Vitamin D plasma binding protein. Turnover and fate in the rabbit

Vitamin D: sources, physiological role, biokinetics, deficiency, therapeutic use, toxicity, and overview of analytical methods for detection of vitamin D and its metabolites

Vitamin D has a well-known role in the calcium homeostasis associated with the maintenance of healthy bones. It increases the efficiency of the intestinal absorption of dietary calcium, reduces calcium losses in urine, and mobilizes calcium stored in the skeleton. However, vitamin D receptors are present ubiquitously in the human body and indeed, vitamin D has a plethora of non-calcemic functions. In contrast to most vitamins, sufficient vitamin D can be synthesized in human skin. However, its production can be markedly decreased due to factors such as clothing, sunscreens, intentional avoidance of the direct sunlight, or the high latitude of the residence. Indeed, more than one billion people worldwide are vitamin D deficient, and the deficiency is frequently undiagnosed. The chronic deficiency is not only associated with rickets/osteomalacia/osteoporosis but it is also linked to a higher risk of hypertension, type 1 diabetes, multiple sclerosis, or cancer. Supplementation of vitamin D may be hence beneficial, but the intake of vitamin D should be under the supervision of health professionals because overdosing leads to intoxication with severe health consequences. For monitoring vitamin D, several analytical methods are employed, and their advantages and disadvantages are discussed in detail in this review.

D-VITylation: Harnessing the biology of vitamin D to improve the pharmacokinetic properties of peptides and small proteins

Peptides have great potential to be potent and specific therapeutics, yet their small size leads to rapid glomerular filtration, which severely limits therapeutic applications. Although conjugation of small proteins to large polymers typically results in longer residence times, these conjugates often have a significant loss of biological activity due to steric hindrance. Here, we improve the pharmacokinetics (PK) of peptide therapeutics by harnessing the biology of vitamin D. Attachment of a small vitamin D-based molecule (D-VITylation) protects the conjugated peptide or protein from renal clearance by virtue of reversible binding to the serum-circulating vitamin D binding protein (DBP), without compromising bioactivity. Varying the conjugation site on vitamin D affects the binding to DBP, with higher affinity corresponding to a longer plasma half-life. We also demonstrate the important contribution of the peptide to the overall PK, likely due to alternative clearance mechanisms such as protease degradation and receptor-mediated cellular uptake. With a Fab antibody fragment, for which these alternate clearance mechanisms are not significant, D-VITylation increases the half-life of elimination from 14 to 61 h in rats. The PK profile in minipigs and projected lifetime in humans suggest that D-VITylation is a viable strategy to achieve once-weekly dosing of peptide therapeutics in humans.

Vitamin D and Skeletal Muscle: Current Concepts From Preclinical Studies

Muscle weakness has been recognized as a hallmark feature of vitamin D deficiency for many years. Until recently, the direct biomolecular effects of vitamin D on skeletal muscle have been unclear. Although in the past, some reservations have been raised regarding the expression of the vitamin D receptor in muscle tissue, this special issue review article outlines the clear evidence from preclinical studies for not only the expression of the receptor in muscle but also the roles of vitamin D activity in muscle development, mass, and strength. Additionally, muscle may also serve as a dynamic storage site for vitamin D, and play a central role in the maintenance of circulating 25-hydroxy vitamin D levels during periods of low sun exposure. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Implications of Vitamin D Research in Chickens can Advance Human Nutrition and Perspectives for the Future

The risk of vitamin D insufficiency in humans is a global problem that requires improving ways to increase vitamin D intake. Supplements are a primary means for increasing vitamin D intake, but without a clear consensus on what constitutes vitamin D sufficiency, there is toxicity risk with taking supplements. Chickens have been used in many vitamin-D-related research studies, especially studies involving vitamin D supplementation. Our state-of-the-art review evaluates vitamin D metabolism and how the different hydroxylated forms are synthesized. We provide an overview of how vitamin D is absorbed, transported, excreted, and what tissues in the body store vitamin D metabolites. We also discuss a number of studies involving vitamin D supplementation with broilers and laying hens. Vitamin D deficiency and toxicity are also described and how they can be caused. The vitamin D receptor (VDR) is important for vitamin D metabolism; however, there is much more to understand about VDR in chickens. Potential research aims involving vitamin D and chickens should explore VDR mechanisms that could lead to newer insights into VDR. Utilizing chickens in future research to help elucidate vitamin D mechanisms has great potential to advance human nutrition. Finding ways to increase vitamin D intake will be necessary because the coronavirus disease 2019 (COVID-19) pandemic is leading to increased risk of vitamin D deficiency in many populations. Chickens can provide a dual purpose with addressing pandemic-caused vitamin D deficiency: 1) vitamin D supplementation gives chickens added-value with the possibility of leading to vitamin-D-enriched meat and egg products; and 2) using chickens in research provides data for translational research. We believe expanding vitamin-D-related research in chickens to include more nutritional aims in vitamin D status has great implications for developing better strategies to improve human health.

