Biological activity of 1,25-dihydroxycholecalciferol

Alterations in vitamin D metabolite, parathyroid hormone and fibroblast growth factor-23 concentrations in sclerostin-deficient mice permit the maintenance of a high bone mass

Humans with mutations of the sclerostin (SOST) gene, and knockout animals in which the Sost gene has been experimentally deleted, exhibit an increase in bone mass. We review the mechanisms by which Sost knockout mice are able to accrete increased amounts of calcium and phosphorus required for the maintenance of a high bone mass. Recently published information from our laboratory, shows that bone mass is increased in Sost-deficient mice through an increase in osteoblast and a decrease in osteoclast activity, which is mediated by activation of β-catenin and an increase in prostacyclin synthesis in osteocytes and osteoblasts. The increases in calcium and phosphorus retention required for enhanced bone mineral accretion are brought about by changes in the vitamin D endocrine system, parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23). Thus, in Sost knockout mice, concentrations of serum 1,25-dihydroxyvitamin D (1,25(OH)2D) are increased and concentrations of FGF-23 are decreased thereby allowing a positive calcium and phosphorus balance. Additionally, in the absence of Sost expression, urinary calcium is decreased, either through a direct effect of sclerostin on renal calcium handling, or through its effect on the synthesis of 1,25(OH)2D. Adaptations in vitamin D, PTH and FGF-23 physiology occur in the absence of sclerostin expression and mediate increased calcium and phosphorus retention required for the increase in bone mineralization. This article is part of a Special Issue entitled '17th Vitamin D Workshop'.

Sensitive method for the determination of paricalcitol by liquid chromatography and mass spectrometry and its application to a clinical pharmacokinetic study

A highly sensitive, specific and rapid LC-ESI-MS/MS method has been developed and validated for the quantification of paricalcitol (PAR) in human plasma (500 μL) using paricalcitol-d6 (PAR-d6 ) as an internal standard (IS) as per regulatory guidelines. A liquid-liquid extraction method was used to extract the analyte and IS from human plasma. Chromatography was achieved on Zorbax SB C18 column using an isocratic mobile phase in a gradient flow. The total chromatographic run time was 6.0 min and the elution of PAR and PAR-d6 occurred at ~2.6 min. A linear response function was established for the range of concentrations 10-500 pg/mL in human plasma. The intra- and inter-day accuracy and precision values for PAR met the acceptance criteria. The validated assay was applied to quantitate PAR concentrations in human plasma following oral administration of 4 µg capsules to humans.

Reduced renal calcium excretion in the absence of sclerostin expression: evidence for a novel calcium-regulating bone kidney axis

The kidneys contribute to calcium homeostasis by adjusting the reabsorption and excretion of filtered calcium through processes that are regulated by parathyroid hormone (PTH) and 1α,25-dihydroxyvitamin D3 (1α,25[OH]2D3). Most of the filtered calcium is reabsorbed in the proximal tubule, primarily by paracellular mechanisms that are not sensitive to calcium-regulating hormones in physiologically relevant ways. In the distal tubule, however, calcium is reabsorbed by channels and transporters, the activity or expression of which is highly regulated and increased by PTH and 1α,25(OH)2D3. Recent research suggests that other, heretofore unrecognized factors, such as the osteocyte-specific protein sclerostin, also regulate renal calcium excretion. Clues in this regard have come from the study of humans and mice with inactivating mutations of the sclerostin gene that both have increased skeletal density, which would necessitate an increase in intestinal absorption and/or renal reabsorption of calcium. Deletion of the sclerostin gene in mice significantly diminishes urinary calcium excretion and increases fractional renal calcium reabsorption. This is associated with increased circulating 1α,25(OH)2D3 levels, whereas sclerostin directly suppresses 1α-hydroxylase in immortalized proximal tubular cells. Thus, evidence is accumulating that sclerostin directly or indirectly reduces renal calcium reabsorption, suggesting the presence of a novel calcium-excreting bone-kidney axis.

Impaired vitamin D metabolism in CKD

Vitamin D metabolism consists of both production and catabolism, which are enzymatically driven and highly regulated. Renal vitamin D metabolism requires filtration and tubular reabsorption of 25-hydroxyvitamin D and is regulated by parathyroid hormone, fibroblast growth factor-23, and 1,25-dihydroxyvitamin D. In chronic kidney disease, renal production of 1,25-dihydroxyvitamin D from 25-hydroxyvitamin D is reduced. In addition, pharmacokinetic studies and epidemiologic studies of 24,25-dihydroxyvitamin D, the most abundant product of 25-hydroxyvitamin D catabolism by CYP24A1, suggest that vitamin D catabolism also is reduced. New insights into the mechanisms and regulation of vitamin D metabolism may lead to novel approaches to assess and treat impaired vitamin D metabolism in chronic kidney disease.

Neurotransmitters in the CNS of the vitamin D deficient, hypocalcemic rat

Sprague-Dawley rats were made chronically vitamin D deficient (VDD) and hypocalcemic, or VDD and normocalcemic. Rickets, severely reduced body weight, hair shedding, lethargy, muscular paralysis and a high mortality rate are characteristic features of the male VDD/hypocalcemic animals. An assessment was made of the neurotransmitter status of the VDD-hypocalcemic and VDD/normocalcemic animals. In nine out of eleven regions of the CNS studied, the increase in GABA induced by the GABA-T inhibitor ethanolamine sulphate (EOS) was significantly higher (P < 0.05 or P < 0.01) in the VDD/hypocalcemic group vs the normal controls. However, the EOS-mediated increase in GABA was similar in the VDD/normocalcemic and normal control groups suggesting that hypocalcemia is the likely cause of the increased GABA turnover in the VDD/hypocalcemic rats. Glutamate, dopamine, dihydroxyphenylacetic acid, homovanillic acid and norepinephrine, were also analysed in representative regions of the CNS. Their concentrations were not affected in any consistent way in either the VDD/hypocalcemic group or VDD/normocalcemic groups vs the normal controls. Therefore, despite the chronic, severe pathology induced by vitamin D deficiency and hypocalcemia, the neurotransmitters studied appeared to be normal in the CNS of these animals.

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.

