/p63/p73) in response to a large number of alterations in cell homeostasis, including UV-induced DNA damage, a hallmark for skin photocarcinogenesis. Considering the relevance of the vitamin D endocrine system for carcinogenesis of skin cancer, it is not surprising that low 25(OH)D serum concentrations and genetic variants (SNPs) of the vitamin D endocrine system have been identified as potential risk factors for occurrence and prognosis of skin malignancies. In conclusion, an increasing body of evidence now convincingly supports the concept that the vitamin D endocrine system is of relevance for photocarcinogenesis and progression of NMSC and that its pharmacologic modulation by vitamin D, 1,25(OH)₂D_3, and analogs represents a promising new strategy for prevention and/or treatment of these malignancies.

25-Hydroxyvitamin D3 Synthesis by Enzymatic Steroid Side-Chain Hydroxylation with Water

The hydroxylation of vitamin D3 (VD3, cholecalciferol) side chains to give 25-hydroxyvitamin D3 (25OHVD3) is a crucial reaction in the formation of the circulating and biologically active forms of VD3 . It is usually catalyzed by cytochrome P450 monooxygenases that depend on complex electron donor systems. Cell-free extracts and a purified Mo enzyme from a bacterium anaerobically grown with cholesterol were employed for the regioselective, ferricyanide-dependent hydroxylation of VD3 and proVD3 (7-dehydrocholesterol) into the corresponding tertiary alcohols with greater than 99 % yield. Hydroxylation of VD3 strictly depends on a cyclodextrin-assisted isomerization of VD3 into preVD3 , the actual enzymatic substrate. This facile and robust method developed for 25OHVD3 synthesis is a novel example for the concept of substrate-engineered catalysis and offers an attractive alternative to chemical or O2  /electron-donor-dependent enzymatic procedures.

Metabolism of 20-hydroxyvitamin D3 and 20,23-dihydroxyvitamin D3 by rat and human CYP24A1

CYP11A1 hydroxylates vitamin D3 producing 20S-hydroxyvitamin D3 [20(OH)D3] and 20S,23-dihydroxyvitamin D3 [20,23(OH)2D3] as the major and most characterized metabolites. Both display immuno-regulatory and anti-cancer properties while being non-calcemic. A previous study indicated 20(OH)D3 can be metabolized by rat CYP24A1 to products including 20S,24-dihydroxyvitamin D3 [20,24(OH)2D3] and 20S,25-dihydroxyvitamin D3, with both producing greater inhibition of melanoma colony formation than 20(OH)D3. The aim of this study was to characterize the ability of rat and human CYP24A1 to metabolize 20(OH)D3 and 20,23(OH)2D3. Both isoforms metabolized 20(OH)D3 to the same dihydroxyvitamin D species with no secondary metabolites being observed. Hydroxylation at C24 produced both enantiomers of 20,24(OH)2D3. For rat CYP24A1 the preferred initial site of hydroxylation was at C24 whereas the human enzyme preferred C25. 20,23(OH)2D3 was initially metabolized to 20S,23,24-trihydroxyvitamin D3 and 20S,23,25-trihydroxyvitamin D3 by rat and human CYP24A1 as determined by NMR, with both isoforms showing a preference for initial hydroxylation at C25. CYP24A1 was able to further oxidize these metabolites in a series of reactions which included the cleavage of C23-C24 bond, as indicated by high resolution mass spectrometry of the products, analogous to the catabolism of 1,25(OH)2D3 via the C24-oxidation pathway. Similar catalytic efficiencies were observed for the metabolism of 20(OH)D3 and 20,23(OH)2D3 by human CYP24A1 and were lower than for the metabolism of 1,25(OH)2D3. We conclude that rat and human CYP24A1 metabolizes 20(OH)D3 producing only dihydroxyvitamin D3 species as products which retain biological activity, whereas 20,23(OH)2D3 undergoes multiple oxidations which include cleavage of the side chain.

Vitamin D: Photobiological and Ecological Aspects

Vitamin D was discovered as a result of its ability to cure rickets, but recently a wide range of other functions for it in the human body has been suggested. Vitamin D is not a vitamin in the strict sense as it can be synthesised in the human body following exposure of the skin to ultraviolet radiation. Provitamin D (7-dehydrocholesterol) is converted to previtamin D which is further modified by a series of reactions to the active form, 1,25-dihydroxyvitamin D. This Chapter summarises the discovery of vitamin D and reviews the chemistry and photochemistry of its precursors, transformations and metabolites. The production of vitamin D in various human populations is described, and how to assess vitamin D status. The skeletal and non-skeletal effects of vitamin D are discussed, particularly its role in immunomodulation with consequences for protection against a variety of human diseases. The Chapter concludes with evolutionary aspects, the occurrence and role of vitamin D in the plant kingdom, biogeographical considerations, and the nonphotochemical production of vitamin D in certain plants.