Skeletal Muscle and the Maintenance of Vitamin D Status

Vitamin D, unlike the micronutrients, vitamins A, E, and K, is largely obtained not from food, but by the action of solar ultraviolet (UV) light on its precursor, 7-dehydrocholesterol, in skin. With the decline in UV light intensity in winter, most skin production of vitamin D occurs in summer. Since no defined storage organ or tissue has been found for vitamin D, it has been assumed that an adequate vitamin D status in winter can only be maintained by oral supplementation. Skeletal muscle cells have now been shown to incorporate the vitamin D-binding protein (DBP) from blood into the cell cytoplasm where it binds to cytoplasmic actin. This intracellular DBP provides an array of specific binding sites for 25-hydroxyvitamin D (25(OH)D), which diffuses into the cell from the extracellular fluid. When intracellular DBP undergoes proteolytic breakdown, the bound 25(OH)D is then released and diffuses back into the blood. This uptake and release of 25(OH)D by muscle accounts for the very long half-life of this metabolite in the circulation. Since 25(OH)D concentration in the blood declines in winter, its cycling in and out of muscle cells appears to be upregulated. Parathyroid hormone is the most likely factor enhancing the repeated cycling of 25(OH)D between skeletal muscle and blood. This mechanism appears to have evolved to maintain an adequate vitamin D status in winter.

The Role of Skeletal Muscle in Maintaining Vitamin D Status in Winter

The status of vitamin D is determined mainly by its formation in skin by the photochemical action of solar UVB light (wavelength 290-320 nm) on the precursor 7-dehydrocholesterol. Because of seasonal variation in intensity of solar UV light, vitamin D status falls in winter and rises in summer. It has been presumed that there is no functional store of vitamin D. Thus, to avoid deficiency, a nutritional supply would be required in winter. However, there is now evidence that the main circulating metabolite of vitamin D, 25-hydroxyvitamin D, accumulates in skeletal muscle cells, which provide a functional store during the winter months. The mechanism is mediated by muscle cell uptake of circulating vitamin D-binding protein (DBP) through a megalin-cubilin membrane transport process. DBP then binds to cytoplasmic actin to provide an array of high-affinity binding sites for 25-hydroxyvitamin D [25(OH)D]. The repeated passage of 25(OH)D into and out of muscle cells would account for its long residence time in blood.

Vitamin D Binding Protein: A Historic Overview

Vitamin D and all its metabolites are bound to a specific vitamin D binding protein, DBP. This protein was originally first discovered by its worldwide polymorphism and called Group-specific Component (GC). We now know that DBP and GC are the same protein and appeared early in the evolution of vertebrates. DBP is genetically the oldest member of the albuminoid family (including albumin, α-fetoprotein and afamin, all involved in transport of fatty acids or hormones). DBP has a single binding site for all vitamin D metabolites and has a high affinity for 25OHD and 1,25(OH)2D, thereby creating a large pool of circulating 25OHD, which prevents rapid vitamin D deficiency. DBP of higher vertebrates (not amphibians or reptiles) binds with very high affinity actin, thereby preventing the formation of polymeric actin fibrils in the circulation after tissue damage. Megalin is a cargo receptor and is together with cubilin needed to reabsorb DBP or the DBP-25OHD complex, thereby preventing the urinary loss of these proteins and 25OHD. The total concentrations of 25OHD and 1,25(OH)2D in DBP null mice or humans are extremely low but calcium and bone homeostasis remain normal. This is the strongest argument for claiming that the "free hormone hypothesis" also applies to the vitamin D hormone, 1,25(OH)2D. DBP also transports fatty acids, and can play a role in the immune system. DBP is genetically very polymorphic with three frequent alleles (DBP/GC 1f, 1s, and 2) but in total more than 120 different variants but its health consequences, if any, are not understood. A standardization of DBP assays is essential to further explore the role of DBP in physiology and diseases.