L-alpha-hydroxycholecalciferol treatment of adults with chronic renal failure

Five adult patients with chronic renal failure and associated renal osteodystrophy have been treated for 6 months with 1-alpha-hydroxycholecalciferol (1 alpha-OH-D3), a synthetic vitamin D analogue. All 5 patients had severe metabolic bone changes as estimated by bone scintigraphy. Three patients were hypocalcemic, 4 had elevated serum alkaline phosphatases, 5 had elevated serum immunoreactive parathyroid hormone (i-PTH) concentration and 3 had bone pains. During treatment serum calcium increased in all patients (mean 11.4%) and 3 originally hypocalcemic patients became normocalcemic. Serum alkaline phosphatases decreased (mean 27.3%) and became normal in 4 patients, who initially had elevated values. A pronounced decline in the serum concentration of i-PTH (mean 53%) was seen in all patients and 1 patient obtained normal i-PTH levels after 4 months of treatment. The intestinal calcium absorption, which was low initially, even when calcium intake was considered, rose almost threefold (mean 273%) and reached normal values in all cases. The bone mineral content increased in all patients, but the changes were small (mean 4.9%) and insignificant. Finally, bone pain disappeared in 2 patients and improved in 1 of 3 patients exhibiting this symptom. A linear correlation (r = 0.48, p less than 0.001) was found between the dose of 1 alpha-OH-D3 and serum calcium. But in spite of this and the frequent control, all patients developed one episode of hypercalcemia. This disappeared within 48 hours after discontinuing the drug. It is concluded that treatment with 1 alpha-OH-D3 appears to be of therapeutic value in metabolic bone disease associated with chronic renal failure, but frequent control of blood biochemistry seems mandatory.

1-Alpha-hydroxycholecalciferol-induced changes in the renal heandling of phosphate and the serum parathyroid hormone level

The effect of 1-alpha-hydroxycholecalciferol (1alpha-OH-D3) on the renal handling of phosphate and the immunoreactive parathyroid hormone in serum (i-PTH) has been studied in 10 patients with a wide range of glomerular filtration rate (GFR), maximal tubular reabsorption of phosphate (TmP) and i-PTH. The patients were treated with 2 mug 1alpha-OH-D3 per day for approximately 80 days. Before and after this period of treatment, the TmP, i-PTH, 51Cr EDTA clearance, extracellular volume, standard bicarbonate, and serum calcium were measured in each patient. The TmP/GFR ratio was used as an index of the renal handling of phosphate. The index increased significantly (mean 26.5%, p less than 0.01) during the treatment, while i-PTH decreased significantly (mean 37.0%. p less than 0.01). An inverse significant correlation was demonstrated between TmP/GFR index and i-PTH both before (r = -0.87, p less than 0.001) and after (r = -0.79, p less than 0.01) the administration o alpha-OH-D3, while none of the other factors investigated were correlated to the index. It is concluded that 1alpha-OH-D3 increases the TmP/GFR index and reduces i-PTH in a parallel manner and it is therefore suggested that the 1alpha-OH-D3-induced changes in the renal handling of phosphate may be explained as being mediated solely via the suppression of i-PTH.

Effects of dietary lactose on long-term high-fat-diet-induced obesity in rats

OBJECTIVE

In this study, we examined the effects of lactose on long-term high-fat-diet-induced obesity in rats.

RESEARCH METHODS AND PROCEDURES

A total of 112 Sprague-Dawley strain female rats (6 weeks old) were divided into four groups: a basic control diet group (Cont), 10% lactose diet group (Lac), high-fat diet group (Fat), and high-fat with 10% lactose diet group (Fat+Lac). After 0, 7, 14, and 84 days from starting the experimental diet, the animals were fasted overnight and killed by bleeding from the abdominal aorta under anesthesia (n = 8 or 9/group).

RESULTS

After 84 days, the addition of lactose to the high-fat diet decreased the final body weight, body weight gain, fat accumulation, and the levels of serum leptin, serum triglycerides, and serum glucose significantly (p < 0.05). Although there was no significant difference in the levels of serum calcium and phosphorus between the Fat and Fat+Lac groups, lumbar vertebral bone mineral density was significantly higher in the Fat+Lac group than in the Cont group on Day 82. Interestingly, the level of serum 1alpha, 25-dihydroxyvitamin D(3) in the Fat+Lac group on Day 84 was reduced by 74% compared with the Fat group (p < 0.01), while there was no significant difference in serum parathyroid hormone levels between the Fat and Fat+Lac groups.

DISCUSSION

This is the first study to suggest that the addition of lactose to a long-term high-fat diet may regulate not only calcium metabolism but also fat deposition. Further studies on the mechanism of dietary lactose in the regulation of adiposity would provide valuable data for the prevention of long-term high-fat-diet-induced obesity.

Effects of lα-hydroxylated metabolites of cholecalciferol on intestinal radiocalcium absorption in goats

1. Intestinal absorption of47Ca was measured by a double-isotope technique in goats treated with 1, 5 or 25 μg of 1,25-dihydroxycholecalciferol (1,25(OH)2D3). The effects of giving 1,25(OH)2D3by intravenous (iv) infusion for 30–36 h were compared at each dose level with the effects obtained by oral administration of 1,25(OH)2D3either in ethanol or protected against rumen degradation in fatty acid pellets.

2. Dose-dependent increments in absorption followed the treatments, with a doubling of absorption at the 1 μg dose and three- to fivefold increases with the 5 and 25 μg doses.47Ca absorption was equally stimulated 2 and 6 d after treatment but had returned to pretreatment levels 12–14 d after treatment.

3. Intravenous and protected oral administration of 1,25(OH)2D3stimulated47Ca absorption to the same extent, in spite of two- to fivefold higher plasma concentrations of 1,25(OH)2D3after iv treatment. Somewhat lower increments in47Ca absorption were seen using ethanol as the vehicle for oral administration.

4. The naturally occurring metabolites 1,24(R),25-trihydroxycholecalciferol and 1,25(S),26-trihydroxy-cholecalciferol had only one-tenth to one-fifteenth the potency of 1,25(OH)2D3in stimulating47Ca absorption, while synthetic lα-hydroxycholecalciferol appeared to be twice as effective as 1,25(OH)2D3when tested at a high (10μg) dose.

Intestinal radiocalcium absorption in the goat: measurement by a double-isotope technique

1. Intestinal radiocalcium absorption was measured in goats by a double-isotope technique involving injection of45CaCl2intravenously and47CaCl2into the abomasum. Cumulative absorption of radiocalcium was calculated by deconvolution analysis form curves of plasma radioactivity.

2. Repeated measurements at 2 d intervals gave highly reproducible results (r0.94,P< 0.001). No systematic difference between two consecutive measurements was observed. A good agreement between absorption of radiocalcium from simultaneously administered47CaCl2and45Ca-labelled hay (r0.93,P< 0.001) seems to justify the use of inorganic47Ca as a tracer for Ca in ruminant diets.

