24R,25-dihydroxyvitamin D3 and 1α,25-dihydroxyvitamin D3 are both indispensable for calcium and phosphorus homeostasis

The essential role of vitamin D throughout the life of most mammals and birds as a mediator of calcium homeostasis is well established. In view of the complex endocrine system existent for the regulated metabolism of vitamin D₃ to both 1α,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] and 24R,25-dihydroxyvitamin D₃ [24R,25-(OH)₂D₃] (both produced by the kidney), an intriguing problem is to elucidate whether only one or both of these dihydroxyvitamin D₃ metabolites is required for the generation of all the biological responses mediated by the parent vitamin D₃. In contrast to the accumulated knowledge concerning the short term actions of 1,25(OH)₂-D₃ on stimulating intestinal calcium absorption and bone calcium reabsorption, relatively little is known of the biological function of 24,25(OH)₂D₃. We report now the results of a nine month study in which chicks were raised on a vitamin D-deficient diet from hatching to sexual maturity and received as their sole source of “vitamin D” either 24,25(OH)₂D₃ or 1,25(OH)₂D₃ singly or in combination. Specifically we are describing the integrated operation of the vitamin D endocrine system as quantitated by the individual measurement in all birds of 22 variables related to “vitamin D status” and as evaluated by the statistical procedure of multivariate discriminant analysis. Twelve of these variables involved detailed analysis of the bone including quantitative histology and the other 10 variables reflect various manifestations of vitamin D action, e.g. serum Ca²⁺ and Pi levels, vitamin D-dependent calcium binding protein (CaBP) in the intestine and kidney, egg productivity etc. As evaluated by the multivariate analysis, it is clear that 24,25(OH)₂D₃ and 1,25(OH)₂D₃ are simultaneously required for normalization of calcium homeostasis.     

Effect of dietary calcium and phosphorus on intestinal calcium absorption and vitamin D metabolism.

To understand better dietary regulation of intestinal calcium absorption, a quantitative assessment of the metabolites in plasma and duodenum of rats given daily doses of radioactive vitamin D₃ and diets differing in calcium and phosphorus content was made. All known vitamin D metabolites were ultimately identified by high-pressure liquid chromatography. In addition to the known metabolites (25-hydroxyvitamin D₃, 24,25-dihydroxyvitamin D₃, 1,25-dihydroxyvitamin D₃, 25,26-dihydroxyvitamin D₃, and 1,24,25-trihydroxyvitamin D₃), several new and unidentified metabolites were found. In addition to 1,25-dihydroxyvitamin D₃ and 1,24,25-trihydroxyvitamin D₃, the levels of some of the unknown metabolites could be correlated with intestinal calcium transport. However, whether or not any of these metabolites plays a role in the stimulation of intestinal calcium absorption by low dietary calcium or low dietary phosphorus remains unknown.     

The vitamin D system in iguanian lizards

The apparent plasma concentration of vitamin D binding protein (DBP) in an iguanian lizard, Pogona barbata, and the affinity of this protein for 25-hydroxyvitamin D₃ (25(OH)D₃), 25-hydroxyvitamin D₂ (25(OH)D₂), and 1,25-dihydroxyvitamin D₃ (1,25(OH)D₃) was found to resemble more closely that of the domestic hen than that of the human. The characteristics of Pogona DBP, the pattern of vitamin D metabolites derived from injected radioactive vitamin D₃ and the plasma concentrations of endogenous 25-hydroxyvitamin D (25(OH)D) in a range of iguanian lizards have been examined. The findings suggest that 25-hydroxyvitamin D (25(OH)D) is the major metabolite of vitamin D, and that it may represent the storage form of vitamin D in these species in the same way as in mammals. High concentrations of vitamin D within iguanian embryos and egg yolks suggest a role for this compound in embryogenesis in these species, and perhaps indicates that there is a mechanism for vitamin D delivery to eggs comparable to that found in the domestic chicken.     

Chemical synthesis and biological activity of 3-deoxy-1alpha-hydroxyvitamin D3 an analog of 1alpha,25-(OH)2-D3, the active form of vitamin D3

The chemical synthesis of 3-deoxy-1α-hydroxyvitamin D₃ from cholesterol is described. This steroid is a highly important analog of the hormonally active form of vitamin D, 1α, 25-dihydroxyvitamin D₃; it is the only analog presently available for structure-function studies which lacks the 3β-hydroxyl but retains the key 1α-hydroxyl of 1α, 25-dihydroxyvitamin D₃. The new steroid is highly biologically active; it stimulated intestinal calcium absorption significantly more rapidly than vitamin D₃ and as rapidly as 1α, 25-dihydroxyvitamin D₃.     

