Isolation, identification, and biological activity of 25-hydroxy-24-oxovitamin D3: a new metabolite of vitamin D3 generated by in vitro incubations with kidney homogenates

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Synthesis of putative metabolites of 1alpha,25-dihydroxy-2beta-(3-hydroxypropoxy)vitamin D(3) (ED-71)

1alpha,25-Dihydroxy-2beta-(3-hydroxypropoxy)vitamin D(3) (ED-71), an analog of active vitamin D(3), 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] is under phase III clinical trials in Japan for the treatment of osteoporosis and bone fracture prevention. Since ED-71 has a substituent at the 2beta-position of the A-ring, it is recognized that the metabolic pathway of ED-71 might be more complicated than 1,25(OH)(2)D(3) because of metabolism at the 2beta-position substituent in addition to the inherent metabolism of the side chain. To clarify the metabolism of hydroxypropoxy substituent of the 2beta-positon and a combination of metabolism between side chain and 2beta-positon, four putative metabolites of ED-71 have been prepared as authentic samples. The metabolites at the 2beta-positon, the methyl ester derivative considered as an ester standard of the oxidized metabolite and the tetraol derivative as the truncated metabolite were synthesized from alpha-epoxide, a key intermediate of ED-71 synthesis. The combination metabolites between side chain and 2beta-positon, the 24(S)- and 24(R)-pentaols were synthesized using Trost's convergent method.

Current understanding of the molecular actions of vitamin D

The important reactions that occur to the vitamin D molecule and the important reactions involved in the expression of the final active form of vitamin D are reviewed in a critical manner. After an overview of the metabolism of vitamin D to its active form and to its metabolic degradation products, the molecular understanding of the 1alpha-hydroxylation reaction and the 24-hydroxylation reaction of the vitamin D hormone is presented. Furthermore, the role of vitamin D in maintenance of serum calcium is reviewed at the physiological level and at the molecular level whenever possible. Of particular importance is the regulation of the parathyroid gland by the vitamin D hormone. A third section describes the known molecular events involved in the action of 1alpha,25-dihydroxyvitamin D3 on its target cells. This includes reviewing what is now known concerning the overall mechanism of transcriptional regulation by vitamin D. It describes the vitamin D receptors that have been cloned and identified and describes the coactivators and retinoid X receptors required for the function of vitamin D in its genomic actions. The presence of receptor in previously uncharted target organs of vitamin D action has led to a study of the possible function of vitamin D in these organs. A good example of a new function described for 1alpha,25-dihydroxyvitamin D3 is that found in the parathyroid gland. This is also true for the role of vitamin D hormone in skin, the immune system, a possible role in the pancreas, i.e., in the islet cells, and a possible role in female reproduction. This review also raises the intriguing question of whether vitamin D plays an important role in embryonic development, since vitamin D deficiency does not prohibit development, nor does vitamin D receptor knockout. The final section reviews some interesting analogs of the vitamin D hormone and their possible uses. The review ends with possible ideas with regard to future directions of vitamin D drug design.

In vivo metabolism of 24R,25-dihydroxyvitamin D3: structure of its major bile metabolite

