25,26-Dihydroxycholecalciferol: a precursor in the renal synthesis of 25-hydroxycholecalciferol-26,23-lactone

Synthesis and determination of configuration of natural 25-hydroxyvitamin D(3) 26,23-lactone

The four stereoisomers of 25-hydroxyvitamin D(3) 26,23-lactone were synthesized by a stereoselective lactonization method. Natural 25-hydroxyvitamin D(3) 26,23-lactone was produced from 25-hydroxy-[3alpha-(3)H]vitamin D(3) by in vitro incubation of kidney homogenate prepared from vitamin D-supplemented chickens or was isolated from the serum of rats given 1,25-dihydroxyvitamin D(3) and 25-hydroxy-[3alpha-(3)H]vitamin D(3). The four synthetic isomers and the naturally produced 25-hydroxyvitamin D(3) 26,23-lactone were subjected to high-performance liquid chromatography in a system capable of separating the four isomers. The natural lactone comigrated with synthetic (23S,25R)-25-hydroxyvitamin D(3) 26,23-lactone, establishing it as the natural vitamin D(3) metabolite.

In vitro metabolism of the vitamin D analog, 22-oxacalcitriol, using cultured osteosarcoma, hepatoma, and keratinocyte cell lines

Using four cultured cell models representing liver, keratinocyte, and osteoblast, we have demonstrated that the vitamin D analog, 22-oxacalcitriol is degraded into a variety of hydroxylated and side chain truncated metabolites. Four of these metabolic products have been rigorously identified by high pressure liquid chromatography, diode array spectrophotometry, and gas chromatography-mass spectrometry analysis as 24-hydroxylated and 26-hydroxylated derivatives as well as the cleaved molecules, hexanor-1alpha,20-dihydroxyvitamin D3 and hexanor-20-oxo-1alpha-hydroxyvitamin D3. Comparison with chemically synthesized standards has revealed the stereochemistry of the biological products. Although differences exist in the amounts of products formed with the different cell types, it is apparent that 22-oxacalcitriol is subject to metabolism by both vitamin D-inducible and noninducible enzymes. Time course studies suggest that the truncated 20-alcohol is derived from a side chain hydroxylated molecule via a hemiacetal intermediate and the 20-oxo derivative is likely formed from the 20-alcohol. Biological activity measurements of the metabolites identified in our studies are consistent with the view that these are catabolites and that the biological activity of 22-oxacalcitriol is due to the parent compound. These results are also consistent with recent findings of others that the biliary excretory form of 22-oxacalcitriol is a glucuronide ester of the truncated 20-alcohol.

(23S)-1,23,25-Trihydroxyvitamin D3: its biologic activity and role in 1 alpha,25-dihydroxyvitamin D3 26,23-lactone biosynthesis

(23S)-1,23,25-Trihydroxyvitamin D3 was isolated from bovine kidney homogenates incubated with 1,25-dihydroxyvitamin D3 by sequential chromatography through one Sephadex LH-20 column and three high-performance liquid chromatography systems. Ultraviolet absorption spectroscopy and mass spectrometry confirmed the structural assignment. One high-performance liquid chromatography system separated the R and S epimers of 1,23,25-trihydroxyvitamin D3 and indicated that the natural product had the S configuration. Plasma pharmacokinetic studies in rats showed that (23S)-1,23,25-trihydroxy[3H]vitamin D3 was rapidly cleared from plasma (t1/2 = 60 min). 1 alpha,25-Dihydroxy[3H]vitamin D3 26,23-lactone appeared concurrently with the disappearance of (23S)-1,23,25-trihydroxy[3H]vitamin D3. Experiments with radioinert compounds showed that 1,25-dihydroxyvitamin D3 and (23S)-1,23,25-trihydroxyvitamin D3 were efficient precursors to 1,25-dihydroxyvitamin D3 26,23-lactone both in intact and in nephrectomized rats. (25S)-1,25,26-Trihydroxyvitamin D3, however, was ineffective at raising plasma 1,25-dihydroxyvitamin D3 26,23-lactone concentrations. These results confirm the presence of extrarenal 1,25-dihydroxyvitamin D3 23(S)-hydroxylase(s) and demonstrate that C-23 hydroxylation of 1,25-dihydroxyvitamin D3 precedes C-26 hydroxylation in the formation of 1,25-dihydroxyvitamin D3 26,23-lactone. (23S)-1,23,25-Trihydroxyvitamin D3 had no intestinal calcium absorptive or bone calcium resorptive activity when dosed to vitamin D deficient rats at levels up to 500 ng. C-23 oxidation, therefore, appears to be a physiologic pathway of 1,25-(OH)2D3 metabolism and a major pathway for the deactivation of pharmacologic levels of 1,25-dihydroxyvitamin D3.