Diet induced obesity modifies vitamin D metabolism and adipose tissue storage in mice

Low circulating levels of total and free 25-hydroxyvitamin D (25(OH)D) indicative of vitamin D status have been associated with obesity in humans. Moreover, obesity is thought to play a causal role in the reduction of 25(OH)D levels, and several theories have been put forward to explain this relationship. Here we tested the hypothesis that obesity disrupts vitamin D homeostasis in key organs of vitamin D metabolism. Male C57BL6 mice were fed for 7 or 11 weeks on either a control diet (control, 10% energy from fat) or a high-fat diet (HF, 60% energy from fat) formulated to provide equivalent vitamin D3 intake in both groups. After 7 weeks, there was a transient increase of total 25(OH)D together with a significant decrease of plasma vitamin D3 that could be related to the induction of hepatic genes involved in 25-hydroxylation. After 11 weeks, there was no change in total 25(OH)D but a significant decrease of free 25(OH)D and plasma vitamin D3 levels. We also quantified an increase of 25(OH)D in adipose tissue that was inversely correlated to the free 25(OH)D. Interestingly, this accumulation of 25(OH)D in adipose tissue was highly correlated to the induction of Cyp2r1, which could actively participate in vitamin D3 trapping and subsequent conversion to 25(OH)D in adipose tissue. Taken together, our data strongly suggest that the enzymes involved in vitamin D metabolism, notably in adipose tissue, are transcriptionally modified under high-fat diet, thus contributing to the obesity-related reduction of free 25(OH)D.

Sunlight exposure is just one of the factors which influence vitamin D status

Studies on the determinants of vitamin D status have tended to concentrate on input - exposure to ultraviolet B radiation and the limited sources in food. Yet, vitamin D status, determined by circulating concentrations of 25-hydroxyvitamin D (25(OH)D), can vary quite markedly in groups of people with apparently similar inputs of vitamin D. There are small effects of polymorphisms in the genes for key proteins involved in vitamin D production and metabolism, including 7-dehydrocholesterol reductase, which converts 7-dehydrocholesterol, the precursor of vitamin D, to cholesterol, CYP2R1, the main 25-hydroxylase of vitamin D, GC, coding for the vitamin D binding protein which transports 25(OH)D and other metabolites in blood and CYP24A1, which 24-hydroxylates both 25(OH)D and the hormone, 1,25-dihydroxyvitamin D. 25(OH)D has a highly variable half-life in blood. There is evidence that the half-life of 25(OH)D is affected by calcium intake and some therapeutic agents. Fat tissue seems to serve as a sink for the parent vitamin D, which is released mainly when there are reductions in adiposity. Some evidence is presented to support the proposal that skeletal muscle provides a substantial site of sequestration of 25(OH)D, protecting this metabolite from degradation by the liver, which may help to explain why exercise, not just outdoors, is usually associated with better vitamin D status.

1,25-Dihydroxycholecalciferol (calcitriol) modifies uptake and release of 25-hydroxycholecalciferol in skeletal muscle cells in culture

The major circulating metabolite of vitamin D₃, 25-hydroxycholecalciferol [25(OH)D], has a remarkably long half-life in blood for a (seco)steroid. Data from our studies and others are consistent with the hypothesis that there is a role for skeletal muscle in the maintenance of vitamin D status. Muscle cells internalise vitamin D-binding protein (DBP) from the circulation by means of a megalin/cubilin plasma membrane transport mechanism. The internalised DBP molecules then bind to actin and thus provide an intracellular array of high affinity binding sites for its specific ligand, 25(OH)D. There is evidence that the residence time for DBP in muscle cells is short and that it undergoes proteolytic degradation, releasing bound 25(OH)D. The processes of internalisation of DBP and its intracellular residence time, bound to actin, appear to be regulated. To explore whether 1,25-dihydroxycholecalciferol (calcitriol) has any effect on this process, cell cultures of myotubes and primary skeletal muscle fibers were incubated in a medium containing 10^-10M calcitriol but with no added DBP. After 3h pre-incubation with calcitriol, the net uptake of 25(OH)D by these calcitriol-treated cells over a further 4h was significantly greater than that in vehicle-treated control cells. This was accompanied by a significant increase in intracellular DBP protein. However, after 16h of pre-incubation with calcitriol, the muscle cells showed a significantly depressed ability to accumulate 25(OH)D compared to control cells over a further 4 or 16hours. These effects of pre-incubation with calcitriol were abolished in fibers from VDR-knockout mice. The effect was also abolished by the addition of 4,4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), which inhibits chloride channel opening. Incubation of C2 myotubes with calcitriol also significantly reduced retention of previously accumulated 25(OH)D after 4 or 8h. It is concluded from these in vitro studies that calcitriol can modify the DBP-dependent uptake and release of 25(OH)D by skeletal muscle cells in a manner that suggests some inducible change in the function of these cells.