Regulation of vitamin D metabolism.

Vitamin D can be regarded as a prohormone and its most potent metabolite, 1, 25-dihydroxyvitamin D₃, a hormone which mobilizes calcium and phosphate from bone and intestine. In true hormonal fashion, the biosynthesis of 1, 25-dihydroxyvitamin D₃ by kidney mitochondria is feed-back regulated by serum calcium and serum phosphorus levels. The lack of calcium brings about a secretion of parathyroid hormone which stimulates 1, 25-dihydroxyvitamin D₃ synthesis while low blood phosphorus stimulates 1, 25-dihydroxyvitamin D₃ synthesis even in the absence of the parathyroid glands. For such regulation to occur, vitamin D must be present probably because 1, 25-dihydroxyvitamin D₃ itself is needed for the regulation. The molecular and cellular mechanisms whereby 1, 25-dihydroxyvitamin D₃ synthesis is regulated are unknown despite many recent reports. Likely the elucidation of these mechanisms must await a detailed investigation of the enzymology of the renal 25-hydroxyvitamin D₃-1α-hydroxylase. In addition to the regulation at the 25-hydroxyvitamin D₃-1α-hydroxylase step, vitamin D metabolism is regulated at the hepatic vitamin D-25-hydroxylase level. This regulation is a suppression of the hydroxylase by the hepatic level of 25-hydroxyvitamin D₃ itself by an unknown mechanism. Much remains to be learned concerning the regulation of this newly discovered endocrine system but already the findings are not only relevant to calcium homeostasis but also to an understanding of a variety of metabolic bone diseases.     

Metabolism and Action of the Hormone Vitamin D: Its Relation to Diseases of Calcium Homeostasis

Extensive experimental evidence has established a significant role of calciferol in the maintenance of normal calcium homeostasis. Present knowledge indicates that vitamin D₃ must first be converted to 25-OH-D₃ and then to 1,25(OH)₂D₃, the most active known form of the steroid. Many of the factors regulating the rate of production of this last steroid from its precurser have been evaluated, and the concept that vitamin D functions as a steroid hormone seems to be well established.Deranged action of calciferol, caused by impaired metabolism of the steroid or through altered sensitivity of target tissues, may be involved in the pathophysiology of several disease states with abnormal calcium metabolism.It is noted that liver disease, osteomalacia due to anticonvulsant therapy, chronic renal failure, hypophosphatemic rickets, hypoparathyroidism, hyperparathyroidism, sarcoidosis and idiopathic hypercalciuria have possible relation to alterations in metabolism or action of vitamin D.The future clinical availability of 1,25(OH)₂D₃ and other analogs of this steroid may offer potential therapeutic benefit in the treatment of certain of the disease entities discussed.     

Identification of a highly specific and versatile vitamin D receptor antibody

The active form of vitamin D, 1alpha,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) is critical for regulation of serum calcium and phosphorus levels and for proper maintenance of bone mineralization and neuromuscular function. Biological effects of 1,25(OH)₂D₃ are mediated through a nuclear steroid hormone receptor, known as the vitamin D receptor (VDR). The discovery of VDR in a number of different cell and tissue types, suggests that the physiological role of vitamin D may extend beyond the regulation of calcium homeostasis and bone function. Unfortunately, identification of tissues expressing VDR has been controversial due to low abundance of the receptor and quality of the antibodies used. Therefore, we elected to characterize a panel of commercially available VDR antibodies in order to identify antibodies with high specificity and sensitivity. To address these objectives, we have used multiple immunoassays to determine VDR expression in tissues from several organs from multiple species employing tissues from VDR knockout mice as critical negative controls. Many of the antibodies tested showed nonspecific binding that can account for divergent reports. However, one antibody, identified as D-6, is highly specific and extremely sensitive. The specificity, sensitivity, and versatility of this antibody make it the preferred antibody for identifying VDR expression in target tissues using immunological methods.     

Extremely high circulating levels of 1α,25-dihydroxyvitamin D3 in the marmoset,a new world monkey

Compared to most mammals, the marmoset, a new world monkey, requires particularly large amounts of vitamin D to maintain normal growth. We compared serum concentrations of vitamin D metabolites in marmosets with rhesus monkeys and humans. The circulating levels of 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] in marmosets were 4 to 10 times higher than those in rhesus monkeys and humans. But none of the marmosets exhibited hypercalcemia. In two marmosets which had suffered bone fractures, the 1α,25-(OH)₂D₃ levels were particularly elevated. These results suggest that the marmoset has an end-organ resistance to 1α,25(OH)₂D₃.     