3. Two- to three-fold increases in radiocalcium absorption 48 h after oral treatment with 1,25- dihydroxycholecalciferol or leaves ofSolanum malacoxylonshowed the usefulness of the method in situations of rapidly changing Ca absorption.

4. Endogenous adaptations in intestinal radiocalcium absorption from 20 to 43% were observed in lactating goats when Ca intakes decreased from 12 to 4 g/d.

5. It is concluded that the double-isotope technique is a suitable method for studies of Ca absorption in ruminants when tracer is introduced into the abomasum. The test is completed in 3–4 h and may therefore be used in situations where the absorption of Ca undergoes rapid changes.

Fibroblast growth factor 23, parathyroid hormone, and 1alpha,25-dihydroxyvitamin D in surgically treated primary hyperparathyroidism

OBJECTIVE

To determine whether fibroblast growth factor 23 (FGF23) contributes to the hypophosphatemia of primary hyperparathyroldism.

PATIENTS AND METHODS

Thirteen adult patients with primary hyperparathyroidism had serum collected before and after parathyroidectomy for analysis of inorganic phosphorus, calcium, 1alpha,25-dihydroxyvitamin D (1alpha,25[OH]2D), parathyroid hormone (PTH), FGF23, creatinine, and bone-specific alkaline phosphatase (BSAP). Patients were recruited between July 24, 2003, and February 11, 2004.

RESULTS

Before surgery, patients had elevated serum calcium and PTH concentrations. Serum phosphorus concentrations were in the low-normal range. The FGF23 concentrations were not elevated in patients with primary hyperparathyroidism compared with healthy controls. Within 24 hours of surgery, serum calcium, PTH, 1alpha,25(OH)2D, and BSAP concentrations were lower (P < .002 for all) and phosphorus concentrations were higher (P = .003) than in the preoperative state. The FGF23 concentrations were similar 1 day and 6 weeks after surgery. The FGF23 concentrations did not correlate with serum phosphorus, calcium, PTH, 1alpha,25(OH)2D, creatinine, or BSAP concentrations in the preoperative or postoperative state.

CONCLUSION

Parathyroid hormone is the major regulator of serum phosphorus concentrations in patients with primary hyperparathyroidism. Fibroblast growth factor 23 does not appear to play a role in phosphorus homeostasis in patients with surgically treated primary hyperparathyroidism.

The phosphatonin pathway: New insights in phosphate homeostasis

Serum phosphate concentrations are maintained within a defined range by processes that regulate the intestinal absorption and renal excretion of inorganic phosphate. The hormones currently believed to influence these processes are parathyroid hormone (PTH) and the active metabolite of vitamin D, 1alpha,25-dihydroxyvitamin D (1alpha,25(OH)2D). A new class of phosphate-regulating factors, collectively known as the phosphatonins, have been shown to be associated with the hypophosphatemic diseases, tumor-induced osteomalacia (TIO), X-linked hypophosphatemic rickets (XLH), and autosomal-dominant hypophosphatemic rickets (ADHR). These factors, which include fibroblast growth factor 23 (FGF23) and secreted frizzled-related protein 4 (FRP4), decrease extracellular fluid phosphate concentrations by directly reducing renal phosphate reabsorption and by suppressing 1alpha,25(OH)2D formation through the inhibition of 25-hydroxyvitamin D 1alpha-hydroxylase. The role of these substances under normal or pathologic conditions is not yet clear. For example, it is unknown whether any of the phosphatonins are directly responsible for the decreased concentrations of 1alpha,25(OH)2D observed in chronic and end-stage kidney disease or whether they are induced in an attempt to correct the hyperphosphatemia seen in late stages of chronic renal failure. Future experiments should clarify their physiologic and pathologic roles in phosphate metabolism.

Cloning and expression of the chicken 25-hydroxyvitamin D3 24-hydroxylase cDNA

Using a cDNA probe from the rat 24-hydrovitamin D3 24-hydroxylase, the chicken 25-hydroxyvitamin D3 24-hydroxylase cDNA has been isolated from a chicken kidney lambda gt11 library. The high degree of similarity with the mammalian 24-hydroxylase cDNAs strongly supports the belief that it is the chicken 25-hydroxyvitamin D3 24-hydroxylase cDNA. The deduced amino acid sequences are also very well conserved and 325 of them are identical among the four known 25-hydroxyvitamin D3 24-hydroxylases. This cDNA expressed in E. coli produces 24-hydroxylase activity.

25-hydroxycholecalciferol in poultry nutrition

Vitamin D is a complex of secosteroids that must undergo metabolic alterations to reach optimal biological activity. The parent compounds 1) ergocalciferol (D2) and 2) cholecalciferol (D3) can be synthesized in the leaves of many plants or in the skin of most animals, respectively. Transport of vitamin D steroids after absorption is associated with vitamin D binding proteins (DBP). In general, the relative binding affinities of the vitamin D steroids are: 25-hydroxy vitamin D3 [25-(OH)D3] = 24,25-dihydroxy vitamin D3 [24,25-(OH)2D3] = 25,26-dihydroxy vitamin D3 [25,26-(OH)2D3] > 25-hydroxy vitamin D2 (25-(OH)D2) > 1,25-dihydroxy vitamin D3 [1,25-(OH)2D3] > vitamin D3. The DBP in poultry does not bind D2 forms effectively, and therefore poultry can not use this form of vitamin D adequately. The concentration of 25-(OH)D3 in blood seems to be well correlated with dietary vitamin D intake or exposure to ultraviolet light. The 1 alpha hydroxylase enzyme in the kidney is subject to negative feedback regulation and is critical for formation of the active metabolite 1,25-(OH)2D3. The intracellular vitamin D receptor (VDR) specifically binds 1,25-(OH)2D3 and is necessary for cellular action. Increased levels of two to three orders of magnitude are required for 25-(OH)D3 to compete with 1,25-(OH)2D3 for binding on VDR. Feeding studies with 25-(OH)D3 suggest it has nearly twice the activity of vitamin D3. Hatchability studies have shown that 25-(OH)D3 supports good fertility and hatchability, whereas hens fed only 1,25-(OH)2D3 did not have normal hatchability. Likewise, 1,25-(OH)2D3 seems to reach toxic levels at dietary concentrations only two to three times optimal dietary levels whereas feeding 25-(OH)D3 for extended periods at levels 8 to 10 times requirement seems to have no adverse effects. It seems that 25-(OH)D3 is the most active metabolite of vitamin D3, ultimately capable of supporting both cellular functions and embryonic development in chickens and turkeys when fed as the sole source of vitamin D3.