Effects of sidechain length and composition on the kinetic conversion and product distribution of vitamin D analogs determined by real-time NMR

Novel pregna-5, 7-dienes were synthesized and subjected to UVB irradiation to generate the corresponding pre-D intermediates, tachysterol and lumisterol analogs. The kinetics of the conversion from each of the pre-D intermediates to the corresponding novel D analogs was investigated by using real time 1H NMR measurements inside the NMR magnet. Both the length and composition of the side chains were found to affect the rate of the kinetic conversion from pre-D intermediates to the thermodynamically more stable D analogs. Compound 7cc which has both a long side chain and a tertiary alcohol moiety showed the highest conversion rate, while compound 4a-S which has a very short side chain without the tertiary alcohol had the lowest conversion rate among the 13 tested compounds. We also determined product distributions for these 5,7-dienes upon UVB irradiation followed by thermodynamic equilibration. No clear correlations between product distribution and side chain length or composition were identifiable under the current experimental conditions, suggesting there are other factors affecting the kinetics during the photochemical reactions for these 5,7-dienes. To the best of our knowledge, this is the first time the influences of side chain length and composition on the real time conversion kinetics from pre-D to D are studied. This study could serve as step-stones in future kinetic studies of novel biologically active 5,7-dienes and their corresponding D analogs under more physiologically relevant ex vivo or in vivo conditions, as well as providing important insights into optimizing yields of the desired active products during their organic syntheses.

Sunlight and Vitamin D: A global perspective for health

Vitamin D is the sunshine vitamin that has been produced on this earth for more than 500 million years. During exposure to sunlight 7-dehydrocholesterol in the skin absorbs UV B radiation and is converted to previtamin D3 which in turn isomerizes into vitamin D3. Previtamin D3 and vitamin D3 also absorb UV B radiation and are converted into a variety of photoproducts some of which have unique biologic properties. Sun induced vitamin D synthesis is greatly influenced by season, time of day, latitude, altitude, air pollution, skin pigmentation, sunscreen use, passing through glass and plastic, and aging. Vitamin D is metabolized sequentially in the liver and kidneys into 25-hydroxyvitamin D which is a major circulating form and 1,25-dihydroxyvitamin D which is the biologically active form respectively. 1,25-dihydroxyvitamin D plays an important role in regulating calcium and phosphate metabolism for maintenance of metabolic functions and for skeletal health. Most cells and organs in the body have a vitamin D receptor and many cells and organs are able to produce 1,25-dihydroxyvitamin D. As a result 1,25-dihydroxyvitamin D influences a large number of biologic pathways which may help explain association studies relating vitamin D deficiency and living at higher latitudes with increased risk for many chronic diseases including autoimmune diseases, some cancers, cardiovascular disease, infectious disease, schizophrenia and type 2 diabetes. A three-part strategy of increasing food fortification programs with vitamin D, sensible sun exposure recommendations and encouraging ingestion of a vitamin D supplement when needed should be implemented to prevent global vitamin D deficiency and its negative health consequences.

Tunneling and conformational flexibility play critical roles in the isomerization mechanism of vitamin D

The thermal isomerization reaction converting previtamin D to vitamin D is an intramolecular [1,7]-sigmatropic hydrogen shift with antarafacial stereochemistry. We have studied the dynamics of this reaction by means of the variational transition-state theory with multidimensional corrections for tunneling in both gas-phase and n-hexane environments. Two issues that may have important effects on the dynamics were analyzed in depth, i.e., the conformations of previtamin D and the quantum effects associated with the hydrogen-transfer reaction. Of the large number of conformers of previtamin D that were located, there are 16 that have the right disposition to react. The transition-state structures associated with these reaction paths are very close in energy, so all of them should be taken into account for an accurate calculation of both the thermal rate constants and the kinetic isotope effects. This issue is particularly important because the contribution of each of the reaction paths to the total thermal rate constant is quite sensitive to the environment. The dynamics results confirm that tunneling plays an important role and that model systems that were considered previously to study the hydrogen shift reaction cannot mimic the complexity introduced by the flexibility of the rings of previtamin D. Finally, the characterization of the conformers of both previtamin D and vitamin D allowed the calculation of the thermal equilibrium constants of the isomerization process.

