Short communication: Metabolic synthesis of vitamin D2 by the gut microbiome.

The origin of vitamin D2 in herbivorous animals was investigated in vivo in sheep and in bovine as well as mouse gastrointestinal tracts. A high concentration of 25-hydroxyvitamin D2 in blood plasma of sheep both in summer and winter appeared to be incompatible with the undetectable level of vitamin D2 in the pasture on which the sheep were grazing. Studies with bovine rumen contents from a cow grazing the same pasture as the sheep, demonstrated an increased concentration of vitamin D2 on anaerobic incubation in a 'Rusitec' artificial rumen, which was further enhanced when cellulose powder was added as a fermentation substrate. The colon contents of mice that were fed from weaning on a vitamin D-free diet were found to contain vitamin D2. The results of these comparative studies in 3 animal species indicated that vitamin D2 was being generated by microbial anaerobic metabolism in the gastrointestinal tract.

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 [...].

Optimization of ultrasound-assisted production of ergosterol from Penicillium brevicompactum by Taguchi statistical method.

Ergosterol as a primary metabolite and precursor of vitamin D2, is the most plentiful mycosterols in fungal cell membrane. Process optimization to increase the yield and productivity of biological products is a topic of interest. Ultrasonic waves have many applications in biotechnology, like cell disruption, and enhancement of primary and secondary metabolites production. This study disclosed an optimal condition for ultrasound-assisted production (UAP) of ergosterol from Penicillium brevicompactum MUCL 19,011 using L9 Taguchi statistical method. The intensity (IS), time of sonication (TS), treatment frequency (TF), and number of days of treatment (DT) were allocated to study the effects of ultrasound on ergosterol production. The results were analyzed using Minitab version 19. The maximum ergosterol, 11 mg/g cell dry weight (CDW), was produced on the tenth day while all factors were at a low level. The days of treatment with a contribution of 45.48% was the most significant factor for ergosterol production. For the first time, this study revealed the positive effect of ultrasound on the production of ergosterol. Ergosterol production increased 73% (4.63 mg/g CDW) after process optimization. Finally, a mathematical model of ultrasound factors with a regression coefficient of R2 = 0.978 was obtained for the ergosterol production during ultrasound treatment.

Investigation of Vitamin D2 and Vitamin D3 Hydroxylation by Kutzneria albida.

The active vitamin D metabolites 25-OH-D and 1α,25-(OH)2 -D play an essential role in controlling several cellular processes in the human body and are potentially effective in the treatment of several diseases, such as autoimmune diseases, cardiovascular diseases and cancer. The microbial synthesis of vitamin D2 (VD2 ) and vitamin D3 (VD3 ) metabolites has emerged as a suitable alternative to established complex chemical syntheses. In this study, a novel strain, Kutzneria albida, with the ability to form 25-OH-D2 and 25-OH-D3 was identified. To further improve the conversion of the poorly soluble substrates, several solubilizers were tested. 100-fold higher product concentrations of 25-OH-D3 and tenfold higher concentrations of 25-OH-D2 after addition of 5 % (w/v) 2-hydroxypropyl ß-cyclodextrin (2-HPßCD) were reached. Besides the single-hydroxylation products, the human double-hydroxylation products 1,25-(OH)2 -D2 and 1,25-(OH)2 -D3 and various other potential single- and double-hydroxylation products were detected. Thus, K. albida represents a promising strain for the biotechnological production of VD2 and VD3 metabolites.

Vitamin D Analogs Regulate the Vitamin D System and Cell Viability in Ovarian Cancer Cells.