Intestinal calcium-binding protein (CaBP) and bone calcium mobilization in response to 25R,26 AND 25S,26-dihydroxycholecalciferol in intact and nephrectomized rats

Since intestinal calcium-binding protein (CaBP) can he regarded as an expression of the hormone-like action of 1,25-dihydroxyvitamin D₃ (1,25-(OH)₂D₃) on the duodenal enterocyte we have investigated the potential biological activity of 25R and 25S,26-(OH)₂D₃ (two recently synthesized epimers of vitamin D₃ metabolite) to promote intestinal CaBP production as compared to bone calcium mobilization in vitamin D and calcium-deficient rats. In our assay steroids exhibited a 72 hour calcemic response. Our results show a linear relationship between CaBP synthesis and the logarithm of the dose (130–2080 pmol dose range) of either 25R or 25S epimer. The CaBP response was comparable for both epimers. Similarly bone calcium mobilization response was dose related as a linear function of the logarithm of the administered dose. Again, calcemic response was comparable for both epimers. In our model these two epimers were about as active on intestine to increase CaBP amount as on bone to elevate serum calcium level. Bilateral nephrectomy abolished CaBP response to a large dose (1040 pmol) of either 25R or 25S epimer but did not abolish it to a 130 pmol dose of 1α, 25-(OH)₂D₃.     

Correlating Pre-Operative Vitamin D Status with Post-Thyroidectomy Hypocalcemia

Objective: To examine the relationship between pre-operative vitamin D status and post-thyroidectomy hypocalcemia.Methods: Retrospective study examining 264 total and completion thyroidectomies conducted between 2007 and 2011. Subjects included had a recorded 25-hydroxyvitamin D (25[OH]D) level within 21 days prior to or 1 day following surgery, did not have a primary parathyroid gland disorder, and were not taking 1,25-dihydroxyvitamin D₃ (calcitriol) prior to surgery. Some subjects were repleted with vitamin D pre-operatively if a low 25(OH)D level (typically below 20 ng/mL) was identified. Pre-operative 25(OH)D, concurrent neck dissection, integrity of parathyroid glands, final pathology, postoperative parathyroid hormone (PTH), calcium nadir and repletion, and length of stay were examined.Results: The mean pre-operative 25(OH)D for all subjects was 25 ng/mL, and the overall rate of post-operative hypocalcemia was 37.5%. Lower pre-operative 25(OH)D did not predict postoperative hypocalcemia (P =.96); however, it did predict the need for postoperative 1,25-dihydroxyvitamin D₃ administration (P =.01). Lower postoperative PTH levels (P =.001) were associated with postoperative hypocalcemia.Conclusion: Pre-operative 25(OH)D did not predict a postoperative decrease in serum calcium, although it did predict the need for 1,25-dihydroxyvitamin D₃ therapy in hypocalcemic subjects. We recommend that 25(OH)D be assessed and, if indicated, repleted pre-operatively in patients undergoing total thyroidectomy.Abbreviations: 25(OH)D = 25-hydroxyvitamin D PTH = parathyroid hormone     

Quantitation of vitamin D and its metabolites and their plasma concentrations in five species of animals

Chromatographic methods suitable for the resolution of 24,25-dihydroxyvitamin D₃, 24,25-dihydroxyvitamin D₂, 25-hydroxyvitamin D₃-26,23 lactone, and 25,26-dihydroxyvitamin D₂ are described. These four metabolites comigrated in high-pressure liquid chromatography on silicic acid columns developed in 11:89 isopropanol:hexane. Adequate resolution was achieved by subjecting the four-metabolite complex to high-pressure liquid chromatography column developed in 2:98 isopropanol:methylene chloride. This additional chromatographic step, coupled with modifications of assay procedures previously described, allowed for the estimation of plasma concentrations of vitamin D₂, vitamin D₃, 25-hydroxyvitamin D₂, 25-hydroxyvitamin D₃, 24,25-dihydroxyvitamin D₂, 24,25-dihydroxyvitamin D₃, 25,26 dihydroxyvitamin D₂, 25,26-dihydroxyvitamin D₃, 25-hydroxyvitamin D₃-26,23 lactone, and 1,25-dihydroxyvitamin D (1,25-dihydroxyvitamin D₂ plus 1,25-dihydroxyvitamin D₃). The samples automatically were introduced onto the high-pressure liquid chromatography columns with a Waters 710A “intelligent” processor. The metabolites were automatically collected with the aid of a programmable timer that advanced a fraction collector at predetermined intervals. The assays were used to determine the plasma vitamin D and vitamin D metabolite concentrations in five species of adult farm animals.     