Gene Expression Pattern in Response to Cholecalciferol Supplementation Highlights Cubilin as a Major Protein of 25(OH)D Uptake in Adipocytes and Male Mice White Adipose Tissue

It is well established that the active form of vitamin D (i.e., 1,25-dihydroxyvitamin D [1,25(OH)2D]) regulates the expression of genes involved in its own metabolism and transport in the kidney and possibly in the liver. However, little is known about the transcriptional impact of cholecalciferol supplementation on white adipose tissue (WAT) and adipocytes, which are a major site of vitamin D and 25-hydroxyvitamin D [25(OH)D] storage in the organism. To fill this gap, we investigated the impact of cholecalciferol supplementation in WAT via a panel of genes coding for enzymes and proteins involved in vitamin D metabolism and uptake. Mice supplemented with cholecalciferol (15,000 IU/kg of body weight per day) for 4 days showed decreased messenger RNA (mRNA) levels of proteins involved in cholecalciferol metabolism (Cyp24a1, Cyp27a1) and decreased cubilin mRNA levels in WAT. These data were partly confirmed in 3T3-L1 adipocytes incubated with 1,25(OH)2D. The downregulation of cubilin mRNA observed in WAT and in 3T3-L1 was confirmed at the protein level in WAT and at the mRNA level in human primary adipocytes. Vitamin D receptor (VDR) agonist (EB1089) and RNA interference approaches demonstrated that VDR was involved in this regulation. Furthermore, chemical inhibitor and RNA inference analysis demonstrated that cubilin was involved in 25(OH)D uptake by adipocytes. This study established an overall snapshot of the genes regulated by cholecalciferol in mouse WAT and cell-autonomously in adipocytes. We highlighted that the regulation of cubilin expression was mediated by a VDR-dependent mechanism, and we demonstrated that cubilin was involved in 25(OH)D uptake by adipocytes.

The effect of parathyroid hormone on the uptake and retention of 25-hydroxyvitamin D in skeletal muscle cells

Data from our studies, and those of others, support the proposal that there is a role for skeletal muscle in the maintenance of vitamin D status. We demonstrated that skeletal muscle is able to internalise extracellular vitamin D binding protein, which then binds to actin in the cytoplasm, to provide high affinity binding sites which accumulate 25-hydroxyvitamin D₃ (25(OH)D₃) [1]. This study investigated the concentration- and time-dependent effects of parathyroid hormone (PTH) on the capacity of muscle cells to take up and release ³H-25(OH)D₃. Uptake and retention studies for ³H-25(OH)D₃ were carried out with C2C12 cells differentiated into myotubes and with primary mouse muscle fibers as described [1]. The presence of PTH receptors on mouse muscle fibers was demonstrated by immunohistochemistry and PTH receptors were detected in differentiated myotubes, but not myoblasts, and on muscle fibers by Western blot. Addition of low concentrations of vitamin D binding protein to the incubation media did not alter uptake of 25(OH)D₃. Pre-incubation of C2 myotubes or primary mouse muscle fibers with PTH (0.1 to 100 pM) for 3h resulted in a concentration-dependent decrease in 25(OH)D₃ uptake after 4 or 16h. These effects were significant at 0.1 or 1pM PTH (p<0.001) and plateaued at 10pM, with 25(OH)D₃ uptake reduced by over 60% (p<0.001) in both cell types. In C2 myotubes, retention of 25(OH)D₃ was decreased after addition of PTH (0.1 to 100pM) in a concentration-dependent manner by up to 80% (p<0.001) compared to non-PTH treated-C2 myotubes. These data show that muscle uptake and retention of 25(OH)D₃ are modulated by PTH, a physiological regulator of mineral homeostasis, but the cell culture model may not be a comprehensive reflection of vitamin D homeostatic mechanisms in whole animals.

