23, 25, 26-trihydroxycholecalciferol. A 25-hydroxycholecalciferol-26,23-lactone precursor

(23S)-23,25-Dihydroxycholecalciferol was converted into at least five metabolites in kidney homogenates prepared from 1,25-dihydroxycholecalciferol-treated chickens. One of these has been positively identified as 23,25,26-trihydroxycholecalciferol by u.v.-absorbance analysis, mass spectrometry and chemical formation of derivatives. 23,25,26-Trihydroxycholecaciferol produces 25-hydroxycholecalciferol-26,23-lactone when incubated in chick kidney homogenates.

Metabolism and molecular mechanism of action of vitamin D: 1981

Cholecalciferol must be regarded as a pro-hormone rather than a vitamin, since it is normally produced in skin under the influence of ultraviolet light. Cholecalciferol must be metabolized in liver to 25-hydroxycholecalciferol and subsequently to 1,25-dihydroxycholecalciferol before it can act on intestine, bone and kidney to provide calcium and phosphorus for bone mineralization and neuromuscular activity. 1,25-Dihydroxycholecalciferol is metabolized in liver and intestine to a C-23-carboxylic acid that is inactive, 25-Hydroxycholecalciferol is metabolized to a variety of metabolic products, including 23S,25-dihydroxycholecalciferol, 23S,25R-25-hydroxycholecalciferol-26,23-lactone, 24R,25-dihydroxycholecalciferol and 25,26-dihydroxycholecalciferol. These metabolites are not involved in the known actions of vitamin D. 1,25-Dihydroxycholecalciferol localizes in the nuclei of target organs through a receptor mechanism. It is believed to initiate transcription of DNA that codes for calcium and phosphorus transport proteins, the nature of which is undetermined. Production of 1,25-dihydroxycholecalciferol is stimulated by low plasma calcium through parathyrin and by low plasma phosphorus. During pregnancy and lactation, 1,25-dihydroxycholecalciferol levels are greatly increased to meet calcium demands. However, vitamin D is not absolutely essential for reproduction. It is likely that some other hormone, possibly prolactin, functions at these periods to mobilize calcium. The clinical application of the vitamin D hormone and its analogues to the treatment of bone disease is presented to illustrate the application of basic science to medical practice. Evidence for each of these points is presented.

Isolation and chemical characterization of two new vitamin D metabolites produced by the intestine. 1,25-Dihydroxy-23-oxo-vitamin D3 and 1,25,26-trihydroxy-23-oxo-vitamin D3

Two new vitamin D metabolites were isolated in pure form from incubations of 53 nM 1,25-dihydroxyvitamin D3 with homogenates of small intestinal mucosa of vitamin D-replete chicks. The birds were injected intravenously with 8 to 9 nmol of 1,25-dihydroxyvitamin D3/100 g body weight 5 to 8 h before death. The isolation involved methanol-chloroform extraction and four successive chromatographic procedures (Sephadex LH-20 and high performance liquid chromatography). Chemical structures of the metabolites are proposed on the basis of (a) their chromatographic behavior, (b) their mass spectra, and (c) ultraviolet absorption spectra. They are identified as 1 alpha,25-dihydroxy-23-oxo-vitamin D3 and 1 alpha,25,26-trihydroxy-23-oxo-vitamin D3. Neither of the two new metabolites is produced by the intestinal mucosa when 1,25S,26-trihydroxyvitamin D3 is used as a substrate.

Biliary excretion of [3H]-25-hydroxyvitamin D3 in the vitamin D-depleted rat

The biliary excretion of [3H]-25-hydroxyvitamin D3 ([3H]25(OH)D3) was studied in vitamin D-depleted female rats over a 3-h period after intravenous or intraduodenal administration of intact [3H]25(OH)D3 and after the intraduodenal readministration of the [3H]25(OH)D3-derived biliary material. In each group four doses of 25(OH)D3 were administered (0.25, 2.5, 25, and 250 nmol/100 g). Over the dose range studied, the biliary excretion of [3H]25(OH)D3 could not be saturated, indicating that the biliary excretion of 25(OH)D3 is a reliable detoxification mechanism in circumstances of 25(OH)D3 intoxication. Analysis of plasma, liver, and bile suggests that the canalicular membrane seems to be rate limiting in the biliary excretion of 25(OH)D3. The intraduodenal administration of biliary excretion compounds derived from [3H]25(OH)D3 showed that they are efficiently reexcreted in newly secreted bile, confirming the existence of an enterohepatic circulation for 25(OH)D3. In this group of animals, however, the plasma analysis indicates that these compounds reach the systemic circulation in insignificant quantities, suggesting that the enterohepatic circulation probably plays a limited role in the body 25(OH)D3 economy.