Calcium regulating activity of 26,27-dimethyl analog of 24R,25-dihydroxyvitamin D3

To determine the possibility that methyl substitution in 26- and 27-positions of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] alters activities of the original compound, the effects of 24,25(OH)2D3 on calcium (Ca) regulating activity were compared with those of its methyl analog [24,25(OH)2(CH3)2D3] in addition to 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. 24,25(OH)2D3 at 10(-6) M and 24,25(OH)2(CH3)2D3 at 10(-7) M and above significantly stimulated both bone resorption in neonatal mouse calvaria cultures and formation of osteoclast-like multinucleated cells (MNC) in mouse bone marrow cultures. A stimulative effect of 1,25(OH)2D3 on bone resorption and MNC formation was recognized in very low concentrations (10(-11) M and above). Although a potency of 24,25(OH)2(CH3)2D3 in stimulating bone calcium (Ca) mobilization and intestinal Ca transport was higher than that of 24,25(OH)2D3, the potencies of both compounds were similar to that of 1,25(OH)2D3 unlike in vitro experiments. As 1,24R,25-trihydroxy-26,27-dimethylvitamin D3 showed almost the same effect as 24,25(OH)2(CH3)2D3, the dihydroxy form is suggested to be hydroxylated at 1 alpha position and converted to trihydroxy form in vitamin D-deficient rats. From these results, methyl substitution in 26- and 27-position of 24,25(OH)2D3 was found to elevate Ca regulating activity of the original compound. In addition, it is suggested that the basis for a similarity in potency between 1,25(OH)2D3 and 24,25(OH)2D3 or its dimethyl analog in vitamin D-deficient rats is likely the result of 1 alpha-hydroxylation.

Calcium regulating activity of 24a-homo-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 and 26,27-dimethyl-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3

Two fluoro analogs of 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3], 24a-homo-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 [24aF2-homo-1,25(OH)2D3], and 26,27-dimethyl-24,24-difluoro-1 alpha,25-dihydroxyvitamin D3 [24F2-1,25(OH)2(Me)2D3] were examined for calcium (Ca)-regulating activity. The objective of the present study was to determine whether or not fluoro substitution at 24-position would alter activities of the original compounds, that is, 26,27-dimethyl 1 alpha,25-dihydroxyvitamin. D3[1,25(OH)2(Me)2D3] and 24-homo-1 alpha,25-dihydroxyvitamin D3 [24homo-1,25(OH)2D3], respectively. The relative activities of 24aF2-homo-1,25(OH)2D3, 24F2-1,25(OH)2(Me)2D3, and 1,25(OH)2D3 in competing with 1,25(OH)2D3 for binding to chick intestinal cytosol receptor were 0.28:0.5:1.0. The relative potencies of the same series of compounds in competition for the vitamin D-deficient rat serum binding sites were 0.04:0.15:1. Bone-resorbing activities of two fluoro analogs in cultures of neonatal mouse parietal bones were more potent than that of 1,25(OH)2D3. Similar results were recognized in stimulating activities of osteoclast-like cell formation. Responses of two fluoro analogs to intestinal Ca absorption were similar to that of 1,25(OH)2D3. The potencies of 1,25(OH)2D3 and its fluoro analogs in bone Ca mobilization were the highest with 1,25(OH)2D3, followed by 24F2-1,25(OH)2(Me)2D3 and 24aF2-homo-1,25(OH)2D3, in that order. From these results and the data of Paulson et al., fluoro substitution in 24-position of 1,25(OH)2D3 apparently does not alter their activities,hence, the fluoro substitution at 24-position of 1,25(OH)2D3 and the elongation of side chain of 1,25(OH)2D3 may not intensify Ca-regulating activity.

Calcium regulating activity of 26,27-dialkyl analogs of 1 alpha,25-dihydroxyvitamin D3

A series of analogs of 1 alpha,25-dihydroxyvitamin D3[1,25(OH)2D3] with alkyl substitutions in 26- and 27-positions were tested for calcium (Ca) regulating activity. The potencies of dialkyl analogs in stimulating bone resorption in neonatal mouse calvaria cultures were the highest in 1 alpha,25-dihydroxy-26,27-dimethylvitamin D3[1,25(OH)2-(Me)2D3], followed by 1,25(OH)2D3, 1 alpha,25-dihydroxy-26,27-diethylvitamin D3[1,25(OH)2(Et)2D3], and 1 alpha,25-dihydroxy-26,27-dipropylvitamin D3[1,25(OH)2(Pr)2D3] in that order. A similar order of potential regarding formation of osteoclast-like cells in mouse bone marrow cell cultures and on bone Ca mobilization with long-term vitamin D-deficient rats was observed in the same series. The relative potencies of 1,25(OH)2D3, 1,25(OH)2(Me)2D3, 1,25(OH)2(Et)2D3, and 1,25(OH)2(Pr)2D3 in competing with 1,25(OH)2D3 for binding to chick intestinal cytosol receptors were 1:1:0.16:0.036. A similar order of potential in case of intestinal Ca transport in situ was observed in the same series. The potencies of dialkyl analogs in competing with 25-hydroxy-vitamin D3 for binding to rat serum vitamin D binding protein were much lower than that of 1,25(OH)2D3. Effect of 1,25(OH)2(Me)2D3 on osteopenia in rats induced by ovariectomy and right sciatic neurotomy was higher than that of 1,25(OH)2D3. From these results, the lengthening by one carbon at 26- and 27-positions was shown to maintain the Ca regulatory activity of 1,25(OH)2D3.

Putrescine is involved in the vitamin D action in chick intestine

We have reported that a single injection of 1 alpha,25-dihydroxyvitamin D3 into vitamin D-deficient chicks produces a marked increase of putrescine accumulation in the duodenum from two different sources, ornithine and spermidine. In the present study, the effects of putrescine depletion and its supplementation on duodenal villus length and calcium absorption were examined in newborn and 5-week-old chicks. Administering either alpha-difluoromethylornithine, a specific inhibitor of ornithine decarboxylase, or N1,N4-bis(2,3-butadienyl)-1,4-butanediamine, a specific inhibitor of polyamine oxidase, to newborn chicks significantly decreased the duodenal content of putrescine and calcium transport activity. The putrescine depletion also induced shortening of the duodenal villus length. The inhibition of calcium absorption and villus length in the putrescine-depleted chicks was almost completely restored by administering putrescine to the birds. The effect of the putrescine depletion and its supplementation on the duodenal villus length and the calcium absorption was reproduced in 5-week-old vitamin D-deficient chicks given vitamin D3 or 1 alpha,25-dihydroxyvitamin D3. These results clearly indicate that putrescine is somehow involved in the vitamin D action in maintaining the morphological and functional development of the intestinal villus mucosa.