Panther chameleons, Furcifer pardalis, behaviorally regulate optimal exposure to UV depending on dietary vitamin D3 status

Reptiles may bask for reasons other than thermoregulation. One alternative is to regulate homeostasis of vitamin D(3), a hormone essential to the health of many reptiles. Maintenance of vitamin D(3) homeostasis requires either regulation of exposure to ultraviolet radiation (UV) or ingested vitamin D(3). However, the prey items of most vertebrates tend to be low in vitamin D(3). Thus, basking may be the primary opportunity for vitamin D(3) regulation. We tested the hypothesis that panther chameleons (Furcifer pardalis) behaviorally regulate exposure to UV in natural sunlight with high precision, accuracy, and effectiveness. Panther chameleons with low dietary vitamin D(3) intake significantly increased exposure to UV in natural sunlight compared to those with high dietary vitamin D(3) intake. All lizards fed low dietary vitamin D(3) regulated within optimal UV levels with extreme effectiveness (ability to regulate within optimal UV levels relative to available UV). Chameleons of both dietary treatments regulated UV exposure with great precision, exhibiting little variation among individuals within treatments. Our results add to a growing body of literature that empirically demonstrates the importance of basking for nonthermoregulatory purposes and, more specifically, as an integral mechanism for the regulation of a vital hormone, vitamin D(3).

UVB-induced conversion of 7-dehydrocholesterol to 1 alpha,25-dihydroxyvitamin D3 (calcitriol) in the human keratinocyte line HaCaT

We have previously shown that keratinocytes in vitro can convert biologically inactive vitamin D3 to the hormone calcitriol. The present study was initiated to test whether ultraviolet B (UVB)-induced photolysis of provitamin D3 (7-dehydrocholesterol, [7-DHC]) which results in the formation of vitamin D3 also leads to the generation of calcitriol in keratinocytes. Submerged monolayers of HaCaT keratinocytes were preincubated with 7-DHC (25 microM) at 37 degrees C and irradiated with monochromatic UVB at different wavelengths (effective UV-doses: 7.5-60 mJ/cm2), or a narrow-band fluorescent lamp Philips TL-01 (UVB-doses: 125-1500 mJ/cm2). Irradiation with both sources of UVB resulted in the generation of different amounts of previtamin D3 in our in vitro model followed by time-dependent isomerization to vitamin D3 and consecutive formation of calcitriol in the picomolar range. Unirradiated cultures or cultures exposed to wavelengths > 315 nm generated no or only trace amounts of calcitriol. The conversion of vitamin D3 generated after UVB irradiation to calcitriol is inhibited by ketoconazole indicating the involvement of P450 mixed function oxidases in this chemical reaction. The generation of calcitriol was wavelength- and UVB dose dependent and reached approximately 18-fold higher levels after irradiation at 297 nm than at 310 nm (effective UVB dose: 30 mJ/cm2). Hence, keratinocytes may be a potential source of biologically active calcitriol within epidermis, when irradiated with therapeutical doses of UVB.

A liposomal model that mimics the cutaneous production of vitamin D3. Studies of the mechanism of the membrane-enhanced thermal isomerization of previtamin D3 to vitamin D3

We reported previously that the rate of previtamin D3 (preD3) <==> vitamin D3 isomerization was enhanced by about 10 times in the skin compared with that in organic solvents. To elucidate the mechanism by which the rate of this reaction is enhanced in the skin, we developed a liposomal model that mimicked the enhanced isomerization of preD3 to vitamin D3 that was described in human skin. Using this model we studied the effect of changing the polarity of preD3 as well as changing the chain length and the degree of saturation of liposomal phospholipids on the kinetics of preD3 <==> vitamin D3 isomerization. We found that a decrease in the hydrophilic interaction of the preD3 with liposomal phospholipids by an esterification of the 3beta-hydroxy of preD3 (previtamin D3-3beta-acetate) reduced the rate of the isomerization by 67%. The addition of a hydroxyl on C-25 of the hydrophobic side chain (25-hydroxyprevitamin D3), which decreased the hydrophobic interaction of preD3 with the phospholipids, reduced the rate by 87%. In contrast, in an isotropic n-hexane solution, there was little difference among the rates of the conversion of preD3, its 3beta-acetate, and 25-hydroxy derivatives to their corresponding vitamin D3 compounds. We also determined rate constants (k) of preD3 <==> vitamin D3 isomerization in liposomes containing phosphatidylcholines with different carbon chain lengths. The rates of the reaction were found to be enhanced as the number of carbons (Cn) in the hydrocarbon chain of the phospholipids increased from 10 to 18. In conclusion, these results support our hypothesis that amphipathic interactions between preD3 and membrane phospholipids stabilize preD3 in its "cholesterol like" cZc-conformer, the only conformer of preD3 that can convert to vitamin D3. The stronger these interactions were, the more preD3 was likely in its cZc conformation at any moment and the faster was the rate of its conversion to vitamin D3.