Study on the metabolites of 1alpha,25-dihydroxyvitamin D4

Calcitroic Acid-A Review

Calcitroic acid was isolated and characterized almost four decades ago, but little is known about this important vitamin D metabolite. Four reported synthetic strategies to generate calcitroic acid are presented that highlight the scientific progress in the field of chemistry directed to vitamin D analog synthesis. The most recent synthesis described the generation of calcitroic acid with an overall yield of 12.8% in 13 steps. The endogenous formation of calcitroic acid has been demonstrated in perfused rat kidney using 24,25,26,27-tetranor-1,23(OH)₂D₃. Although, the majority of vitamin D metabolism is mediated by 24-hydoxylase (CYP24A1), it is not clear why the formation of calcitroic acid was not observed in the presence of recombinant CYP24A1 enzyme. Furthermore, it is not known if enzyme 1α-hydroxylase (CYP27B1) can convert calcioic acid into calcitroic acid. In addition to the lack of research investigating the endogenous formation of calcitroic acid, the physiological role of calcitroic acid remains unknown. Only a few reports mentioned the biological activity of calcitroic acid in connection with the vitamin D receptor (VDR). When administered subcutaneously, calcitroic acid has anthracitic properties and elevates calcium blood levels when administered intravenously. In vitro, calcitroic acid at higher concentrations has been shown to bind VDR and induce gene transcription. However, these studies were not carried out in cells derived from target organs of calcitroic acid such as kidney, liver, and intestine. We can conclude that our current knowledge of calcitroic acid is limited, and more studies are needed to identify its physiological role.

Vitamin D4 in mushrooms

An unknown vitamin D compound was observed in the HPLC-UV chromatogram of edible mushrooms in the course of analyzing vitamin D(2) as part of a food composition study and confirmed by liquid chromatography-mass spectrometry to be vitamin D(4) (22-dihydroergocalciferol). Vitamin D(4) was quantified by HPLC with UV detection, with vitamin [(3)H] itamin D(3) as an internal standard. White button, crimini, portabella, enoki, shiitake, maitake, oyster, morel, chanterelle, and UV-treated portabella mushrooms were analyzed, as four composites each of a total of 71 samples from U.S. retail suppliers and producers. Vitamin D(4) was present (>0.1 µg/100 g) in a total of 18 composites and in at least one composite of each mushroom type except white button. The level was highest in samples with known UV exposure: vitamin D enhanced portabella, and maitake mushrooms from one supplier (0.2-7.0 and 22.5-35.4 µg/100 g, respectively). Other mushrooms had detectable vitamin D(4) in some but not all samples. In one composite of oyster mushrooms the vitamin D(4) content was more than twice that of D(2) (6.29 vs. 2.59 µg/100 g). Vitamin D(4) exceeded 2 µg/100 g in the morel and chanterelle mushroom samples that contained D(4), but was undetectable in two morel samples. The vitamin D(4) precursor 22,23-dihydroergosterol was found in all composites (4.49-16.5 mg/100 g). Vitamin D(4) should be expected to occur in mushrooms exposed to UV light, such as commercially produced vitamin D enhanced products, wild grown mushrooms or other mushrooms receiving incidental exposure. Because vitamin D(4) coeluted with D(3) in the routine HPLC analysis of vitamin D(2) and an alternate mobile phase was necessary for resolution, researchers analyzing vitamin D(2) in mushrooms and using D(3) as an internal standard should verify that the system will resolve vitamins D(3) and D(4).

Synthesis and Structure-Activity Relationships of Bioactive Compounds Using Sterols

Sterols are widely and abundantly distributed in nature. It is convenient to utilize them for the preparation of useful compounds such as pharmaceuticals with steroid and secosteroid skeletons. This paper describes the synthesis and structure-activity relationships of naturally occurring active forms of vitamin D analogues, sterols having neurite outgrowth activity, and liver X receptor agonist. The active form of vitamin D(4) showed similar biological activities but had higher affinity to the vitamin D-binding protein compared with the corresponding vitamins D(2) and D(3). This shows that the active form of vitamin D(4) is a good candidate for an agent to replace the active forms of vitamins D(2) and D(3). In the course of screening for low molecular-weight compounds that exhibit neurite outgrowth activity in the culture broth, we found that the natural product dictyosterol showed strong activity. From screening of the analogues, it was found that the double bond between C22 and C23 in the side chain of the sterol is essential for its activity. Ergost-22-ene-1alpha,3beta-diol was found to serve as a stronger liver X receptor agonist than 24(S), 25-epoxycholesterol, which regulates the expression of genes involved in lipid metabolism. Structure-function study showed that the 1alpha-hydroxyl group, the saturated steroid structure, and the double bond between C22 and C23 are needed to function as a liver X receptor agonist.