BACKGROUND: Ovarian cancer (OC) is one of the most lethal cancers in women. The active form of vitamin D3, 1,25-dihydroxyvitamin D3 (1,25D3, calcitriol) has anticancer activity in several cancers, including ovarian cancer, but the required pharmacological doses may cause hypercalcemia. We hypothesized that newly developed, low calcemic, vitamin D analogs (an1,25Ds) may be used as anticancer agents instead of calcitriol in ovarian cancer cells. METHODS: We used two patient-derived high-grade serous ovarian cancer (HGSOC) cell lines with low (13781) and high (14433) mRNA expression levels of the gene encoding 1,25-dihydroxyvitamin D3 24-hydroxylase CYP24A1, one of the main target genes of calcitriol. We tested the effect of calcitriol and four structurally related series of an1,25Ds (PRI-1906, PRI-1907, PRI-5201, PRI-5202) on cell number, viability, the expression of CYP24A1, and the vitamin D receptor (VDR). RESULTS: CYP24A1 mRNA expression increased in a concentration-dependent manner after treatment with all compounds. In both cell lines, after 4 h, PRI-5202 was the most potent analog (in 13781 cells: EC50 = 2.98 ± 1.10 nmol/L, in 14433 cells: EC50 = 0.92 ± 0.20 nmol/L), while PRI-1907 was the least active one (in 13781 cells: EC50 = n/d, in 14433 cells: EC50 = n/d). This difference among the analogs disappeared after 5 days of treatment. The 13781 cells were more sensitive to the an1,25Ds compared with 14433 cells. The an1,25Ds increased nuclear VDR levels and reduced cell viability, but only in the 13781 cell line. CONCLUSIONS: The an1,25Ds had different potencies in the HGSOC cell lines and their efficacy in increasing CYP24A1 expression was cell line- and chemical structure-dependent. Therefore, choosing sensitive cancer cell lines and further optimization of the analogs' structure might lead to new treatment options against ovarian cancer.

Highly regio- and stereoselective hydroxylation of vitamin D2 by CYP109E1.

Vitamin D2 is a form of vitamin D derived from mushrooms and plants which is structurally modified in the body due to the action of several enzymes. The resulting metabolites represent important compounds with potential bioactive properties. However, they are poorly studied and their availability is mostly limited. In order to identify new enzymes capable of producing vitamin D2 metabolites, we investigated a bacterial P450 monooxygenase, CYP109E1, which was previously shown to be a vitamin D3 hydroxylase. It was found that CYP109E1 catalyzes a vitamin D2 two-step hydroxylation at positions C24 and C25 resulting in the generation of 24(R),25-diOH VD2. Interestingly, the enzyme showed high selectivity towards vitamin D2, whereas it showed an unselective product pattern for the structurally similar vitamin D3. Our docking results for vitamin D2 and D3 revealed favorable hydroxylation positions for both substrates and suggested an explanation for the high selectivity of CYP109E1 towards vitamin D2. In addition, we established a whole-cell biocatalyst expressing CYP109E1 in Bacillus megaterium to produce 24(R),25-diOH VD2 and a production yield of 12.3 ± 1.2 mg/L was obtained after 48 h. To the best of our knowledge, this is the first report on the generation of 24(R),25-diOH VD2 by a microbial biocatalyst allowing a low-cost and eco-friendly production of this pharmaceutically interesting and expensive metabolite from the relatively cheap substrate, VD2.

Vitamin D Binding Protein (VDBP) and Its Gene Polymorphisms-The Risk of Malignant Tumors and Other Diseases.

Vitamin D is an important component of the endocrine system that controls calcium homeostasis and bone mineralization. Because of the very short half-life of free serum vitamin D it is stabilized and transported to target tissues by being bound to the vitamin D binding protein (VDBP). The most common polymorphisms: rs4588 and rs7041 in the vitamin D binding protein gene may correlate with differences in vitamin D status in the serum. This review presents data that relate to the presence of genetic variants in the VDBP gene in correlation with certain diseases, mostly concerning cancers (breast, prostate, pancreatic, lung, colorectal, basal cell carcinoma cancer and cutaneous melanoma) or other related diseases (thyroid autoimmunity disorders, obesity, diabetes mellitus, bone metabolism, rheumatoid arthritis, ankylosing spondylitis, asthma, chronic obstructive pulmonary disease, tuberculosis and coronary artery diseases).

Production of an active form of vitamin D2 by genetically engineered CYP105A1.

Our previous studies revealed that CYP105A1 can convert vitamin D3 (VD3) to its active form, 1α,25-dihydroxyvitamin D3 (1,25D3). Site-directed mutagenesis of CYP105A1 based on its crystal structure dramatically enhanced its activity; the activity of double variants R73A/R84A and R73A/R84V was more than 100-fold higher than that of the wild type of CYP105A1. In contrast, these variants had a low ability to convert vitamin D2 (VD2) to 1α,25-dihydroxyvitamin D2 (1,25D2), whereas they catalyzed the sequential hydroxylation at positions C25 and C26 to produce 25,26D2. A comparison of the docking models of 25D2 and 25D3 into the substrate-binding pocket of R73A/R84A suggests that the side chain of the Met239 inhibits the binding of 25D2 for 1α-hydroxylation. Therefore, the Met239 residue of R73A/R84A was substituted for Ala. As expected, the triple variant R73A/R84A/M239A showed a 22-fold higher 1α-hydroxylation activity towards 25D2. To the best of our knowledge, this is the first report on the generation of microbial cytochrome P450 that converts VD2 to 1,25D2 via 25D2.