Simultaneous measurement of plasma vitamin D(3) metabolites, including 4β,25-dihydroxyvitamin D(3), using liquid chromatography-tandem mass spectrometry

Simultaneous and accurate measurement of circulating vitamin D metabolites is critical to studies of the metabolic regulation of vitamin D and its impact on health and disease. To that end, we have developed a specific liquid chromatography–tandem mass spectrometry (LC–MS/MS) method that permits the quantification of major circulating vitamin D₃ metabolites in human plasma. Plasma samples were subjected to a protein precipitation, liquid–liquid extraction, and Diels–Alder derivatization procedure prior to LC–MS/MS analysis. Importantly, in all human plasma samples tested, we identified a significant dihydroxyvitamin D₃ peak that could potentially interfere with the determination of 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃] concentrations. This interfering metabolite has been identified as 4β,25-dihydroxyvitamin D₃ [4β,25(OH)₂D₃] and was found at concentrations comparable to 1α,25(OH)₂D₃. Quantification of 1α,25(OH)₂D₃ in plasma required complete chromatographic separation of 1α,25(OH)₂D₃ from 4β,25(OH)₂D₃. An assay incorporating this feature was used to simultaneously determine the plasma concentrations of 25OHD₃, 24R,25(OH)₂D₃, 1α,25(OH)₂D₃, and 4β,25(OH)₂D₃ in healthy individuals. The LC–MS/MS method developed and described here could result in considerable improvement in quantifying 1α,25(OH)₂D₃ as well as monitoring the newly identified circulating metabolite, 4β,25(OH)₂D₃.     

Characterization of the vitamin D endocrine system in human sebocytes in vitro

Sebocytes are sebum-producing cells that form the sebaceous glands. We investigated the role of sebocytes as target cells for vitamin D metabolites and the existence of an enzymatic machinery for the local synthesis and metabolism of 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃, calcitriol], the biologically active vitamin D metabolite, in these cell types. Expression of vitamin D receptor (VDR), vitamin D-25-hydroxylase (25OHase), 25-hydroxyvitamin D-1α-hydroxylase (1αOHase), and 1,25-dihydroxyvitamin D-24-hydroxylase (24OHase) was detected in SZ95 sebocytes in vitro using real time quantitative polymerase chain reaction. Splice variants of 1αOHase were identified by nested touchdown polymerase chain reaction. We demonstrated that incubation of SZ95 sebocytes with 1,25(OH)₂D₃ resulted in a cell culture condition-, time-, and dose-dependent modulation of cell proliferation, cell cycle regulation, lipid content and interleukin-6/interleukin-8 secretion in vitro. RNA expression of VDR and 24OHase was upregulated along with vitamin D analogue treatment. Although several other splice variants of 1αOHase were detected, our findings indicate that the full length product represents the major 1αOHase gene product in SZ95 cells. In conclusion, SZ95 sebocytes express VDR and the enzymatic machinery to synthesize and metabolize biologically active vitamin D analogues. Sebocytes represent target cells for biologically active metabolites. Our findings indicate that the vitamin D endocrine system is of high importance for sebocyte function and physiology. We conclude that sebaceous glands represent potential targets for therapy with vitamin D analogues or for pharmacological modulation of 1,25(OH)₂D₃ synthesis/metabolism.     

1α,25-Dihydroxyvitamin D3 induces differentiation of human myeloid leukemia cells

A human myeloid leukemia cell line [HL-60] could be induced to differentiate into mature myeloid cells by 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃], the active form of vitamin D₃. At 10^?10–10^?8 M, 1α,25(OH)₂D₃ suppressed cell growth in a dose-dependent manner and markedly induced phagocytosis and C3 rosette formation. The potency of 1α,25(OH)₂D₃ in inducing differentiation was nearly equivalent to that of known synthetic inducers such as dimethyl sulfoxide, actinomycin D or a phorbol ester (12-o-tetra-decanoyl-phorbol-13-acetate). These results clearly indicate that 1α,25(OH)₂D₃, besides its well known biological effect in enhancing intestinal calcium transport and bone mineral mobilization activities, is involved in the cell grwoth and differentiation of HL-60 cells.     