Vitamin D metabolites in human milk

The concentrations of unconjugated 25-OHD, 24, 25(OH)2D, and 1,25(OH)2D were measured in human milk by competitive protein-binding radioassays following successive preparative Sephadex LH-20 chromatography and HPLC. The mean (+/- SE) concentration of 25-OHD was 0.37 +/- 0.03 ng/ml, of 24,25(OH)2D was 24.8 +/- 1.9 pg/ml, and of 1,25(OH)2D was 2.2 +/-0.1 pg/ml. The concentration of 25-OHD3 in milk as determined by HPLC and UV detection at 254 nm was 0.27 +/- 0.08 ng/ml. The milk concentrations of vitamin D metabolites did not correlate with the maternal serum 25-OHD levels. The total amounts of unconjugated vitamin D metabolites correspond to the known low bioassayable vitamin D antirachitic activity in human milk.

Evidence for 24,25-dihydroxycholecalciferol receptors in long bones of newborn rats

Several reports have appeared that suggest that 24,25-dihydroxycholecalciferol has a possible biological role in bone formation. We have utilized competition studies, saturation analysis, sucrose-density-gradient sedimentation and DEAE-cellulose chromatography to demonstrate that long bones of vitamin D-depleted newborn rats contain cytoplasmic and possibly nuclear receptors that bind 24,25-dihydroxycholecalciferol with specificity and high affinity (Kd = 1.79 nM). Sucrose-density-gradient analysis of the cytoplasmic 24,25-dihydroxycholecalciferol-binding component showed a single binding macromolecule for 24,25-dihydroxycholecalciferol with a sedimentation coefficient of 3.1 S. DEAE-cellulose chromatography showed a [3H]24,25, dihydroxycholecalciferol-macromolecular complex that binds to DEAE-cellulose and elutes between 0.15 and 0.21 M-KCl. The finding of 24,25-dihydroxycholecalciferol receptors in long bones of newborn rats suggests a possible involvement of 24,25-dihydroxycholecalciferol in the metabolism of developing skeletal tissues.

Chemical synthesis, biological activity, and metabolism of 25-hydroxy-24-oxovitamin D3

25-Hydroxy-24-oxovitamin D3 (25(OH)24-oxo-D3), a metabolite of 25-hydroxyvitamin D3, has been chemically synthesized. The ultraviolet, mass, infrared, and proton nuclear magnetic resonance spectra of the 25(OH)24-oxo-D3 were identical with those of the natural product isolated from chick kidney incubates. The oxo compound showed biological activity similar to 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) in vitamin D-deficient chicks in enhancing intestinal calcium transport and bone calcium mobilization activities. Although 25(OH)24-oxo-D3 partially restored the impaired eggshell weights of Japanese quails fed a vitamin D-deficient diet, it was much less potent than 25-hydroxyvitamin D3 or 1 alpha, 25-dihydroxyvitamin D3. In addition, there was no effect on the calcification of medullary bone. When 25(OH)24-oxo[3H]D3 was incubated with kidney homogenates from vitamin D-deficient chicks, it was metabolized to [3H]1 alpha, 24,25-trihydroxyvitamin D3 and a metabolite which was eluted in a region between authentic 24R,25(OH)2D3 and 1 alpha, 25-dihydroxyvitamin D3 on high pressure liquid chromatography. In the incubates of kidney homogenates from vitamin D-supplemented chicks, those metabolites were not detected. In vitamin D-supplemented chicks, the recovery of radioactivity in the chloroform phase extracted by the method of Bligh and Dyer was only 50%, while that in vitamin D-deficient chicks was 87%. Moreover, the radioactivity eluted in the 25(OH)24-oxo-D3 fraction from vitamin D-supplemented chicks was only one-fifth of that from vitamin D-deficient birds. The present results indicate that the 24-oxidation of 24,25(OH)2D3 may be a route of inactivation of vitamin D3.

Do anticonvulsant drugs commonly induce osteomalacia?

Twenty adult epileptic out-patients who had received anticonvulsant therapy for a mean duration of 15 years were assessed for clinical, biochemical, radioisotopic and bone biopsy evidence of osteomalacia. Occasional biochemical abnormalities were demonstrated but no individual subject was found to have osteomalacia. There is increasing evidence to cast doubt on the existence of a strong relationship between anticonvulsant drugs and osteomalacia. This evidence is reviewed and it is concluded that in the presence of adequate sunlight exposure anticonvulsant therapy alone is most unlikely to lead to osteomalacia in adults.

