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

Mitochondrial oxysterol biosynthetic pathway gives evidence for CYP7B1 as controller of regulatory oxysterols

The aim of this paper was to more completely study the mitochondrial CYP27A1 initiated acidic pathway of cholesterol metabolism. The mitochondrial CYP27A1 initiated pathway of cholesterol metabolism (acidic pathway) is known to synthesize two well-described vital regulators of cholesterol/lipid homeostasis, (25R)-26-hydroxycholesterol (26HC) and 25-hydroxycholesterol (25HC). Both 26HC and 25HC have been shown to be subsequently 7α-hydroxylated by Cyp7b1; reducing their regulatory abilities and furthering their metabolism to chenodeoxycholic acid (CDCA). Cholesterol delivery into the inner mitochondria membrane, where CYP27A1 is located, is considered the pathway's only rate-limiting step. To further explore the pathway, we increased cholesterol transport into mitochondrial CYP27A1 by selectively increased expression of the gene encoding the steroidogenic acute transport protein (StarD1). StarD1 overexpression led to an unanticipated marked down-regulation of oxysterol 7α-hydroxylase (Cyp7b1), a marked increase in 26HC, and the formation of a third vital regulatory oxysterol, 24(S)-hydroxycholesterol (24HC), in B6/129 mice livers. To explore the further metabolism of 24HC, as well as, 25HC and 26HC, characterizations of oxysterols and bile acids using three murine models (StarD1 overexpression, Cyp7b1^-/-, Cyp27a1^-/-) and human Hep G2 cells were conducted. This report describes the discovery of a new mitochondrial-initiated pathway of oxysterol/bile acid biosynthesis. Just as importantly, it provides evidence for CYP7B1 as a key regulator of three vital intracellular regulatory oxysterol levels.

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).

Contemporary diagnosis and treatment of vitamin D-related disorders

Plasma 25(OH)D has emerged as a valuable biomarker for the many varied health-related effects of vitamin D in the clinic mainly because of the recognition of the importance of the enzyme, CYP27B1, or the 25(OH)D-alpha-hydroxylase in the extrarenal, target cell production of calcitriol. This review briefly assesses current methodology for plasma 25(OH)D assay focusing mainly on currrent controversies surrounding the definition of the normal range and performance characteristics of the assay, separate measurement of both 25(OH)D(2) and 25(OH)D(3), and quality assurance tesing of laboratories offering the test. Clinicians have two main types of 25(OH)D assay based on either high-performance liquid chromatography with UV or mass detection or higher throughput kits based on protein (competitive protein binding assay or radioimmunoassay) binding. Based on 30 yr of experience with measuring 25(OH)D levels, it is concluded that, in the hands of appropriately trained experts, both types of assay provide reliable and accurate results, but all laboratories providing 25(OH)D data need frequent external quality assurance service to ensure that this performance is maintained.

Structure elucidation of retinoids in biological samples using postsource decay laser desorption/ionization mass spectrometry after high-performance liquid chromatography separation

Retinoids [retinol (vitamin A) and its metabolites] function in the visual cycle, embryonic development, cellular differentiation, and tissue homeostasis. Notwithstanding pivotal roles of retinoids in mammals, the limited number of commercially available retinoid standards is a major roadblock to identifying and studying retinoids in biological samples. Therefore, a need exists for improved methods to identify retinoid metabolites. We analyzed polar and nonpolar retinoids, including retinoic acid, retinol, retinyl acetate, and other retinyl esters, using postsource decay laser desorption/ionization mass spectrometry (PSD-LDI MS). PSD analysis was employed to examine the PSD fragmentation patterns of retinoids, as these patterns can be used for the characterization of retinoids from biological samples without the need for matching retention time with a commercially available or synthetic retinoid. Mechanisms for the formation of these PSD fragment ions are proposed. The feasibility of employing PSD after HPLC separation was demonstrated by characterizing the endogenous retinoids in canine kidney epithelial cell extracts and in mouse lung. We show that the PSD-LDI MS approach described here can facilitate the identification and characterization of retinoids from mammalian cells and tissues.

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.