Metabolism and binding properties of 24,24-difluoro-25-hydroxyvitamin D3

To evaluate possible functional roles for 24,25-dihydroxyvitamin D₃, 24,24-difluoro-25-hydroxyvitamin D₃ has been synthesized and shown to be equally as active as 25-hydroxyvitamin D₃ in all known functions of vitamin D. The use of the difluoro compound for this purpose is based on the assumption that the C-F bonds are stable in vivo and that the fluorine atom does not act as hydroxyl in biological systems. No 24,25-dihydroxyvitamin D₃ was detected in the serum obtained from vitamin D-deficient rats that had been given 24,24-difluoro-25-hydroxyvitamin D₃, while large amounts were found when 25-hydroxyvitamin D₃ was given. Incubation of the 24,24-difluoro compound with kidney homogenate prepared from vitamin D-replete chickens failed to produce 24,25-dihydroxyvitamin D₃, while the same preparations produced large amounts of 24,25-dihydroxyvitamin D₃ from 25-hydroxyvitamin D₃. Kidney homogenate prepared from vitamin D-deficient chickens produced 24,24-difluoro-1,25-dihydroxyvitamin D₃ from 24,24-difluoro-25-hydroxyvitamin D₃ and 1,25-dihydroxyvitamin D₃ from 25-hydroxyvitamin D₃. In binding to the plasma transport protein for vitamin D compounds, 24,24-difluoro-25-hydroxyvitamin D₃ is less active than 25-hydroxyvitamin D₃ and 24R,25-dihydroxyvitamin D₃. In binding to the chick intestinal cytosol receptor, 24,24-difluoro-25-hydroxyvitamin D₃ is more active than 25-hydroxyvitamin D₃ which is itself more active than 24R,25-dihydroxyvitamin D₃. The 24,24-difluoro-1,25-dihydroxyvitamin D₃ is equal to 1,25-dihydroxyvitamin D₃, and both are 10 times more active than 1,24R,25-trihydroxyvitamin D₃ in this system. These results provide strong evidence that the C-24 carbon of 24,24-difluoro-25-hydroxyvitamin D₃ cannot be hydroxylated in vivo, and, further, the 24-F substitution acts similar to H and not to OH in discriminating binding systems for vitamin D compounds.     

Vitamin D analog 25-(OH)-16,23E-diene-26,27-hexafluoro-vitamin D3 induces differentiation of HL60 cells with minimal effects on cellular calcium homeostasis

Numerous vitamin D₃ analogs (VDAs) can inhibit the proliferation of cells from several types of human malignancies. The physiologically active form of vitamin D₃, 1,25-dihydroxyvitamin D₃(1,25D₃), is formed by successive hydroxylations of cholecalciferol at the 25 and 1α positions. In this study we examined the effects of the absence of the 1α(OH) group, introduction of a double bond in position 16, and further modifications at the 23, 26, and 27 positions in the side chain on the potency of the VDAs. The parameters studied were the rapidity of the induction of monocytic differentiation, the cell cycle traverse, and the effects of VDAs on intracellular calcium homeostasis in HL60 cells. The results show that (1) 1,25D₃ derivatives which lack the 1α(OH) group have little differentiation-inducing activity, (2) hexafluorination (6F) of the terminal methyl groups in the side chain partially restores the activity of 1α-desoxy compounds and potentiates the activity of 1α hydroxylated compounds, and (3) 25-(OH)-16,23E-diene-26,27-hexafluoro-vitamin D₃ (Ro25-9887) alone among the twelve compounds tested induces differentiation with only minimal changes in the basal levels of intracellular calcium and store-dependent calcium influx in HL60 cells. Addition of 1α(OH) group to this compound increases its differentiation-inducing activity but also elevates basal calcium level. The results suggest that altered calcium homeostasis is not an obligatory component of HL60 leukemia cell differentiation, and that Ro25-9887 and related VDAs may be suitable for testing as components of anti-leukemic therapy. © 1996 Wiley-Liss, Inc.     

