Stereoselective Synthesis of Four Calcitriol Lactone Diastereomers at C23 and C25

Discovery and Structural Analysis of Metabolites of Vitamin D3 Lactones

25-Hydroxyvitamin D3-23,26-lactone (1) and 1α,25-dihydroxyvitamin D3-23,26-lactone (2) have long been considered as among the end metabolites of vitamin D3. Recently, however, we found that these lactones exhibit biological activity related to the β-oxidation of fatty acids. We hypothesized that a metabolic pathway might exist to inactivate their physiological activity. Here, by means of metabolic experiments with a variety of cytochrome P450 (CYP) enzymes, we show that CYP3A4 metabolizes the lactones. The metabolites were presumed to be hydroxylated at C4 based on the previous reports showing that metabolism of 25-hydroxyvitamin D3 by CYP3A4 along with the current LC-MS analysis. To confirm this, we chemically synthesized 4α,25(OH)2D3-23,26-lactone (3), 4β,25(OH)2D3-23,26-lactone (4), 1α,4α,25(OH)3D3-23,26-lactone (5), and 1α,4β,25(OH)3D3-23,26-lactone (6). HPLC analysis using these authentic compounds as standards revealed that 1 was metabolized to 3 and 4, while 2 was metabolized exclusively to 6 by CYP3A4. Docking studies suggest that the hydroxyl group at C1 in 2 forms hydrogen bonds with Ser119 and Arg212 of CYP3A4, contributing to the fixation of C4β on heme iron in the CYP, thereby resulting in stereoselective hydroxylation at C4.

Differential gene regulation by a synthetic vitamin D receptor ligand and active vitamin D in human cells

Vitamin D (VD) exerts a wide variety of biological functions including calcemic activity. VD nutritional status is closely associated with the onset and development of chronic diseases. To develop a VD analog with the desired VD activity but without calcemic activity, we screened synthetic VDR antagonists. We identified 1α,25-dihydroxyvitamin D3-26-23-lactams (DLAM)-2a-d (DLAM-2s) as nuclear vitamin D receptor (VDR) ligands in a competitive VDR binding assay for 1α,25(OH)2 vitamin D3 (1α,25(OH)2D3), and DLAM-2s showed an antagonistic effect on 1α,25(OH)2 D3-induced cell differentiation in HL60 cells. In a luciferase reporter assay in which human VDR was exogenously expressed in cultured COS-1 cells, DLAM-2s acted as transcriptional antagonists. Consistently, DLAM-2s had an antagonistic effect on the 1α,25(OH)2D3-induced expression of a known VD target gene [Cytochrome P450 24A1 (CYP24A1)], and VDR bound DLAM-2s was recruited to an endogenous VD response element in chromatin in human keratinocytes (HaCaT cells) endogenously expressing VDR. In an ATAC-seq assay, the effects of 1α,25(OH)2 D3 and DLAM-2b on chromatin reorganization were undetectable in HaCaT cells, while the effect of an androgen receptor (AR) antagonist (bicalutamide) was confirmed in prostate cancer cells (LNCaP) expressing endogenous AR. However, whole genome analysis using RNA-seq and ATAC (Assay for Transposase Accessible Chromatin)-seq revealed differential gene expression profiles regulated by DLAM-2b versus 1α,25(OH)2D3. The upregulated and downregulated genes only partially overlapped between cells treated with 1α,25(OH)2D3 and those treated with DLAM-2b. Thus, the present findings illustrate a novel VDR ligand with gene regulatory activity differing from that of 1α,25(OH)2D3.

Peroxygenase-Catalyzed Selective Synthesis of Calcitriol Starting from Alfacalcidol

Calcitriol is an active analog of vitamin D3 and has excellent physiological activities in regulating healthy immune function. To synthesize the calcitriol compound, the concept of total synthesis is often adopted, which typically involves multiple steps and results in an overall low yield. Herein, we envisioned an enzymatic approach for the synthesis of calcitriol. Peroxygenase from Agrocybe aegerita (AaeUPO) was used as a catalyst to hydroxylate the C-H bond at the C-25 position of alfacalcidol and yielded the calcitriol in a single step. The enzymatic reaction yielded 80.3% product formation in excellent selectivity, with a turnover number up to 4000. In a semi-preparative scale synthesis, 72% isolated yield was obtained. It was also found that AaeUPO is capable of hydroxylating the C-H bond at the C-1 position of vitamin D3, thereby enabling the calcitriol synthesis directly from vitamin D3.

Synthesis of Deuterium-Labeled Vitamin D Metabolites as Internal Standards for LC-MS Analysis

Blood levels of the vitamin D₃ (D₃) metabolites 25-hydroxyvitamin D₃ (25(OH)D₃), 24R,25-dihydroxyvitamin D₃, and 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) are recognized indicators for the diagnosis of bone metabolism-related diseases, D₃ deficiency-related diseases, and hypercalcemia, and are generally measured by liquid-chromatography tandem mass spectrometry (LC-MS/MS) using an isotope dilution method. However, other D₃ metabolites, such as 20-hydroxyvitamin D₃ and lactone D₃, also show interesting biological activities and stable isotope-labeled derivatives are required for LC-MS/MS analysis of their concentrations in serum. Here, we describe a versatile synthesis of deuterium-labeled D₃ metabolites using A-ring synthons containing three deuterium atoms. Deuterium-labeled 25(OH)D₃ (2), 25(OH)D₃-23,26-lactone (6), and 1,25(OH)₂D₃-23,26-lactone (7) were synthesized, and successfully applied as internal standards for the measurement of these compounds in pooled human serum. This is the first quantification of 1,25(OH)₂D₃-23,26-lactone (7) in human serum.