Pharmacological induction of ferritin prevents osteoblastic transformation of smooth muscle cells.

Vascular calcification is a frequent complication of atherosclerosis, diabetes and chronic kidney disease. In the latter group of patients, calcification is commonly seen in tunica media where smooth muscle cells (SMC) undergo osteoblastic transformation. Risk factors such as elevated phosphorus levels and vitamin D3 analogues have been identified. In the light of earlier observations by our group and others, we sought to inhibit SMC calcification via induction of ferritin. Human aortic SMC were cultured using ß-glycerophosphate with activated vitamin D3 , or inorganic phosphate with calcium, and induction of alkaline phosphatase (ALP) and osteocalcin as well as accumulation of calcium were used to monitor osteoblastic transformation. In addition, to examine the role of vitamin D3 analogues, plasma samples from patients on haemodialysis who had received calcitriol or paricalcitol were tested for their tendency to induce calcification of SMC. Addition of exogenous ferritin mitigates the transformation of SMC into osteoblast-like cells. Importantly, pharmacological induction of heavy chain ferritin by 3H-1,2-Dithiole-3-thione was able to inhibit the SMC transition into osteoblast-like cells and calcification of extracellular matrix. Plasma samples collected from patients after the administration of activated vitamin D3 caused significantly increased ALP activity in SMC compared to the samples drawn prior to activated vitamin D3 and here, again induction of ferritin diminished the osteoblastic transformation. Our data suggests that pharmacological induction of ferritin prevents osteoblastic transformation of SMC. Hence, utilization of such agents that will cause enhanced ferritin synthesis may have important clinical applications in prevention of vascular calcification.

Sequential hydroxylation of vitamin D2 by a genetically engineered CYP105A1.

Our previous studies revealed that the double variants of CYP105A1- R73A/R84A and R73V/R84A-show high levels of activity with respect to conversion of vitamin D3 to its biologically active form, 1α,25-dihydroxyvitamin D3 (1α,25(OH)2D3). In this study, we found that both the double variants were also capable of converting vitamin D2 to its active form, that is, 1α,25-dihydroxyvitamin D2 (1α,25(OH)2D2), via 25(OH)D2, whereas its 1α-hydroxylation activity toward 25(OH)D2 was much lower than that toward 25(OH)D3. Comparison of the wild type and the double variants revealed that the amino acid substitutions remarkably enhanced both 25- and 26-hydroxylation activity toward vitamin D2. After 25-hydroxylation of vitamin D2, further hydroxylation at C26 may occur frequently without the release of 25(OH)D2 from the substrate-binding pocket. Thus, the double variants of CYP105A1 are quite useful to produce 25,26(OH)2D2 that is one of the metabolites of vitamin D2 detected in human serum.

Cholecalciferol v. ergocalciferol for 25-hydroxyvitamin D (25(OH)D) repletion in chronic kidney disease: a randomised clinical trial.