Faecal tritium excretion after intravenous administration of 3H-25-hydroxyvitamin D3 in control subjects and in patients with malabsorption

Faecal tritium excretion after intravenous 3H-25-hydroxyvitamin D3 administration was measured in three control subjects and in six patients with small intestine resection or bypass. The mean daily faecal tritium excretion over four to six days ranged from 0.8-1.6% of the injected dose in the controls (mean 1.2) and 0.9-6.8% in the patients (mean 3.7). There was a significant positive correlation between stool volume and the mean daily faecal tritium excretion. No correlation was found between the faecal tritium excretion and the plasma 25-hydroxyvitamin D concentration. Between 2.5 and 19.0% of faecal radioactivity eluted as 3H-25-hydroxyvitamin D3 on silicic acid chromatography. We conclude that faecal loss of endogenous 25-hydroxyvitamin D may be increased after small intestinal resection or bypass. Although the amount lost by this route is relatively small, it may contribute to the development of vitamin D deficiency in patients with malabsorption when endogenous vitamin D3 synthesis is also reduced.

Vitamin D compounds in cows' milk

The milk from cows fed normal levels of vitamin D has been found to contain approximately 40 IU per liter of vitamin D activity. A 14-fold increase in dietary vitamin D intake causes only a doubling of the amount of vitamin D in milk. This was determined by measuring stimulation of intestinal calcium transport in the vitamin D-deficient rat. Four vitamin D compounds were then isolated from cow's milk using a combination of conventional chromatography on Sephadex LH-20 and Lipidex 5000 followed by high-performance liquid chromatography. 24,25-Dihydroxycholecalciferol and 1,25-dihydroxycholecalciferol were measured using binding protein assays. One liter of milk contained 27 ng and 4.9 ng, respectively, of these two metabolites. Together these account for about 15% of the vitamin D activity. Cholecalciferol was found to be present at a concentration of 281 ng/liter or 11 IU/liter of biological activity. The milk contained 145 ng/liter 25-hydroxycholecalciferol or 29 IU/liter of activity. Therefore the known vitamin D compounds fully account for the biological activity observed in milk. It is therefore clear that no evidence could be found for the existence of a highly active water-soluble form of vitamin D in milk.

Brain target sites for 1,25-dihydroxyvitamin D3

Autoradiographic studies with 3H-labeled 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] demonstrate, in certain neurons of rat forebrain, hindbrain, and spinal cord, a nuclear retention and concentration of radioactivity, which can be prevented by treatment with 1,25(OH)2D3, but not with 25-hydroxyvitamin D3. These results indicate the presence of brain receptors in addition to pituitary receptors for 1,25(OH)2D3 and suggest a central modulation of calcium homeostasis and other central effects for this hormone. The existence of a brain-pituitary axis for certain 1,25(OH)2D3-mediated endocrine-autonomic effects is postulated.

1,25-Dihydroxyvitamin D3 receptors and functions in cultured pig kidney cells (LLC PK1). Regulation of 24,25-dihydroxyvitamin D3 production

Although 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) regulates the renal metabolism of 25-hydroxyvitamin D3 (25-OH-D3), the mechanism is not well understood. The established pig kidney cell line, LLC PK1, was used to study this feedback regulation. These cells possess a receptor for 1,25-(OH)2D3 with a sedimentation coefficient of 3.2 S. Scatchard analysis of 1,25-(OH)2D3 binding to cell cytosol yielded an apparent Kd of 0.12 nM and an Nmax of 26 fmol/mg of cytosol protein. LLC PK1 cells respond to 1,25-(OH)2D3 by changes in the metabolism of 25-OH-D3. When incubated with 25-OH-[3H]D3, homogenates of untreated cells did not produce detectable 1,25-(OH)2[3H]D3 or 24,25-(OH)2[3H]D3. However, after treatment of cell monolayers with 1,25-(OH)2D3 for 8 h, homogenates converted substantial 25-OH-[3H]D2 substrate to 24,25-(OH)2[3H]D3. The appearance of this 24-hydroxylase activity in response to 1,25-(OH)2D3 was time- and dose-dependent. Half-maximal levels of enzyme activity were achieved with 0.13 nM 1,25-(OH)2D3, a concentration almost identical to the Kd of the 1,25-(OH)2D3 receptor. The stimulation of 24-hydroxylase activity was shown to be an induction event; treatment of monolayers with 13 nM 1,25-(OH)2D3 for a 4-h pulse was sufficient to induce maximal activity assayed at h. The presence of the transcriptional inhibitor, actinomycin D, during the 4-h pulse abolished the induction of 24-hydroxylase activity. These results demonstrate for the first time the presence of both 1,25-(OH)2D3 receptors and stimulation of 24-hydroxylase activity within the same established mammalian kidney cell line.