In vivo metabolism of 24R,25-dihydroxyvitamin D3 (24,25-(OH)2D3) in female dogs has been studied thoroughly, and its major bile metabolite identified. After single oral administration of 24,25-(OH)2 [6,19,19-3H]D3 the plasma concentrations of radioactive metabolites were monitored for 504 h, and the metabolites in the bile collected and analyzed. The concentration of 24,25-(OH)2D3 in plasma reached a maximum after 6 h and decayed in two distinct phases; a fast-phase with a half-life of 17 h, followed by a slow-phase with a 17-day half-life. The area under the concentration/time curve (AUC) was 78-84% (0-504 h). The only detectable metabolite in the plasma was 25-hydroxy-24-oxovitamin D3 whose AUC was less than 5%. At 504 h, about 50% of administered radioactivity has been excreted, of which about 90% was found in the feces, indicating most of the administered 24,25-(OH)2D3 to be excreted in bile. A major metabolite, which constituted 23% of the total bile radioactivity at 504 h, was found in the bile. This metabolite was efficiently deconjugated by beta-glucuronidase to afford an aglycone which was identified as 23S,25-dihydroxy-24-oxovitamin D3 (23S,25-(OH)2-24-oxo-D3), by co-chromatography on HPLC with synthetic standards. The glucuronide was isolated from the bile of dogs given large doses of 24,25-(OH)2D3, and the structure determined being 23-(beta-glucuronide) of 23S,25-(OH)2-24-oxo-D3, by analyzing its negative ion mass spectrum and the positive ion mass spectrum of its derivatives. Thus it was concluded that, in dogs, 24,25-(OH)2D3 is a long lasting vitamin D metabolite, is mainly excreted in bile when metabolized to 23S,25-(OH)2-24-oxo-D3 and is conjugated at 23-OH as glucuronide.

Further oxidation of hydroxycalcidiol by calcidiol 24-hydroxylase. A study with the mature enzyme expressed in Escherichia coli

The coding region of the cDNA for rat kidney calcidiol 24-hydroxylase (P450cc24), which is involved in calcium homeostasis in animals, was inserted into an expression vector pKK223-3. The recombinant plasmid was formed in a specific manner without deletion or substitution of any parts of the coding region of the cDNA. When the resulting plasmid was introduced into Escherichia coli JM109, the recombinant cells produced a protein which was immunoreactive to an antibody against P450cc24. When the cell-free extract of the transformed cells was incubated with calcidiol together with bovine adrenodoxin and NADPH-adrenodoxin reductase, not only hydroxycalcidiol but also other metabolites such as oxocalcidiol and oxohydroxycalcidiol were produced. Similarly, calcitriol was converted not only to calcitetrol but also to oxocalcitriol and oxohydroxycalcitriol. These results indicate that a single enzyme expressed in the bacteria is responsible for all these successive reactions.

Syntheses of 24R,25-dihydroxy-[6,19,19-3H]vitamin D3 and 24R,25-dihydroxy-[6,19,19-2H]vitamin D3

24R,25-Dihydroxy-[6,19,19-3H]vitamin D3 with a specific activity of 54 Ci/mmol and 24R,25-dihydroxy-[6,19,19-2H]vitamin D3 with 2.6 deuterium atoms/mol were synthesized in four steps starting from 24R,25-Dihydroxyvitamin D3 via its sulfur dioxide adduct.

Ring hydroxylation of 25-hydroxycholecalciferol by rat renal microsomes

Two metabolites have been isolated from rat renal microsomes incubated with 25-hydroxycholecalciferol. Postmitochondrial supernatant fractions from kidneys of thyroidectomized and parathyroidectomized rats were incubated with magnesium acetate, potassium acetate, an NADPH generating system, and 25-hydroxycholecalciferol at a level of 20 micrograms/ml postmitochondrial supernatant for 60 min at 30 degrees C. Lipid extracts of the incubation mixtures were purified by silica gel TLC and HPLC. Two peaks were obtained. Metabolite chi 2 eluted at 18 min and metabolite chi 1 at 23 min when chromatographed on a silica column developed with hexane-isopropanol. Metabolites chi 1 and chi 2 were found to have maximal absorbance at 265 nm. Both metabolites were periodate sensitive, indicating vicinal hydroxyl groups. Mass spectral analysis of metabolite chi 2, which was isolated in greater quantity than metabolite chi 1, indicates that metabolite chi 2 had resulted from hydroxylation of the A ring. Results indicate that 25-hydroxycholecalciferol is hydroxylated on carbon 2 or carbon 4 by renal microsomes. Metabolites chi 1 and chi 2, because of similarity in chromatographic migration and periodate sensitivity, are, perhaps, isomers or 2- and 4-hydroxylated metabolites.