Patients with chronic kidney disease (CKD) demonstrate complex mineral metabolism derangements and a high prevalence of vitamin D deficiency. However, the optimal method of 25-hydroxyvitamin D (25(OH)D) repletion is unknown, and trials analysing the comparative efficacy of cholecalciferol and ergocalciferol in this population are lacking. We conducted a randomised clinical trial of cholecalciferol 1250µg (50 000 IU) weekly v. ergocalciferol 1250µg (50 000 IU) weekly for 12 weeks in forty-four non-dialysis-dependent patients with stage 3-5 CKD. The primary outcome was change in total 25(OH)D from baseline to week 12 (immediately after therapy). Secondary analyses included the change in 1,25-dihydroxyvitamin D (1,25(OH)2D), parathyroid hormone (PTH), D2 and D3 sub-fractions of 25(OH)D and 1,25(OH)2D and total 25(OH)D from baseline to week 18 (6 weeks after therapy). Cholecalciferol therapy yielded a greater change in total 25(OH)D (45·0 (sd 16·5) ng/ml) v. ergocalciferol (30·7 (sd 15·3) ng/ml) from baseline to week 12 (P<0·01); this observation partially resulted from a substantial reduction in the 25(OH)D3 sub-fraction with ergocalciferol. However, following cessation of therapy, no statistical difference was observed for total 25(OH)D change from baseline to week 18 between cholecalciferol and ergocalciferol groups (22·4 (sd 12·7) v. 17·6 (sd 8·9) ng/ml, respectively; P=0·17). We observed no significant difference between these therapies with regard to changes in serum PTH or 1,25(OH)2D. Therapy with cholecalciferol, compared with ergocalciferol, is more effective at raising serum 25(OH)D in non-dialysis-dependent CKD patients while active therapy is ongoing. However, levels of 25(OH)D declined substantially in both arms following cessation of therapy, suggesting the need for maintenance therapy to sustain levels.

Protective Effects of Tinospora cordifolia on Hepatic and Gastrointestinal Toxicity Induced by Chronic and Moderate Alcoholism.

AIMS: Heavy alcohol intake depletes the plasma vitamins due to hepatotoxicity and decreased intestinal absorption. However, moderate alcohol intake is often thought to be healthy. Therefore, effects of chronic moderate alcohol intake on liver and intestine were studied using urinary vitamin levels. Furthermore, effects of Tinospora cordifolia water extract (TCE) (hepatoprotective) on vitamin excretion and intestinal absorption were also studied. METHODS: In the study, asymptomatic moderate alcoholics (n = 12) without chronic liver disease and healthy volunteers (n = 14) of mean age 39 ± 2.2 (mean ± SD) were selected and divided into three groups. TCE treatment was performed for 14 days. The blood and urine samples were collected on Day 0 and 14 after treatment with TCE and analyzed. RESULTS: In alcoholics samples, a significant increase in the levels of gamma-glutamyl transferase, aspartate transaminase, alanine transaminase, Triglyceride, Cholesterol, HDL and LDL (P < 0.05) was observed but their level get downregulated after TCE intervention. Multivariate analysis of metabolites without missing values showed an increased excretion of 7-dehydrocholesterol, orotic acid, pyridoxine, lipoamide and niacin and TCE intervention depleted their levels (P < 0.05). In contrast, excretion of biotin, xanthine, vitamin D2 and 2-O-p-coumaroyltartronic acid (CA, an internal marker of intestinal absorption) were observed to be decreased in alcoholic samples; however, TCE intervention restored the CA and biotin levels. Vitamin metabolism biomarkers, i.e. homocysteine and xanthurenic acid, were also normalized after TCE intervention. CONCLUSION: Overall data depict that moderate alcohol intake is also hepatotoxic and decreases intestinal absorption. However, TCE treatment effectively increased the intestinal absorption and retaining power of liver that regulated alcohol-induced multivitamin deficiency.

25-hydroxyvitamin D circulates in different fractions of calf plasma if the parent compound is vitamin D2 or vitamin D3, respectively.

Vitamin D has become one of the most discussed nutrients in human nutrition, which has led to an increased interest in milk as a vitamin D source. Problems related to fortifying milk with synthetic vitamin D can be avoided by securing a high content of natural vitamin D in the milk by supplying dairy cows with sufficient vitamin D. However, choosing the most efficient route and form of supplementation requires insight into how different vitamin D metabolites are transported in the body of cattle. There are two forms of vitamin D: vitamin D2 (D2) and vitamin D3 (D3). Vitamin D2 originates from fungi on roughage. Vitamin D3 originates either from endogenous synthesis in the skin or from feed supplements. Vitamin D2 is chemically different from, and less physiologically active than, D3. Endogenous and dietary D3 is chemically similar but dietary D3 is toxic, whereas endogenous D3 appears well regulated in the body.

A significant induction of neutrophilic chemoattractants but not RANKL in synoviocytes stimulated with interleukin 17.