Disorders of cholecalciferol metabolism in old egg-laying hens

It has been reported that the rate of cracked or soft-shelled eggs markedly increases in old laying hens. We investigated the effect of age on cholecalciferol metabolism in different age groups of laying hens. The egg production rate in hens more than 500 days old was maintained within a range of about 70% of that in young hens (230-320 days old), whereas the rate of cracked or soft-shelled eggs increased markedly with age. When kidney homogenates from the different age groups were incubated with [3H]-25-hydroxyvitamin D-3, renal 25-hydroxyvitamin D-3-1 alpha-hydroxylase activity was found to decrease markedly with age. When birds were given intravenously either [3H]-25-hydroxyvitamin D-3 or [3H]-1 alpha,25-dihydroxyvitamin D-3, the accumulation of [3H]-1 alpha,25-dihydroxyvitamin D-3 in plasma and target tissue also decreased with age. Forced molting performed in old hens restored eggshell quality. The treatment also restored, though partially, the in vivo accumulation of [3H]-l alpha,25-dihydroxyvitamin D-3 in the target tissues. These results suggest that the increased rate of cracked or soft-shelled eggs seen in older birds is associated with disorders of vitamin D-3 metabolism.

[Cholecalciferol metabolism of pregnant rats with vitamin D poisoning]

Four and 24 hours after administering toxic doses of vitamin D3 the concentration of unchanged cholecalciferol in the blood plasma, liver, kidneys, osseous and muscle tissues in pregnant rats was lower than in the control animals. The content of 25-hydroxycholecalciferol in all test tissues of the pregnant rats was higher than in non-pregnant animals. Particularly great amounts of this metabolite are accumulated by the osseous tissue, kidneys and intestine, i.e. by vitamin D target tissues. During vitamin D poisoning the content of 25-hydroxycholecalciferol in embryos and placenta is fairly high, the content in the placenta being greater as compared to the test tissues of the pregnant rats.

Extraction of vitamin D metabolites by bones of normal adult dogs

Using the isolated perfused canine tibia we examined the extraction of [(3)H]25(OH)D(3), [(3)H]1,25(OH)(2)D(3) and [(3)H]24,25(OH)(2)D(3) by bone of normal adult dogs. The studies were performed with and without vitamin D binding protein (DBP) in the perfusate to examine the effect of protein binding on the extraction of the vitamin D metabolites. An average of 48+/-2% of [(3)H]25(OH)D(3) was extracted by bone, when no DBP was present. However, addition of only a small amount of DBP ( approximately 720 ng/ml of perfusate) nearly completely abolished the extraction of [(3)H]25(OH)D(3) by bone. No degradation and/or transformation of the labeled 25(OH)D(3) could be demonstrated during passage through the isolated perfused bone. The extraction of [(3)H]24,25(OH)(2)D(3) in a DBP-free medium averaged 33+/-5%. Addition of 720 ng of DBP/ml of perfusate completely inhibited the extraction of this metabolite. The extraction of [(3)H]1,25(OH)(2)D(3) averaged 30+/-3% in a DBP free medium and no inhibition of the extraction was demonstrated after addition of DBP (720 ng/ml of perfusate). However, addition of DBP in a concentration of 14.4 mug/ml of perfusate reduced the extraction of 1,25(OH)(2)D(3) to 8+/-2%, a value still significantly higher than that seen after addition of 20 times less DBP to perfusions with 25(OH)D(3) and 24,25(OH)(2)D(3). It is concluded that the isolated perfused bone of normal dogs can extract significant amounts of 25(OH)D(3), 1,25(OH)(2)D(3), and 24,25(OH)(2)D(3). Small concentrations of DBP (720 ng/ml) in the perfusate significantly inhibited the extraction of 25(OH)D(3) and 24,25(OH)(2)D(3). A carrier role for DBP is suggested and it is proposed that the levels of free vitamin D are important for extraction of the metabolites by bone. Therefore, due to the different affinities of DBP for the various metabolites of vitamin D, only 1,25(OH)(2)D(3) is extracted in vitro in significant amounts by bone of normal adult dogs, in the presence of DBP.