Side-chain hydroxylation of vitamin D3 and its physiological implications

Evidence is accumulating that, in vivo and in vitro, both 25-OH-D3 and 1,25-(OH)2D3 undergo side-chain modification leading to side-chain cleaved metabolites lacking the 24, 25, 26, and 27 carbons. The enzymes involved are D-dependent and are located in the kidney, bone, intestine, and perhaps other sites. We speculate that the extra-renal side-chain pathway may be primarily for target organ destruction of 1,25-(OH)2D3, whereas the renal pathway may be primarily for destruction of 25-OH-D3 formed in large amounts in hypervitaminosis D.

Differences in the side-chain metabolism of vitamin D3 between chickens and rats

In vitro metabolism of 25-hydroxy-24-oxovitamin D3 was studied in kidney homogenates from vitamin D-supplemented chickens and rats. In chicken homogenates, 25-hydroxy-24-oxovitamin D3 was converted predominantly to 23,25-dihydroxy-24-oxovitamin D3, 24,25-dihydroxyvitamin D3, and 23,24,25-trihydroxyvitamin D3. In rat homogenates, 25-hydroxy-24-oxovitamin D3 was not converted to either 24,25-dihydroxyvitamin D3 or 23,24,25-trihydroxyvitamin D3, but it was converted to 23,25-dihydroxy-24-oxovitamin D3 and 23-hydroxy-24,25,26,27-tetranorvitamin D3. The latter metabolite was not produced by the chicken preparations. The stereochemical configuration at C-24 of the 24,25-dihydroxyvitamin D3 produced by chicken homogenates was determined to be S. This contrasts with the R configuration of 24,25-dihydroxyvitamin D3 produced by 24-hydroxylation of 25-hydroxyvitamin D3. These results suggest that chickens have an enzyme that can reduce the 24-oxo group to 24S-hydroxyl group, whereas rats do not.

Naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of both C-24R and C-24S epimers

Tritium-labeled 24,25-dihydroxyvitamin D3 was prepared both in vitro, by using chick kidney homogenates, and in vivo in rats from [26,27-methyl-3H]25-hydroxyvitamin D3. These compounds were mixed with synthetic 24(R),25- and 24(S),25-dihydroxyvitamin D3, converted to the corresponding trimethylsilyl ether derivatives, and analyzed by a high-pressure liquid chromatography procedure that separates the derivatized isomers. The tritium-labeled 24,25-dihydroxyvitamin D3 derivatives were found to be a mixture of both the 24(R) and 24(S) epimers; the ratio was found to be 96.4:3.6 in chick kidney homogenates and 96.8:3.2 in the serum of rats under physiological conditions. In addition, nonradioactive 24,25-dihydroxyvitamin D3 isolated from the serum of rats given large doses of vitamin D3 was shown to be an 89.5:10.5 mixture of the 24(R) and 24(S) isomers. When 25-hydroxy-24-oxo-vitamin D3 was utilized as a substrate, it was found to be more selectively reduced to 24(S),25-dihydroxyvitamin D3 than 24(R),25-dihydroxyvitamin D3 by the renal enzyme. The 24(S),25-dihydroxyvitamin D3 has been identified by ultraviolet absorption spectrophotometry, cochromatography with an authentic standard, and mass spectrometry. The reduced metabolites of 25-hydroxy-24-oxo-vitamin D3 were a 1:50 mixture of the 24(R) and 24(S) epimers. There are two known metabolic pathways leading to 24,25-dihydroxyvitamin D3 from 25-hydroxyvitamin D3; one is 24(R)-hydroxylation of 25-hydroxyvitamin D3 and the other is reduction of 25-hydroxy-24-oxo-vitamin D3. In contrast, 24(S),25-dihydroxyvitamin D3 is produced only by reduction of 25-hydroxy-24-oxo-vitamin D3 in the kidney. Therefore, naturally occurring 24,25-dihydroxyvitamin D3 is a mixture of the 24(R) and 24(S) isomers, and not just the 24(R) isomer as reported previously.