Interleukin 17 (IL-17) is a cytokine implicated in the promotion of osteoclastogenesis. Its effect has been believed not to be directly exerted on osteoclast precursors, but rather indirectly carried out via an induction of receptor activator of NF-κB ligand (RANKL), the osteoclast differentiation factor, on osteoclast-supporting cells, which in turn exert an effect on osteoclast precursors. The mechanistic details, however, remain unclear. In this study, we first performed a transcriptome analysis of synoviocytes derived from a patient with rheumatoid arthritis cultured in the presence or absence of IL-17. We discovered that most of the genes significantly induced by IL-17 were chemokines with a chemotactic effect on neutrophils. We confirmed these results by quantitative RT-PCR and ELISA. Unexpectedly, the stimulation with IL-17 alone did not induce the expression of RANKL either at the mRNA or the protein level. The induction of RANKL was observed when IL-17 was added in combination with 1,25-dihydroxyvitamin D3 and prostaglandin E2, well-known inducers of RANKL, although the exact mechanism of this synergistic effect remains unclear. IL-6 and monocyte chemoattractant protein-1 were also significantly induced by IL-17 at both the mRNA and protein levels. Thus, it appears that IL-17 induces the migration of neutrophils and monocytes/macrophages through the activation of synoviocytes, and enhances a positive feedback loop composed of proinflammatory cytokines IL-6 and IL-17.

Extrarenal vitamin D activation and interactions between vitamin D2, vitamin D3, and vitamin D analogs.

Our understanding of the mechanism of action of vitamin D has been broadened by the discovery of the extrarenal 1α-hydroxylase (CYP27B1) in various vitamin D target tissues around the body and the implications of this for the putative paracrine actions of 1α,25-dihydroxyvitamin D3. This review updates our current knowledge of the cytochrome P450-mediated steps of vitamin D activation (CYP2R1, CYP27B1) and inactivation (CYP24A1, CYP3A4) and the newest physiological roles of vitamin D. The review goes on to examine how well exogenously supplied vitamin D compounds, whether dietary vitamin D2 supplements or prescribed vitamin D analogs, substitute for their natural counterparts; how in some cases vitamin D can be used in conjunction with vitamin D analogs; and the overall impact of these supplements and drugs on the components of the vitamin D signal transduction machinery.

Dietary vitamin D2--a potentially underestimated contributor to vitamin D nutritional status of adults?

It has been suggested that vitamin D2 is not very prevalent in the human food chain. However, data from a number of recent intervention studies suggest that the majority of subjects had measurable serum 25-hydroxyvitamin D2 (25(OH)D2) concentrations. Serum 25(OH)D2, unlike 25(OH)D3, is not directly influenced by exposure of skin to sun and thus has dietary origins; however, quantifying dietary vitamin D2 is difficult due to the limitations of food composition data. Therefore, the present study aimed to characterise serum 25(OH)D2 concentrations in the participants of the National Adult Nutrition Survey (NANS) in Ireland, and to use these serum concentrations to estimate the intake of vitamin D2 using a mathematical modelling approach. Serum 25(OH)D2 concentration was measured by a liquid chromatography-tandem MS method, and information on diet as well as subject characteristics was obtained from the NANS. Of these participants, 78.7 % (n 884) had serum 25(OH)D2 concentrations above the limit of quantification, and the mean, maximum, 10th, 50th (median) and 90th percentile values of serum 25(OH)D2 concentrations were 3.69, 27.6, 1.71, 2.96 and 6.36 nmol/l, respectively. To approximate the intake of vitamin D2 from these serum 25(OH)D2 concentrations, we used recently published data on the relationship between vitamin D intake and the responses of serum 25(OH)D concentrations. The projected 5th to 95th percentile intakes of vitamin D2 for adults were in the range of 0.9-1.2 and 5-6 µg/d, respectively, and the median intake ranged from 1.7 to 2.3 µg/d. In conclusion, the present data demonstrate that 25(OH)D2 concentrations are present in the sera of adults from this nationally representative sample. Vitamin D2 may have an impact on nutritional adequacy at a population level and thus warrants further investigation.

Triple quadrupole liquid chromatography-mass spectrometry-mediated evaluation of vitamin D2 accumulation potential, antioxidant capacities, and total polyphenol content of white jelly mushroom (Tremella fuciformis Berk.).