Rat cytochrome P450C24 (CYP24) does not metabolize 1,25-dihydroxyvitamin D2 to calcitroic acid

1alpha-Hydroxy-23 carboxy-24,25,26,27-tetranorvitamin D(3) (calcitroic acid) is known to be the major water-soluble metabolite produced during the deactivation of 1,25-(OH)(2)D(3). This deactivation process is carried out exclusively by the multicatalytic enzyme CYP24 and involves a series of oxidation reactions at C(24) and C(23) leading to side-chain cleavage and, ultimately, formation of the calcitroic acid. Like 1,25-(OH)(2)D(3), 1alpha,25-1,25-(OH)(2)D(2) is also known to undergo side-chain oxidation and side-chain cleavage to form calcitroic acid (Zimmerman et al. [2001]. 1,25-(OH)(2)D(2) differs from 1,25-(OH)(2)D(3) by the presence of a double bond at C(22) and a methyl group at C(24). To date, there have been no studies detailing the participation of CYP24 in the production of calcitroic acid from 1,25-(OH)(2)D(2). We, therefore, studied the metabolism of 1,25-(OH)(2)D(3) and 1,25-(OH)(2)D(2) using a purified rat CYP24 system. Lipid and aqueous-soluble metabolites were prepared for characterization. Aqueous-soluble metabolites were subjected to reverse-phase high-pressure liquid chromatography (HPLC) analysis. As expected, 1,23(OH)(2)-24,25,26,27-tetranor D and calcitroic acid were the major lipid and aqueous-soluble metabolites, respectively, when 1,25-(OH)(2)D(3) was used as substrate. However, when 1,25-(OH)(2)D(2) was used as substrate, 1,24(R),25-(OH)(3)D(2) was the major lipid-soluble metabolite with no evidence for the production of either 1,23(OH)(2)-24,25,26,27-tetranor D or calcitroic acid. Apparently, the CYP24 was able to 24-hydroxylate 1,25-(OH)(2)D(2), but was unable to effect further changes, which would result in side-chain cleavage. These data suggest that the presence of either the double bond at C(22) or the C(24) methyl group impedes the metabolism of 1,25-(OH)(2)D(2) to calcitroic acid by CYP24 and that enzymes other than CYP24 are required to effect this process.

Use of vitamin D(4) analogs to investigate differences in hepatic and target cell metabolism of vitamins D(2) and D(3)

In this study, we used molecules with either of the structural differences in the side chains of vitamin D(2) and vitamin D(3) to investigate which feature is responsible for the significant differences in their respective metabolism, pharmacokinetics and toxicity. We used two cell model systems-HepG2 and HPK1A-ras-to study hepatic and target cell metabolism, respectively. Studies with HepG2 revealed that the pattern of 24- and 26-hydroxylation of the side chain reported for 1alpha-hydroxyvitamin D(2) (1alpha-OH-D(2)) but not for 1alpha-OH-D(3) is also observed in both 1alpha-OH-D(4) and Delta(22)-1alpha-OH-D(3) metabolism. This suggests that the structural feature responsible for targeting the enzyme to the C24 or C26 site could be either the C24 methyl group or the 22-23 double bond. In HPK1A-ras cells, the pattern of metabolism observed for the 24-methylated derivative, 1alpha,25-(OH)(2)D(4), was the same pattern of multiple hydroxylations at C24, C26 and C28 seen for vitamin D(2) compounds without evidence of side chain cleavage observed for vitamin D(3) derivatives, suggesting that the C24 methyl group plays a major role in this difference in target cell metabolism of D(2) and D(3) compounds. Novel vitamin D(4) compounds were tested and found to be active in a variety of in vitro biological assays. We conclude that vitamin D(4) analogs and their metabolites offer valuable insights into vitamin D analog design, metabolic enzymes and maybe useful clinically.