Synthesis and biological activity of (22E,25R)- and (22,25S)-22-dehydro- 1 alpha,25-dihydroxy-26-methylvitamin D3

Both 25-epimers of (22E)-22-dehydro-1 alpha,25-dihydroxy-26-methylvitamin D3 [22-dehydro-26-methyl-1,25-(OH)2D3] were synthesized. The biological activity of these compounds was tested in binding affinity to chick intestinal receptor protein of 1 alpha,25-dihydroxy-vitamin D3 [1,25-(OH)2D3] and in stimulating for intestinal calcium transport and bone calcium mobilization with vitamin D-deficient rats. The relative potency of (25R)- and (25S)-22-dehydro-26-homo-1,25-(OH)2D3 and 1,25-(OH)2D3 in competing for the intestinal cytosolic binding was 1.7:1.5:1. A similar order of activity was observed on intestinal calcium transport and bone calcium mobilization. In the ability for stimulation of intestinal calcium transport, (25R)- and (25S)-22-dehydro-26-methyl-1,25-(OH)2D3 were about 3.6 and 2.1 times as active as 1,25-(OH)2D3, respectively. In bone calcium mobilization tests, (25R)- and (25S)-22-dehydro-26-methyl-1,25-(OH)2D3 were estimated to be 2.2 and 1.6 times as potent as 1,25-(OH)2D3, respectively.

Purification and characterization of spermidine N1-acetyltransferase from chick duodenum

We have reported that spermidine N1-acetyltransferase has a larger role than ornithine decarboxylase in putrescine synthesis in chick duodenum induced by 1 alpha,25-dihydroxycholecalciferol (calcitriol) [Shinki, T., Kadofuku, T., Sato, T. and Suda, T. (1986) J. Biol. Chem. 261, 11712-11716]. In the present study, spermidine N1-acetyltransferase was purified from the duodenal cytosol of calcitriol-treated chicks to homogeneity judged by SDS/polyacrylamide gel electrophoresis. The purified enzyme converted spermidine only to N1-acetyl-spermidine. The apparent molecular mass of the purified spermidine N1-acetyltransferase was found to be 36 kDa by gel filtration on Sephacryl S-200 and 18 kDa by SDS/polyacrylamide gel electrophoresis. When duodenal crude 105,000 x g extracts were directly applied to a Sephacryl S-200 column without prior purification, three peaks with spermidine N1-acetyltransferase activity appeared. The first peak was in the void volume, the second peak was in the fraction corresponding to an apparent molecular mass of 70 kDa, and the third peak was in the fraction corresponding to 36 kDa. These results suggest that spermidine N1-acetyltransferase exists as a dimer of the 18 kDa subunits and is stabilized in (a) form(s) bound to other components or proteins in intact cells.

Major pathway for putrescine synthesis induced by 1 alpha,25-dihydroxyvitamin D3 in chick duodenum

We have reported that a single injection of 1 alpha,25-dihydroxyvitamin D3 into vitamin D-deficient chicks produces a marked accumulation of putrescine in the duodenum by an interconversion pathway. In the present study, we examined the effect of N1,N2-bis(2,3-butadienyl)-1,4-butanediamine, a specific irreversible inhibitor of polyamine oxidase, on the duodenal putrescine synthesis induced by 1 alpha,25-dihydroxyvitamin D3. Addition of N1,N2-bis(2,3-butadienyl)-1,4-butanediamine to an assay mixture completely inhibited the activity of duodenal polyamine oxidase in vitro. Prior administration of N1,N2-bis(2,3-butadienyl)-1,4-butanediamine to chicks completely blocked the 1 alpha,25-dihydroxyvitamin D3-induced increase in duodenal accumulation of putrescine in vivo. The increase of the duodenal accumulation of putrescine by 1 alpha,25-dihydroxyvitamin D3 in vitamin D-deficient chicks coincided quantitatively with the amount of N1-acetylspermidine synthesized from spermidine after the injection of the vitamin into the chicks pretreated with the inhibitor of polyamine oxidase. These results clearly indicate that spermidine N1-acetyltransferase plays a preferential role in the increase in duodenal putrescine synthesis by 1 alpha,25-dihydroxyvitamin D3. The rapidly proliferating and maturing epithelium of small intestines will provide a good model for investigating the role of the interconversion of polyamine metabolism in cell growth and differentiation.

A new tritium-release assay for 25-hydroxyvitamin D-1 alpha-hydroxylase

A new, rapid assay for 1 alpha-hydroxylase has been developed using 25-hydroxy-[1 alpha-3H]vitamin D3 as the substrate. Using the solubilized and reconstituted chick 1 alpha-hydroxylase, conversion of this substrate to 1,25-dihydroxyvitamin D3 causes the release of tritium into the aqueous medium. This 3H2O can be easily separated from the labeled substrate by passing the reaction mixture through a reverse-phase silica cartridge. The release of tritium is stereospecific as evidenced by the lack of 3H2O formed when 25-hydroxy-[1 beta-3H]vitamin D3 is used as the substrate. In parallel reactions containing the 25-hydroxy-[26,27-3H]vitamin D3 substrate, production of labeled 1,25-dihydroxyvitamin D3 was assessed by extraction and high-performance liquid chromatography and found to agree very closely with the amount of 3H2O produced from 25-hydroxy-[1 alpha-3H]vitamin D3, validating the accuracy of the new assay. Finally, a major advantage of the tritium-release assay for 1 alpha-hydroxylase is that the results are not affected by further metabolism of the 1,25-dihydroxyvitamin D formed in the incubations.