Effects of the vitamin D3 analog 1α,25-dihydroxyvitamin D3-3β-bromoacetate on rat osteosarcoma cells: Comparison with 1α,25-dihydroxyvitamin D3

The actions of the hormonal form of vitamin D, 1α,25-dihydroxyvitamin D₃ [1α,25-(OH)₂D₃], are mediated by both genomic and nongenomic mechanisms. Several vitamin D synthetic analogs have been developed in order to identify and characterize the site(s) of action of 1α,25-(OH)₂D₃ in many cell types including osteoblastic cells. We have compared the effects of 1α,25-(OH)₂D₃ and a novel 1α,25-(OH)₂D₃ bromoester analog (1,25-(OH)₂-BE) that covalently binds to vitamin D receptors. Rat osteosarcoma cells that possess (ROS 17/2.8) or lack (ROS 24/1) the classic intracellular vitamin D receptor were studied to investigate genomic and nongenomic actions. In ROS 17/2.8 cells plated at low density, the two vitamin D compounds (1 × 10⁻⁸ M) caused increased cell proliferation, as assessed by DNA synthesis and total cell counts. Northern blot analysis revealed that the mitogenic effect of both agents was accompanied by an increase in steady-state osteocalcin mRNA levels, but neither agent altered alkaline phosphatase mRNA levels in ROS 17/2.8 cells. ROS 17/2.8 cells responded to 1,25-(OH)₂-BE but not the natural ligand with a significant increase in osteocalcin secretion after 72, 96, 120, and 144 hr of treatment. Treatment of ROS 17/2.8 cells with the bromoester analog also resulted in a significant decrease in alkaline phosphatase-specific activity. To compare the nongenomic effects of 1α,25-(OH)₂D₃ and 1,25-(OH)₂-BE, intracellular calcium was measured in ROS 24/1 cells loaded with the fluorescent calcium indicator Quin 2. At 2 × 10⁻⁸ M, both 1α,25-(OH)₂D₃ and 1,25-(OH)₂-BE increased intracellular calcium within 5 min. Both the genomic and nongenomic actions of 1,25-(OH)₂-BE are similar to those of 1α,25-(OH)₂D₃, and since 1,25-(OH)₂-BE has more potent effects on osteoblast function than the naturally occurring ligand due to more stable binding, this novel vitamin D analog may be useful in elucidating the structure and function of cellular vitamin D receptors. © 1996 Wiley-Liss, Inc.     

Circulating vitamin D3 and 25-hydroxyvitamin D in humans: An important tool to define adequate nutritional vitamin D status

Circulating 25-hydroxyvitamin D [25(OH)D] is generally considered the means by which we define nutritional vitamin D status. There is much debate, however, with respect to what a healthy minimum level of circulation 25(OH)D should be. Recent data using various biomarkers such as intact parathyroid hormone (PTH), intestinal calcium absorption, and skeletal density measurements suggest this minimum level to be 80 nmol (32 ng/mL). Surprisingly, the relationship between circulating vitamin D₃ and its metabolic product—25(OH)D₃ has not been studied. We investigated this relationship in two separate populations: the first, individuals from Hawaii who received significant sun exposure; the second, subjects from a lactation study who received up to 6400 IU vitamin D₃/day for 6 months.Results (1) the relationship between circulating vitamin D₃ and 25(OH)D in both groups was not linear, but appeared saturable and controlled; (2) optimal nutritional vitamin D status appeared to occur when molar ratios of circulating vitamin D₃ and 25(OH)D exceeded 0.3; at this point, the V_max of the 25-hydroxylase appeared to be achieved. This was achieved when circulating 25(OH)D exceeded 100 nmol.We hypothesize that as humans live today, the 25-hydroxylase operates well below its V_max because of chronic substrate deficiency, namely vitamin D₃. When humans are sun (or dietary) replete, the vitamin D endocrine system will function in a fashion as do these other steroid synthetic pathways, not limited by substrate. Thus, the relationship between circulating vitamin D and 25(OH)D may represent what “normal” vitamin D status should be.     

Studies on the mode of action of calciferol: Biological activity of 1α,24R,25-trihydroxyvitamin D3 in the chick

The biological activity of 1α,24R,25-trihydroxyvitamin D₃ [1α,24R,25(OH)₃D₃] was elevated in comparison to the hormonally active form of vitamin D₃, 1α,25-dihydroxyvitamin D₃ [1α,25(OH)₂D₃], in the rachitic chick in terms of its ability to (a) stimulate intestinal calcium absorption, (b) mobilize bone calcium, (c) induce intestinal calcium binding protein, (d) modulate the level of enzyme activity of the renal 25-OH-D₃-1-hydroxylase system, and (e) interact with the intestinal cystosol-chromatin receptor system for the 1α,25(OH)₂D₃ receptor system. In each of these assays, the relative ratio of activity of 1α,24R,25(OH)₃D₃ to 1α,25(OH)₂D₃was (a) 25–50, (b) ca. 20, (c) 10, (d) 50, and (e) 36%, respectively.