Proteomics of acute heart failure in a rat post-myocardial infarction model

The aim of the present study was to identify the mechanisms underlying the development of post-myocardial infarction (post-MI) heart failure. The left anterior descending coronary artery of rats was occluded to mimic human ischemic heart disease. Linear Trap Quadropole OrbiTrap mass spectrometry was used to profile the expressions of energy metabolism‑associated and calcium‑binding proteins in the post‑MI and control groups. Using the online Protein Analysis Through Evolutionary Relationships classification system, 78 differentially expressed proteins were identified, including 50 downregulated proteins and 28 upregulated proteins in post‑MI group when compared with the control group. The differentially expressed proteins were closely associated with energy metabolism, contractile function, calcium handling, pathological hypertrophy and cardiac remodeling. These results were further validated using western blotting. At different postoperative time points (1st and 14th day following surgery) during the progression of advanced heart failure post‑MI, dynamic alterations in differential protein expression were identified. The expression of the vitamin D protein was significantly upregulated on the 1st day post‑MI however, was then downregulated with progression of the disease on the 14th day post‑MI. These results identified various target proteins associated with the disease, which may be used as diagnostic markers.

Uptake of 25-hydroxyvitamin D by muscle and fat cells

Vitamin D status, measured as serum 25-hydroxyvitamin D (25OHD) concentration, is determined by rates of input and of degradation. The half-life of 25OHD is surprisingly long for a steroid and much longer than its blood transporter, vitamin D binding protein. There is some evidence to suggest that vitamin D itself is stored in fat, whereas 25OHD concentrations are usually related to muscle-related parameters such as lean body mass and exercise. Both muscle and fat cells come from the mesenchymal cell lineage. We recently published evidence for net uptake of 25OHD into differentiated muscle cells, in a process that was megalin dependent, and speculated that this uptake might contribute to the extended half-life of 25OHD. Whether 25OHD is also taken up into cells of the adipocyte lineage is not clear. In the current study, we used the C2 muscle cell line as a source of myoblasts that were differentiated in culture to myotubes and 3T3-L1 pre-adipocytes that were differentiated into adipocytes in culture. We incubated the cells with trititated 25OHD and measured net uptake 4 and 16h afterwards. Differentiated myotubes took up labeled 25OHD in a time-dependent process to a far greater extent than myoblasts. In contrast, pre-adipocytes, but not differentiated adipocytes, accumulated labeled 25OHD in a time-dependent manner, though to a lesser extent than myotubes. Myotubes, but not myoblasts, showed megalin expression by immunohistochemistry. Pre-adipocytes, but not adipocytes, also showed expression of megalin. Since skeletal muscle consists mainly of differentiated muscle cells, while adipose tissue is mainly differentiated fat cells, it seems likely that muscle, but not fat tissue, provides a large extravascular pool through which 25OHD circulates and that this protects 25OHD from degradation. This article is part of a Special Issue entitled '16th Vitamin D Workshop'.

The vitamin D receptor (VDR) is expressed in skeletal muscle of male mice and modulates 25-hydroxyvitamin D (25OHD) uptake in myofibers

Vitamin D deficiency is associated with a range of muscle disorders, including myalgia, muscle weakness, and falls. In humans, polymorphisms of the vitamin D receptor (VDR) gene are associated with variations in muscle strength, and in mice, genetic ablation of VDR results in muscle fiber atrophy and motor deficits. However, mechanisms by which VDR regulates muscle function and morphology remain unclear. A crucial question is whether VDR is expressed in skeletal muscle and directly alters muscle physiology. Using PCR, Western blotting, and immunohistochemistry (VDR-D6 antibody), we detected VDR in murine quadriceps muscle. Detection by Western blotting was dependent on the use of hyperosmolar lysis buffer. Levels of VDR in muscle were low compared with duodenum and dropped progressively with age. Two in vitro models, C2C12 and primary myotubes, displayed dose- and time-dependent increases in expression of both VDR and its target gene CYP24A1 after 1,25(OH)2D (1,25 dihydroxyvitamin D) treatment. Primary myotubes also expressed functional CYP27B1 as demonstrated by luciferase reporter studies, supporting an autoregulatory vitamin D-endocrine system in muscle. Myofibers isolated from mice retained tritiated 25-hydroxyvitamin D3, and this increased after 3 hours of pretreatment with 1,25(OH)2D (0.1 nM). No such response was seen in myofibers from VDR knockout mice. In summary, VDR is expressed in skeletal muscle, and vitamin D regulates gene expression and modulates ligand-dependent uptake of 25-hydroxyvitamin D3 in primary myofibers.