Synthesis and biological activity of 1 alpha,23,25,26-tetrahydroxyvitamin D3

The metabolic pathway from 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2D3] to 1 alpha,25-dihydroxyvitamin D3-26,23-lactone includes the formation of 1 alpha,23,25-26-tetrahydroxyvitamin D3 [1 alpha,23,25,26-(OH)4D3]. The aim of the current study was to explore the as yet unknown biological properties of this vitamin D3 sterol. The four diastereoisomers of 1 alpha,23,25,26-(OH)4D3 were chemically synthesized. They were compared to 1 alpha,25-(OH)2D3 in terms of their affinity for the chick intestinal 1 alpha,25-(OH)2D3 receptor and their biologic activity in vivo (stimulation of intestinal calcium absorption and mobilization of calcium from bone in vitamin D-deficient rats). The 1,25-(OH)2D3 receptor binding affinities of 1 alpha,23(R)25(R)26-(OH)4D3, 1 alpha,23(S)25(S)26-(OH)4 D3, 1 alpha,23(S)25(R)26-(OH)4D3, and 1 alpha,23(R)25(S)26-(OH)4D3 were 11, 100, 216, and 443 times weaker than the binding affinity of 1 alpha,25-(OH)2D3, respectively. Compared to 1 alpha,25-(OH)2D3, the relative capacities of the 1 alpha,23,25,26-(OH)4D3 compounds to stimulate intestinal calcium absorption were 1/4 for 1 alpha,23(R)25(R)26-(OH)4D3; 1/19 for 1 alpha,23(S)25(S)26-(OH)4D3; 1/90 for 1 alpha,23(S)25(R)26-(OH)4D3; and 1/136 for 1 alpha,23(R)25(S)26-(OH)4D3. Maximal stimulation of intestinal calcium transport occurred 8 h after administration of vitamin D3 metabolites. Mobilization of calcium from bone was quantitated by serum calcium concentration measurements. The activities of 1 alpha,23(R)25(R)26-(OH)4D3, 1 alpha,23(S)25(S)26-(OH)4D3, 1 alpha,23(S)25(R)26-(OH)4D3, and 1 alpha,23(R)25(S)26-(OH)4D3 to increase serum calcium were estimated to be 4, 13, 43, and 69 times weaker than that of 1 alpha,25-(OH)2D3, respectively. These results illustrate the stereospecificity of the chicken intestine 1 alpha,25-(OH)2D3 receptor for binding of 1 alpha,23,25,26-(OH)4D3 and suggest that the 1 alpha,23,25,26-(OH)4D3 exerts its biological activity in the rat through an interaction with 1,25-(OH)2D3 receptors. In summary, the 1 alpha,23,25,26-(OH)4D3 had a markedly lower biological activity than 1 alpha,25-(OH)2D3.

Effect of cortisol on [3H] 1,25-dihydroxyvitamin D3 uptake and 1,25-dihydroxyvitamin D3-induced DNA-dependent RNA polymerase activity in chick intestinal cells

The influence of cortisol on intestinal DNA-dependent RNA polymerase activity was studied in purified nuclei of vitamin D-deficient or 1,25-dihydroxyvitamin D3-treated chicks. Six- to 7-week-old vitamin D-deficient cockerels were given 5 mg of cortisol or vehicle intraperitoneally 24 and 48 hours before sacrifice. Three hours before sacrifice, 200 ng of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) was administered intracardially. Cortisol did not alter the uptake or metabolism of 1,25(OH)2D3 in the intestinal mucosa. After a 200 ng dose of 1,25(OH)2D3 the in situ intestinal ligated loop technique revealed a 39% increase in calcium absorption compared to control birds (P less than 0.001). The administration of cortisol (5 mg) to chickens given 1,25(OH)2D3, however, resulted in a significant decrease in intestinal calcium transport in vivo (P less than 0.0025). When intestinal nuclei were prepared from birds treated in a manner identical with that described above, 1,25(OH)2D3-treated and 1,25(OH)2D3 plus cortisol-treated chicks had intestinal RNA polymerase II transcriptional activities that were significantly greater than those of vitamin D-deficient controls (P less than or equal to 0.02, P less than or equal to 0.005). There was no difference between RNA polymerase II and I + III activities of the 1,25(OH)2D3-treated birds and that of the cortisol plus 1,25(OH)2D3-treated birds. Vitamin D-deficient chicks treated with cortisol alone showed RNA polymerase I + III activity that was significantly higher (P less than or equal to 0.01) than that of birds treated with vehicle alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of 25-hydroxy-[26,27-3H]vitamin D2, 1,25-dihydroxy-[26,27-3H]vitamin D2 and their (24R)-epimers

Synthesis of a C-24-epimeric mixture of 25-hydroxy-[26,27-3H]vitamin D2 and a C-24-epimeric mixture of 1,25-dihydroxy-[26,27-3H]vitamin D2 by the Grignard reaction of the corresponding 25-keto-27-nor-vitamin D2 and 1 alpha-acetoxy-25-keto-27-nor-vitamin D3 with tritiated methyl magnesium bromide is described. Separation of epimers by high-performance liquid chromatography afforded pure radiolabeled vitamins of high specific activity (80 Ci/mmol). The identities and radiochemical purities of 25-hydroxy-[26,27-3H[vitamin D2 and 1,25-dihydroxy-[26,27-3H]vitamin D2 D2 were established by cochromatography with synthetic 25-hydroxyvitamin D2 or 1,25-dihydroxyvitamin D2. Biological activity of 25-hydroxy-[26,27-3H]vitamin D2 was demonstrated by its binding to the rat plasma binding protein for vitamin D compounds, and by its in vitro conversion to 1,25-dihydroxy-[26,27-3H]vitamin D2 by kidney homogenate prepared from vitamin D-deficient chickens. The biological activity of 1,25-dihydroxy-[26,27-3H]vitamin D2 was demonstrated by its binding to the chick intestinal receptor for 1,25-dihydroxyvitamin D3.