Biological activity assessment of the 26,23-lactones of 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 and their binding properties to chick intestinal receptor and plasma vitamin D binding protein

The binding of the natural and unnatural diastereoisomers 25-hydroxyvitamin D3-26,23-lactone and 1,25 dihydroxyvitamin D3-26,23-lactone to the vitamin D-binding protein (DBP) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] chick intestinal receptor have been investigated. Also, the biological activities, under in vivo conditions, of these compounds, in terms of intestinal calcium absorption (ICA) and bone calcium mobilization (BCM), in the chick are reported. The presence of the lactone ring in the C23-C26 position of the seco-steroid side chain increased two to three times the ability of both 25(OH)D3 and 1,25(OH)2D3 to displace 25(OH)[3H]D3 from the D-binding protein; however, the DBP could not distinguish between the various diastereoisomers. In contrast, the unnatural form (23R,25S) of the 25-hydroxy-lactone was found to be 10-fold more potent than the natural form, and the unnatural (23R,25S)1,25(OH)2D3-26,23-lactone three times more potent than the natural 1,25-dihydroxy-lactone in displacing 1,25(OH)2[3H]D3 from its intestinal receptor. While studying the biological activity of these lactone compounds, it was found that the natural form of the 25-hydroxy-lactone increased the intestinal calcium absorption 48 h after injection (16.25 nmol), while bone calcium mobilization was decreased by the same dose of the 25-hydroxy-lactone. The 1,25-dihydroxyvitamin D3-26,23-lactone in both its natural and unnatural forms was found to be active in stimulating ICA and BCM. These results suggest that the 25-hydroxy-lactone has some biological activity in the chick and that 1,25(OH)2D3-26,23-lactone can mediate ICA and BCM biological responses, probably through an interaction with 1,25-(OH)2D3 specific receptors in these target tissues.

Stereo-retained and stereo-selective lactonization of four diastereoisomers of 23,25,26-trihydroxyvitamin D3 in homogenates of kidney from vitamin D-supplemented chicks

To elucidate the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone, various vitamin D3 derivatives were incubated individually with kidney homogenates prepared from vitamin D3-supplemented chicks, a preparation known to produce the 25-hydroxyvitamin D3-26,23-lactone from 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3-26, 23-lactone produced in vitro was then separated, purified, identified, and quantitated by consecutive analysis by high-pressure liquid chromatography. The naturally occurring 23(S), 25(R)-25-hydroxyvitamin D3-26,23-lactone was produced from 23(S),25-dihydroxyvitamin D3, 25(R),26-dihydroxyvitamin D3, and 23(S),25(R),26-trihydroxyvitamin D3. 23(S),25 (S)-25-Hydroxyvitamin D3-26,23-lactone was synthesized from 25(S),26-dihydroxyvitamin D3 and 23(S),25(S),26-trihydroxyvitamin D3. The relative amounts of 25-hydroxyvitamin D3-26,23-lactones generated from the following vitamin D3 derivatives used as substrate (23(S),25(S),26-trihydroxyvitamin D3; 23(R),25(R),26-trihydroxyvitamin D3; 23(S),25(R),26-trihydroxyvitamin D3; 23(R),25(S),26-trihydroxyvitamin D3; 23(S), 25-dihydroxyvitamin D3; 23(R),25-dihydroxyvitamin D3; 25(S),26-dihydroxyvitamin D3; and 25(R),26-dihydroxyvitamin D3) are, respectively, 15:1.7:24:3.3:2.5:0:1:1.7. These results indicate that when the lactonization at C-23 and C-26 positions of various vitamin D3 derivatives occurred the stereochemical configuration at their C-23 and/or C-25 positions was not changed and the difference of the stereochemical configurations determined the rate of lactonization.