Evidence for a specific uptake and retention mechanism for 25-hydroxyvitamin D (25OHD) in skeletal muscle cells

Little is known about the mechanism for the prolonged residence time of 25-hydroxyvitamin D (25OHD) in blood. Several lines of evidence led us to propose that skeletal muscle could function as the site of an extravascular pool of 25OHD. In vitro studies investigated the capacity of differentiated C2 murine muscle cells to take up and release 25OHD, in comparison with other cell types and the involvement of the membrane protein megalin in these mechanisms. When C2 cells are differentiated into myotubes, the time-dependent uptake of labeled 25OHD is 2-3 times higher than in undifferentiated myoblasts or nonmuscle osteoblastic MG63 cells (P < .001). During in vitro release experiments (after 25OHD uptake), myotubes released only 32% ± 6% stored 25OHD after 4 hours, whereas this figure was 60% ± 2% for osteoblasts (P < .01). Using immunofluorescence, C2 myotubes and primary rat muscle fibers were, for the first time, shown to express megalin and cubilin, endocytotic receptors for the vitamin D binding protein (DBP), which binds nearly all 25OHD in the blood. DBP has a high affinity for actin in skeletal muscle. A time-dependent uptake of Alexafluor-488-labeled DBP into mature muscle cells was observed by confocal microscopy. Incubation of C2 myotubes (for 24 hours) with receptor-associated protein, a megalin inhibitor, led to a 40% decrease in 25OHD uptake (P < .01). These data support the proposal that 25OHD, after uptake into mature muscle cells, is held there by DBP, which has been internalized via membrane megalin and is retained by binding to actin.

Cellular uptake of steroid carrier proteins--mechanisms and implications

Steroid hormones are believed to enter cells solely by free diffusion through the plasma membrane. However, recent studies suggest the existence of cellular uptake pathways for carrier-bound steroids. Similar to the clearance of cholesterol via lipoproteins, these pathways involve the recognition of carrier proteins by endocytic receptors on the surface of target cells, followed by internalization and cellular delivery of the bound sterols. Here, we discuss the emerging concept that steroid hormones can selectively enter steroidogenic tissues by receptor-mediated endocytosis, and we discuss the implications of these uptake pathways for steroid hormone metabolism and action in vivo.

Vitamin D: a growing perspective

Vitamin D deficiency has been widely reported in all age groups in recent years. Rickets has never been eradicated in developed countries, and it most commonly affects children from recent immigrant groups. There is much evidence that current vitamin D guidelines for the neonatal period, 5-10 microg (200-400 IU)/day, prevent rickets at the typical calcium intakes in developed countries. The annual incidence of vitamin D-deficiency rickets in developed countries ranges between 2.9 and 7.5 cases per 100,000 children. The prevalence of vitamin D deficiency in mothers and their neonates is remarkable, and the results of one study suggest that third-trimester 25-hydroxyvitamin D (25(OH)D) is associated with fetal bone mineral accrual that may affect prepubertal bone mass accumulation. Beyond infancy, the evidence indicates that 5 microg (200 IU)/day of vitamin D has little effect on vitamin D status as measured by the serum 25(OH)D concentration. Two randomized clinical trials show that higher vitamin D intake improves one-year gain in bone density in adolescent girls. The functions of vitamin D extend beyond bone to include immune system regulation and anti-proliferative effects on cells. Early life vitamin D inadequacy is implicated in the risk of bone disease, autoimmune disease, and certain cancers later in life; however, long-term interventional studies do not exist to validate the widespread implementation of greater vitamin D consumption. Here we review the available data concerning vitamin D status and health effects of vitamin D in pregnancy through to and including adolescence.

Changes with malnutrition in the concentration of plasma vitamin D binding protein in growing rats

The work presented here examines the possible effects of nutritional deficiencies on the characteristics of the plasma transport protein for vitamin D and its metabolites (vitamin D binding protein, DBP) in the growing rat. Deficiencies in both dietary protein intake and dietary energy intake may decrease the concentration of DBP in the circulation, although plasma DBP was not affected by dietary Ca deficiency. None of the dietary factors examined appears to influence the affinity of DBP for its major ligand, 25-hydroxycholecalciferol (25(OH)D3). Protein-deficient rats seemed to have difficulty in maintaining adequate concentrations of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) in the circulation. The sensitivity of DBP to dietary protein and energy intake may constitute a novel mechanism that may help to explain the observed associations between malnutrition and the development of metabolic bone disease, through alterations to the cellular availability of vitamin D ligands to DBP.