Effect of vitamin D metabolites on the expression of alkaline phosphatase activity by epiphyseal hypertrophic chondrocytes in primary cell culture

The effects of three vitamin D3 metabolites, 25-hydroxyvitamin D3 (25-(OH)D3), 1 alpha,25-dihydroxyvitamin D3 (1 alpha, 25-(OH)2D3), and 24R,25-dihydroxyvitamin D3 (24R,25-(OH)2D3) on the activity of alkaline phosphatase (AP), a key enzyme involved in biomineralization, have been studied in primary cultures of chicken epiphyseal growth plate chondrocytes. Dosages of 1 alpha, 25-(OH)2D3 (10(-12) to 10(-7) M) caused a progressive, dosage- and time-dependent decrease in cellular AP levels, IC50 occurring at approximately 10(-12) M. In contrast, 24R,25-(OH)2D3 at 10(-13) to 10(-10) M stimulated cellular AP activity, half-maximal stimulation occurring at about 10(-13) M. At higher levels (10(-10) to 10(-7) M), 24R,25-(OH)2D3 caused progressive reduction in AP activity. Maximal effects of 24R,25-(OH)2D3 were evident 48 h after administration of the metabolite. 25-(OH)D3 initially (24 h) caused a weak, dosage-dependent decrease in cellular AP activity, but after 48-72 h, low levels (10(-13) to 10(-11) M) caused a dosage-dependent increase in AP activity. Higher levels of 25-(OH)D (greater than 10(-10) M) were clearly inhibitory to AP. These findings reveal that the AP activity of growth plate chondrocytes is exquisitely sensitive to both 1 alpha,25- and 24R,25-(OH)2D3 but the response to each is in opposite directions. The paradoxical response of the cells to 25-(OH)D3 can be explained if the metabolite is slowly metabolized by a 24-hydroxylase to 24R,25-(OH)2D3 leading to stimulation of cellular AP.(ABSTRACT TRUNCATED AT 250 WORDS)

The difference of biological activity among four diastereoisomers of 1 alpha,25-dihydroxycholecalciferol-26,23-lactone

All four possible diastereoisomers of 1 alpha,25-dihydroxycholecalciferol-26,23-lactone (1 alpha,25-(OH)2D3-26,23-lactone) were chemically synthesized and were compared to 1 alpha,25-dihydroxycholecalciferol (1 alpha,25(OH)2D3) in terms of their stimulation, in vivo, of intestinal calcium transport and mobilization of calcium from bone in vitamin D-deficient rats (the two classic vitamin D-mediated responses), and their relative binding to the chick intestinal cytosol receptor for 1 alpha,25-(OH)2D3. The receptor binding affinity results are expressed as relative competitive index (RCI), where the RCI is defined as 100 for 1 alpha,25(OH)2D3. The RCI obtained for 23(S)25(S)-1 alpha,25(OH)2D3-26,23-lactone was 7.90, for 23(R)25(R)-1 alpha,25(OH)2D3-26,23-lactone was 2.27, 23(S)25(R)-1 alpha,25(OH)2D3-26,23-lactone was 0.17, for 23(R)25(S)-1 alpha,25(OH)2D3-26,23-lactone 0.22 and for the in vivo produced 1 alpha,25(OH)2D3-26,23-lactone the RCI was only 0.17. Also the four diastereoisomers of 1 alpha,25(OH)2D3-26,23-lactone all stimulated intestinal calcium transport, reaching a maximum 8 h after administration. Compared with the stimulation of intestinal calcium transport by 1 alpha,25(OH)2D3, 23(S)25(S)-1 alpha,25(OH)2D3-26,23-lactone was 1/4 as effective, 23(R)25(R)-1 alpha,25(OH)2D3-26,23-lactone was 1/20 as effective, 23(S)25(R)-1 alpha,25(OH)2D3-26,23-lactone was 1/74 as effective and 23(R)25(S)-1 alpha,25(OH)2D3-26,23-lactone was 1/53 as effective. Similarly, 23(S)25(S)-1 alpha,25(OH)2D3-26,23-lactone and 23(R)25(R)-1 alpha,25(OH)2D3-26,23-lactone were estimated to be 3 and 20 times less active than 1 alpha,25-(OH)2D3 in elevation of serum calcium. However, 23(S)25(R)-1 alpha,25(OH)2D3-26,23-lactone and 23(R)25(S)-1 alpha,25(OH)2D3-26,23-lactone decreased the serum calcium levels 24 h after administration. 23(S)25(R)-1 alpha,25(OH)2D3-26,23-lactone reduced serum calcium concentrations to a greater extent than 23(R)25(S)-1 alpha,25(OH)2D3-26,23-lactone. These results indicate that the biological activities of the diastereoisomers of 1 alpha,25(OH)2D3-26,23-lactone were quite different among four stereochemical configurations.

The physiology and pathophysiology of vitamin D

The vitamin D endocrine system plays an important role in the maintenance of normal calcium homeostasis. Abnormalities of this system occur in many conditions, such as rickets, osteomalacia, hypoparathyroidism, and hyperparathyroidism. The diagnosis and treatment of these disorders will be facilitated if the clinician understands the general mechanisms by which defects in vitamin D metabolism and action occur. We review this information and discuss the use and limitations of vitamin D metabolite assays for diagnosis of clinical disorders of mineral metabolism.

The stereochemical configuration of the natural 1 alpha,25-dihydroxyvitamin D3-26,23-lactone

Four possible diastereoisomers of 1 alpha,25-dihydroxyvitamin D3-26,23-lactone were chemically synthesized and compared with the natural metabolite by high-pressure liquid chromatography. The four synthetic diastereoisomers of 1 alpha,25-dihydroxyvitamin D3-26,23-lactone could be separated into three peaks by high-pressure liquid chromatography. The naturally occurring 1 alpha,25-dihydroxyvitamin D3-26,23-lactone isolated from dog serum and in vitro incubation of chick kidney homogenates comigrated with 23(S)25(R)-1 alpha,25-dihydroxyvitamin D3-26,23-lactone. The four diastereoisomers of 1 alpha,25-dihydroxyvitamin D3-26,23-lactone were tested against naturally occurring 1 alpha,25-dihydroxyvitamin D3-26,23-lactone to determine their relative competition in the 1 alpha,25-dihydroxyvitamin D3-specific cytosol receptor binding assay for 1 alpha,25-dihydroxyvitamin D3. 23(S)25(S)-1 alpha,25-Dihydroxyvitamin D3-26,23-lactone was the best competitor followed by 23(R)25(R)-1 alpha,25-dihydroxyvitamin D3-26,23-lactone and 23(R)25(S)-1 alpha,25-dihydroxyvitamin D3-26,23-lactone, and 23(S)25(R)-1 alpha,25-dihydroxyvitamin D3-26,23-lactone was the poorest competitor. Natural 1 alpha,25-dihydroxyvitamin D3-26,23-lactone isolated from dog serum had almost the same binding affinity as that of 23(S)25(R)-1 alpha,25-dihydroxyvitamin D3-26,23-lactone. These data unequivocally demonstrate that the stereochemistry of the natural 1 alpha,25-dihydroxyvitamin D3-26,23-lactone has the 23(S) and 25(R) configuration.