Acquired alterations in vitamin D metabolism in the acidotic state

Classic (type I) renal tubular acidosis in children in attended by growth retardation and rickets, abnormalities that can be corrected by alkali therapy alone. We have employed the NH4Cl-treated rachitic chick as a model to investigate vitamin D metabolism in the acidotic state. NH4Cl ingestion for 96 h was associated with a rise in serum calcium, a significant decrease in blood pH (7.42 +/- 0.08 vs 7.30 +/- 0.08, P less than 0.005), decreased [3H]1,25(OH)2D3 following [3H]25OHD D3 injections, and enhanced metabolic clearance of administered [3H]1,25(OH)2D3. The data collectively suggest that metabolic acidosis in the chick alters the production and degradation of 1,25(OH)2D3.

Comparison of binding of 1,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol in intact tissue and cytosol preparations from bone and other tissues in the fetal rat

The binding of 1,25-dihydroxy[3H]cholecalciferol (1,25(OH)2[3H]D3) and 25-hydroxy-[3H]cholecalciferol (25(OH)[3H]D3) in vitamin D target and non-target tissues from the fetal rat has been compared using two incubation conditions, each followed by charcoal adsorption and sucrose gradient centrifugation. In intact tissue incubations, equilibrium with the ligand was established overnight at 4 °C before cell disruption. In pre-prepared cytosol incubations, cytosol was prepared from the tissue before incubation with ligand. With both ligands, more sterol was bound during pre-prepared cytosol incubation despite the use of fourfold lower ligand concentrations. With 1,25(OH)2[3H]D3, intact tissue incubation led to most marked binding with calvaria, which was preferentially displaced by unlabelled 1,25(OH)2D3; radioactivity sedimented almost entirely as a 3·2–3·7S peak (peak I) which was completely displaced by 100-fold excess 1,25(OH)2D3 but only partially by 25(OH)D3. Fetal small intestine, another putative target tissue, also showed displaceable binding of 1,25(OH)2D3; however, this was much less marked, and was not accompanied by a significant peak on sucrose gradients. Other fetal tissues (large intestine, kidney, skin, brain and heart) did not show significant displaceable binding of 1,25(OH)2D3 in intact tissue. In contrast, when intact calvaria, kidney, brain and heart were incubated with 25(OH)D3 they all showed significant displaceable binding to a 5·0–5·7S peak (peak II).

Incubations with pre-prepared cytosol confirmed ubiquitous binding of both sterols to peak II with the notable exception of that from small intestine; this peak II binding was preferential for 25(OH)D3, and is believed to be to the plasma vitamin D-binding protein (DBP). Only calvaria showed an additional peak I under these conditions, and even here it was dwarfed by peak II. We conclude that, at least in fetal bone, intact tissue incubations selectively detect receptor (peak I) binding of 1,25(OH)2D3 in the presence of the DBP. The substantial increase in binding of both sterols to peak II in pre-prepared cytosol compared with that in the same fetal tissues incubated intact suggests that some DBP is intracellular and therefore relatively inaccessible to extracellular sterol. The apparent absence of both peak I and peak II binding of cytosol from fetal small intestine merits further investigation.

Some characteristics of cytosol binding protein for 1 alpha,25-dihydroxycholecalciferol, 24R,25-dihydroxycholecalciferol and 25-hydroxycholecalciferol in rat parotid gland

1. 1 alpha,25-dihydroxycholecalciferol (1,25-(OH)2VD3), 24R,25-dihydroxycholecalciferol (24,25-(OH)2VD3) and 25-hydroxycholecalciferol (25-OHVD3) binding proteins were existed in cytosol of rat parotid gland. 2. The dissociation constants of binding proteins for 1,25-(OH)2VD3, 24,25-(OH)2VD3 and 25-OHVD3 were 3.15 x 10(-9), 5.05 x 10(-9) and 6.60 x 10(9) M, respectively. 3. The mol wt and the isoelectric point (pI) of binding proteins for 1,25-(OH)2VD3, 24,25-(OH)2VD3, 24,25-(OH)2VD3 and 25-OHVD3 were the same to each other (with mol wt of 155,000 and pI of 4.87). 4. The binding specificity of 1,25-(OH)2VD3 binding protein was similar to that of 24,25-(OH)2VD3 and 25-OHVD3 binding protein. 5. The extent of loss of binding activity for 1,25-(OH)2VD3 by pronase and trypsin was much greater than that of binding activity for 24,25-(OH)2VD3 and 25-OHVD3.