Isolation and identification of 1 alpha,25-dihydroxy-24-oxovitamin D3, 1 alpha,25-dihydroxyvitamin D3 26,23-lactone, and 1 alpha,24(S),25-trihydroxyvitamin D3: in vivo metabolites of 1 alpha,25-dihydroxyvitamin D3

Three new in vivo metabolites of 1 alpha,25-dihydroxyvitamin D3 were isolated from the serum of dogs given large doses (two doses of 1.5 mg/dog) of 1 alpha,25-dihydroxyvitamin D3. The metabolites were isolated and purified by methanol-chloroform extraction and a series of chromatographic procedures. By cochromatography on a high-performance liquid chromatograph, ultraviolet absorption spectrophotometry, mass spectrometry, Fourier-transform infrared spectrophotometry, and specific chemical reactions, the metabolites were identified as 1 alpha,25-dihydroxy-24- oxovitamin D3, 1 alpha,25-dihydroxyvitamin D3 26,23-lactone, and 1 alpha,24(S),25-trihydroxyvitamin D3. According to these procedures, the total amounts of the isolated metabolites were as follows: 1 alpha,25-dihydroxyvitamin D3, 23.6 micrograms; 1 alpha,25-dihydroxy-24- oxovitamin D3, 1.8 micrograms; 1 alpha,25-dihydroxyvitamin D3 26,23-lactone, 9.2 micrograms; 1 alpha,24(R),25-trihydroxyvitamin D3, 15.4 micrograms; 1 alpha,24(S),25-trihydroxyvitamin D3, 1.0 microgram. With recovery corrections, the serum levels of each metabolite were approximately 49 ng/mL for 1 alpha,25-dihydroxyvitamin D3, 3.7 ng/mL for 1 alpha,25-dihydroxy-24- oxovitamin D3, 19 ng/mL for 1 alpha,25-dihydroxyvitamin D3 26,23-lactone, 32 ng/mL for 1 alpha,24(R),25-trihydroxyvitamin D3, and 2.1 ng/mL for 1 alpha,24(S),25-trihydroxyvitamin D3.

23,25-Dihydroxy-24-oxovitamin D3: a metabolite of vitamin D3 made in the kidney

Kidney homogenates of rats produced a new metabolite of 25-hydroxyvitamin D3 which has been isolated in pure form after five column chromatographic steps. It was identified as 23,25-dihydroxy-24-oxovitamin D3 by means of ultraviolet and infrared absorption spectrophotometry, mass spectrometry, and proton nuclear magnetic resonance spectrometry. The stereochemistry at the C-23 position is as yet unknown. 25-Hydroxy-24-oxovitamin D3, which also has been isolated in pure form from this system, was found to be the precursor of the new metabolite in vitro. The production of the new metabolite was induced by two different methods: (a) perfusion of the kidneys with 1,25-dihydroxyvitamin D3 contained in the perfusate and (b) injection of 1,25-dihydroxyvitamin D3 in the intact animal. 23,25-Dihydroxy-24-oxovitamin D3 was not biologically active in an assay for intestinal calcium transport and bone calcium mobilization in the vitamin D deficient chick at a dose level of 5.3 nmol. A metabolic pathway is proposed to describe the results; it leads from 25-hydroxyvitamin D3 leads to 24(R),25-dihydroxyvitamin D3 leads to 25-hydroxy-24-oxovitamin D3 leads to 23,25-dihydroxy-24-oxovitamin D3.

23,24,25-Trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3, and 23-dehydro-25-hydroxyvitamin D3: new in vivo metabolites of vitamin D3

Four new in vivo metabolites of vitamin D3 were isolated from the blood plasma of chicks given large doses of vitamin D3. The metabolites were isolated by methanol-chloroform extraction and a series of chromatographic procedures. By use of mass spectrometry, ultraviolet absorption spectrophotometry, and specific chemical reactions, the metabolites were identified as 23,24,25-trihydroxyvitamin D3, 24,25,26-trihydroxyvitamin D3, 24-keto-25-hydroxyvitamin D3 and 23-dehydro-25-hydroxyvitamin D3.