Molybdate and the 1,25-dihydroxyvitamin D3 receptor from chick intestine

The nature of the 1,25-dihydroxyvitamin D3 receptor from chick intestine was examined in regard to its response to sodium molybdate. Sodium molybdate (10 mM) stabilized the receptor from crude nuclear extract but not that from the supernatant or cytoplasmic fraction, suggesting the molybdate may act by binding to the DNA binding region of the receptor. At a concentration of 50 mM, sodium molybdate prevented aggregation of the nuclear receptor. This concentration of sodium molybdate also inhibited the receptor from binding to DNA cellulose while the same ionic strength KCl (90 mM) did not. These properties also suggest that molybdate interacts with the DNA binding region. Purification of the receptor using DNA cellulose chromatography has also been improved by using a sodium molybdate gradient (0-0.2 M) instead of the KCl gradient used previously.

1 alpha,25-dihydroxyvitamin D3 receptors and their action in embryonic chick chondrocytes

The role of vitamin D in the maturation of epiphyseal chondrocytes was investigated in the developing chick embryo. Cartilage tissues were divided into two parts: resting cartilage and growth cartilage. A cytosol component to which 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) is specifically bound first appeared in the growth cartilage on day 15, rapidly increased, and attained a maximum on day 19. The calcium content of the growth cartilage also began to increase on day 15 and continued to increase in parallel with the 1 alpha,25(OH)2D3 receptor levels. Glycosaminoglycan (GAG) synthesis by the growth cartilage cells increased from day 11-17 and rapidly declined thereafter reciprocally with the increase in calcium and receptor levels. In the resting cartilage, no cytosol receptor for 1 alpha,25(OH)2D3 was detected up to hatching time. The calcium content and GAG synthesis in the resting cartilage were very low and did not change appreciably throughout development. No receptor-like macromolecule for 24R,25-dihydroxyvitamin D3 (24R,25(OH)2D3) was recognized in either the resting or growth cartilage. 1 alpha,25(OH)2D3 added to the culture of chondrocytes from the epiphyseal growth cartilage inhibited GAG synthesis and stimulated its release from the cell layer into the medium in a dose-dependent manner. These in vitro effects of 1 alpha,25(OH)2D3 were not observed in chondrocytes obtained from 13-day-old growth cartilage and 19-day-old resting cartilage. 25-Hydroxyvitamin D3 and 24R,25(OH)2D3 had no effect on chondrocytes in any of the preparations. These results suggest that 1 alpha, 25 (OH)2D3 is directly involved in the maturation of chondrocytes and possibly in the calcification of growth cartilage.

Subcellular distribution of DNA-binding and non-DNA-binding 1,25-dihydroxyvitamin D receptors in chicken intestine

A comparison of 1,25-dihydroxyvitamin D3 receptor concentration and equilibrium dissociation constants of whole tissue, nuclear, and cytosol extracts of vitamin D-deficient chicken intestine has been carried out. The extracts had the following order of receptor concentration: nuclei greater than whole tissue greater than cytosol. The receptors in each preparation had identical equilibrium dissociation constants (Kd) for 1,25-dihydroxyvitamin D3. However, the receptor in the cytosol fraction did not bind to DNA-cellulose, while a large fraction of the receptor population in crude nuclear extract and whole tissue extract bound to DNA-cellulose and could be eluted with 0.22 M KCl, suggesting that the cytosolic form of the receptor does not possess a DNA binding site.

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.

1,25-dihydroxyvitamin D3 rapidly alters the morphology of the duodenal mucosa of rachitic chicks: Evidence for novel effects of 1,25-dihydroxyvitamin D3

When rachitic chicks are given 1,25-dihydroxyvitamin D3 in amounts as low as 50 ng/bird, the appearance of the duodenal mucosa is altered within 2 h of the administration of the hormone. The changes are most readily apparent on scanning electron microscopy and include: a more plump appearance of villi with loss of furrows and pits on their surfaces, elongation of villi and a smoother, more uniform microvillus surface. These changes occur within 2 h of the administration of the hormone and persist for as long as 24 h. The morphological change precedes the increase in calcium absorption induced by 1,25-dihydroxyvitamin D3 in the intestine. These observations suggest that 1,25-dihydroxyvitamin D3 may play an important part in maintenance of the structure of the duodenal mucosa.

Interaction of the 1 alpha,25-dihydroxyvitamin D3 receptor with RNA and synthetic polyribonucleotides

The interaction of the 1 alpha,25-dihydroxyvitamin D3 receptor with RNA and synthetic polynucleotides has been examined by using receptor from rachitic chicken intestine. Total intestinal RNA inhibited the binding of receptor to calf thymus DNA-cellulose with an efficiency equivalent to single-stranded DNA. A comparison of the inhibitory activity of several polyribonucleotides gave the following order of activities: poly(I) = poly(G) greater than or equal to double-stranded DNA greater than single-stranded DNA = poly(U). Poly(A), poly(C), and the ribonucleoside monophosphates AMP, GMP, CMP, and UMP had minimal activity. A preference for single-stranded homopolymers was observed [i.e., poly(I) and poly(U) were active, whereas poly(I):poly(C) and poly(U):poly(A) were not]. The ability of nucleic acids to displace receptor from DNA-cellulose was also measured. Both poly(G) and poly(I) were more active than double-stranded DNA in this assay. Furthermore, differences were noted between intestinal RNA fractions separated on the basis of poly(A) content. Receptor also bound to immobilized intestinal RNA and polynucleotides. The KCl concentration necessary to disrupt binding to a given polynucleotide generally paralleled the activity of that molecule in DNA-cellulose inhibition and displacement assays. These results suggest that the 1 alpha,25-dihydroxyvitamin D3 receptor can interact with RNA as well as DNA.

The brush border cytoskeleton is not static: in vivo turnover of proteins

The shape and stability of intestinal epithelial cell microvilli are maintained by a cytoskeletal core composed of a bundle of actin filaments with several associated proteins. The core filaments are intimately associated with the overlying plasma membrane, in which there occur rapid turnover of proteins and constant incorporation of new membrane. Previous work has shown that starvation or inhibition of protein synthesis results in modulation of microvillar length, which indicates that there may be cytoskeletal protein turnover. We demonstrate herein, by means of in vivo pulse labeling with radioactive amino acids, that turnover of brush border cytoskeletal proteins occurs in mature absorptive cells. Turnover of cytoskeletal proteins appears to be quite slow relative to membrane protein turnover, which suggests that the turnover of these two microvillar compartments is not coupled. We thus conclude that cytoskeletal protein turnover may be a factor used to maintain normal length and stability of microvilli and that the cytoskeleton cannot be considered a static structure.

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.