Urinary and plasma vitamin D3 metabolites in the nephrotic syndrome

Using newly developed and established extraction, Lipidex-5000 chromatography, normal phase gradient HPLC, and ligand binding assay techniques we have directly measured plasma and urine levels of vitamin D3 and its metabolites in seven normal subjects and seven patients with nephrotic syndrome and normal renal function. Significant reductions in the plasma levels of vitamin D3, 24,25(OH)2D3, 25,26(OH)2D3, and 1,25(OH)2D3 were noted in all nephrotic patients. In conjunction with the plasma metabolite abnormalities, direct quantitative analysis of the urine in these patients revealed significant increases in nonconjugated 250HD3, 24,25(OH)2D3 and 1,25(OH)2D3. Significant correlations were noted between the plasma and/or urine metabolites and other mineral homeostatic parameters. The results indicate that the primary basis for the reductions in plasma vitamin D3 and its metabolites in the nephrotic syndrome is enhanced urinary excretion. The findings of normal serum ionized Ca and i-PTH levels in the patients with nephrosis suggest that reductions in bound and not free forms of vitamin D3 metabolites in plasma may occur in the initial stages of the nephrotic syndrome.

Comparison of plasma concentrations of vitamin D and its metabolites in young and aged domestic animals

1. Vitamin D and its metabolites were measured in the plasma of five species of rural domestic animals. 2. Concentration of 1,25-dihydroxyvitamin D was higher (P less than 0.01) in young animals (range 24-118 pg/ml, means +/- SD = 72.0 +/- 30.0) than in adult animals (range 14-67 pg/ml, means +/- SD = 40.2 +/- 22.6). 3. 25-Hydroxyvitamin D3-26,23 lactone was present only in the chick and the pig. 4. Unsheared sheep appeared to be inefficient utilizers of the photochemical conversion of 7-dehydrocholesterol to vitamin D3. 5. Conversion of 25-hydroxyvitamin D to 24,25-dihydroxyvitamin D was most efficient in species with high plasma phosphorus concentrations (pig, sheep).

23,25-Dihydroxyvitamin D3: a natural precursor in the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone

To elucidate the biosynthesis of 25-hydroxyvitamin D3-26,23-lactone, two known metabolites of 25-hydroxyvitamin D3--23,25-dihydroxyvitamin D3 and 25,26-dihydroxyvitamin D3--were incubated individually with kidney homogenate prepared from vitamin D-supplemented chickens, a preparation known to produce the lactone from 25-hydroxyvitamin D3. The 25-hydroxyvitamin D3-26,23-lactone produced in vitro was then separated, purified, identified, and quantitated by consecutive straight-phase and reverse-phase high-performance liquid chromatography. 23,25-Dihydroxyvitamin D3 is a far better substrate for production of 25-hydroxyvitamin D3-26,23-lactone than is 25,26-dihydroxyvitamin D3. Production of lactone is highly selective for the natural 23(S)-hydroxy-23,25-dihydroxyvitamin D3 while both epimers of 25,26-dihydroxyvitamin D3 resulted in small amounts of product comigrating with natural lactone. It appears that 23(S),25-dihydroxyvitamin D3, but not 25,26-dihydroxyvitamin D3, is a natural precursor in the synthesis of 25-hydroxyvitamin D3-26,23-lactone; this result also implies that the configuration of the lactone at C-23 is S.