Controlled lipid β-oxidation and carnitine biosynthesis by a vitamin D metabolite

Lactone-vitamin D3 is a major metabolite of vitamin D3, a lipophilic vitamin biosynthesized in numerous life forms by sunlight exposure. Although lactone-vitamin D3 was discovered 40 years ago, its biological role remains largely unknown. Chemical biological analysis of its photoaffinity probe identified the hydroxyacyl-CoA dehydrogenase trifunctional multienzyme complex subunit alpha (HADHA), a mitochondrial enzyme that catalyzes β-oxidation of long-chain fatty acids, as its selective binding protein. Intriguingly, the interaction of lactone-vitamin D3 with HADHA does not affect the HADHA enzymatic activity but instead limits biosynthesis of carnitine, an endogenous metabolite required for the transport of fatty acids into the mitochondria for β-oxidation. Lactone-vitamin D3 dissociates the protein-protein interaction of HADHA with trimethyllysine dioxygenase (TMLD), thereby impairing the TMLD enzyme activity essential in carnitine biosynthesis. These findings suggest a heretofore undescribed role of lactone-vitamin D3 in lipid β-oxidation and carnitine biosynthesis, and possibly in sunlight-dependent shifts of lipid metabolism in animals.

Chemical Synthesis of Side-Chain-Hydroxylated Vitamin D3 Derivatives and Their Metabolism by CYP27B1

1α,25-Dihydroxyvitamin D₃ (abbreviated here as 1,25D₃ ) is a hormonally active form of vitamin D₃ (D₃ ), and is produced from D₃ by CYP27 A1-mediated hydroxylation at C25, followed by CYP27B1-mediated hydroxylation at C1. Further hydroxylation of 25D₃ and 1,25D₃ occurs at C23, C24 and C26 to generate corresponding metabolites, except for 1,25R,26D₃ . Since the capability of CYP27B1 to hydroxylate C1 of side-chain-hydroxylated metabolites other than 23S,25D₃ and 24R,25D₃ has not been examined, we have here explored the role of CYP27B1 in the C1 hydroxylation of a series of side-chain-hydroxylated D₃ derivatives. We found that CYP27B1 hydroxylates the R diastereomers of 24,25D₃ and 25,26D₃ more effectively than the S diastereomers, but shows almost no activity towards either diastereomer of 23,25D₃ . This is the first report to show that CYP27B1 metabolizes 25,26D₃ to the corresponding 1α-hydroxylated derivative, 1,25,26D₃ . It will be interesting to examine the physiological relevance of this finding.

Differential diagnosis of vitamin D-related hypercalcemia using serum vitamin D metabolite profiling

Genetic causes of vitamin D-related hypercalcemia are known to involve mutation of 25-hydroxyvitamin D-24-hydroxylase CYP24A1 or the sodium phosphate co-transporter SLC34A1, which result in excessive 1,25-(OH)₂ D hormonal action. However, at least 20% of idiopathic hypercalcemia (IH) cases remain unresolved. In this case-control study, we used precision vitamin D metabolite profiling based on liquid chromatography-tandem mass spectrometry (LC-MS/MS) of an expanded range of vitamin D metabolites to screen German and French cohorts of hypercalcemia patients, to identify patients with altered vitamin D metabolism where involvement of CYP24A1 or SLC34A1 mutation had been ruled out and who possessed normal 25-OH-D₃ :24,25-(OH)₂ D₃ ratios. Profiles were compared to those of hypercalcemia patients with hypervitaminosis D, Williams-Beuren syndrome (WBS), CYP24A1 mutation, and normal subjects with a range of 25-OH-D levels. We observed that certain IH and WBS patients exhibited a unique profile comprising eightfold to 10-fold higher serum 23,25,26-(OH)₃ D₃ and 25-OH-D₃ -26,23-lactone than normals, as well as very low serum 1,25-(OH)₂ D₃ (2-5 pg/ml) and elevated 1,24,25-(OH)₃ D₃ , which we interpret implies hypersensitive expression of vitamin D-dependent genes, including CYP24A1, as a general underlying mechanism of hypercalcemia in these patients. Because serum 25-OH-D₃ and 24,25-(OH)₂ D₃ remained normal, we excluded the possibility that the aberrant profile was caused by hypervitaminosis D, but instead points to an underlying genetic cause that parallels the effect of Williams syndrome transcription factor deficiency in WBS. Furthermore, we observed normalization of serum calcium and vitamin D metabolite profiles at follow-up of an IH patient where 25-OH-D was reduced to 9 ng/ml, suggesting that symptomatic IH may depend on vitamin D nutritional status. Other hypercalcemic patients with complex conditions exhibited distinct vitamin D metabolite profiles. Our work points to the importance of serum vitamin D metabolite profiling in the differential diagnosis of vitamin D-related hypercalcemia that can rationalize expensive genetic testing, and assist healthcare providers in selecting appropriate treatment. © 2021 American Society for Bone and Mineral Research (ASBMR).

Introduction of fluorine atoms to vitamin D3 side-chain and synthesis of 24,24-difluoro-25-hydroxyvitamin D3

During our ongoing studies of vitamin D, we focused on the vitamin D₃ side-chain 24-position, which is the major metabolic site of human CYP24A1. In order to inhibit the metabolism of vitamin D₃, 24,24-difluorovitamin D₃analogues are important candidates. In this paper, we report the practical introduction of the difluoro-unit to the 24-position to synthesize 24,24-difluoro-CD ring (1) and 24,24-difluoro-25-hydroxyvitamin D₃ (2).