Tremella fuciformis Berk. (TF), or the white jelly mushroom, is well known for its myriad of pharmacological properties, such as immunomodulatory, anti-inflammatory, antidiabetic, antitumor, and antioxidant activities, and hypocholesterolemic and hepatoprotective effects that boost human health. Most of the studies of TF are concentrated on its polysaccharide (glucuronoxylomannan) composition, which is responsible for its pharmacological as well as rheological properties. It is well established that mushrooms are a great source of dietary vitamin D due to the presence of ergosterol in their cell membrane. There is a lack of published data on TF as a source of vitamin D2. Therefore, this study aimed to evaluate the vitamin D2 composition of the fruiting bodies of TF using triple quadrupole liquid chromatography-mass spectrometry (LC-MS/QQQ). The results showed highest vitamin D2 content (292.02 µg/g dry weight) in the sample irradiated with ultraviolet B (UVB; 310 nm) for 180 min as compared with the control group (52.47 µg/g dry weight) (P ≤ 0.001). The results showed higher accumulation potential of vitamin D2 in TF as compared with published data available for other extensively studied culinary mushrooms, such as Agaricus bisporus, Lentinula edodes, Pleurotus ostreatus, Cordiceps militaris, and Calocybe indica. Moreover, the impact of UV treatment on antioxidant capacities and total polyphenol content of TF was also studied. The accumulation potential of vitamin D in TF reveals a novel commercial source for this nutrient.

FGF23 and mineral metabolism in the early post-renal transplantation period.

BACKGROUND: The relationship between fibroblast growth factor 23 (FGF23) and vitamin D production and catabolism post-renal transplantation has not been characterized. METHODS: Circulating creatinine, calcium, phosphorus, albumin, parathyroid hormone, FGF23, and 1,25(OH)2 vitamin D (calcitriol) values were obtained pre-transplantation, daily post-operatively for 5 days, and at 6 months post-transplantation in 44 patients aged 16.4 ± 0.4 years undergoing renal transplantation at UCLA from 1 August 2005 through to 30 April 2007. 25(OH) Vitamin D and 24,25(OH)2 vitamin D concentrations were obtained at baseline and on post-operative days 5 and 180, and urinary concentrations of creatinine, phosphorus, and FGF23 were measured on post-operative days 1, 3, 5, and 180. RESULTS: Circulating phosphate concentrations declined more rapidly and the fractional excretion of phosphorus was higher in the first week post-transplantation in subjects with higher FGF23 values. Fractional excretion of FGF23 was low at all time-points. Circulating 1,25(OH)2 vitamin D levels rose more rapidly and were consistently higher in patients with lower FGF23 values; however, 25(OH) vitamin D and 24,25(OH)2 vitamin D values were unrelated to FGF23 concentrations. CONCLUSIONS: Inhibition of renal 1α-hydroxylase, rather than stimulation of 24-hydroxylase, may primarily contribute to the relationship between FGF23 values and calcitriol. The rapid decline in FGF23 levels post-transplantation in our patient cohort was not mediated solely by the filtration of intact FGF23 by the new kidney.

Understanding vitamin D formulations.

Treating patients who are vitamin D deficient can be challenging. This article describes various formulations of vitamin D and their indications.

Paricalcitol protects against hydrogen peroxide-induced injury in endothelial cells through suppression of apoptosis.

The vascular endothelium is one of the main targets of oxidative stress which plays an important role in the pathophysiology of vascular damage. Recent studies show that vitamin D can positively regulate endothelial functions in various chronic diseases and in cases of increased oxidative stress. In our study, we investigated the restorative and protective potentials of paricalcitol which is frequently used in patients with chronic renal failure, a vitamin D analogue, in human umbilical vein endothelial cells (HUVEC) before and after H2O2-induced oxidative stress. Paricalcitol treatment after the oxidative stress induced by H2O2 increased cell viability in endothelial cells depending on the dose that was used. While paricalcitol (500 nM) decreased caspase-3 activity and mitochondrial membrane potential loss, it increased nitric oxide (NO) production and reduced glutathione (GSH) levels. Paricalcitol treatment before oxidative stress increased cell viability. It increased NO production and mitochondrial membrane potential while significantly reducing caspase-3 activity. While paricalcitol caused a significant inhibition of protein disulfide isomerase (PDI) reductase activity in healthy endothelial cells, it did not cause a significant change on the PDI reductase activity under oxidative stress conditions. Present study showed that paricalcitol has restorative and protective effects on endothelial cells against oxidative injury, but these effects occur at high concentrations of paricalcitol.