Kidney-specific upregulation of vitamin D3 target genes in ClC-5 KO mice

Mutations in ClC-5 cause Dent's disease, a disorder associated with low molecular weight proteinuria, hyperphosphaturia, and kidney stones. ClC-5 is a Cl(-)/H(+)-exchanger predominantly expressed in the kidney, where it facilitates the acidification of proximal tubular endosomes. The reduction in proximal tubular endocytosis resulting from a lack of ClC-5 raises the luminal concentration of filtered proteins and peptides like parathyroid hormone (PTH). The increase in PTH may explain the hyperphosphaturia observed in Dent's disease. Expression profiling, quantitative reverse transcriptase-polymerase chain reaction (qRT-PCR), and hormone measurements were used to investigate whether the disruption of ClC-5 affects other signalling pathways. Although the upregulation of 25(OH)(2)-vitamin D(3) 1alpha-hydroxylase and downregulation of vitamin D(3) 24-hydroxylase suggested an increased formation of 1,25(OH)(2)-vitamin D(3), the concentration of this active metabolite was reduced in the serum of ClC-5 knockout (KO) mice. However, target genes of 1,25(OH)(2)-vitamin D(3) were upregulated in KO kidneys. Expression analysis of intestine and bone revealed that the upregulation of 1,25(OH)(2)-vitamin D(3) target genes was kidney intrinsic and not systemic. In spite of reduced serum levels of 1,25(OH)(2)-vitamin D(3) in ClC-5 KO mice, 1,25(OH)(2)-vitamin D(3) is increased in later nephron segments as a consequence of impaired proximal tubular endocytosis. This leads to a kidney-specific stimulation of 1,25(OH)(2)-vitamin D(3) target genes that may contribute to the pathogenesis of Dent's disease. The activation of genes in distal nephron segments by hormones that are normally endocytosed in the proximal tubule may extend to other pathways like those activated by retinoic acid.

Biological and clinical aspects of the vitamin D binding protein (Gc-globulin) and its polymorphism

The vitamin D binding protein (DBP) is the major plasma carrier protein of vitamin D and its metabolites. Unlike other hydrophobic hormone-binding systems, it circulates in a considerably higher titer compared to its ligands. Apart from its specific sterol binding capacity, DBP exerts several other important biological functions such as actin scavenging, fatty acid transport, macrophage activation and chemotaxis. The DBP-gene is a member of a multigene cluster that includes albumin, alpha-fetoprotein, and alpha-albumin/afamin. All four genes are expressed predominantly in the liver with overlapping developmental profiles. DBP is a highly polymorphic serum protein with three common alleles (Gc1F, Gc1S and Gc2) and more than 120 rare variants. The presence of unique alleles is a useful tool for anthropological studies to discriminate and to reveal ancestral links between populations. Many studies have discussed the link between DBP-phenotypes and susceptibility or resistance to osteoporosis, Graves' disease, Hashimoto's thyroiditis, diabetes, COPD, AIDS, multiple sclerosis, sarcoidosis and rheumatic fever. This article reviews the general characteristics, functions and clinical aspects of DBP.

Mechanisms for the reduction of 24,25-dihydroxyvitamin D3 levels and bone mass in 24-hydroxylase transgenic rats

24-Hydroxylase (CYP24) is an enzyme distributed in the target tissues of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. Two functions for this enzyme have been reported: One is production of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] and the other is inactivation of 1alpha,25(OH)2D3. To elucidate other physiologic roles of CYP24 in vivo, we previously generated rats that constitutively express the CYP24 gene. These transgenic (Tg) rats developed unexpected phenotypes, such as low plasma levels of 24,25(OH)2D3, lipidemia, and albuminuria. In this study, we elucidated the mechanisms for inducing low plasma 24,25(OH)2D3 levels and bone loss. Tg rats excreted massive amounts of vitamin D binding protein (DBP), which coincided with the loss of albumin. In Tg rats, the renal expression pattern of megalin, which serves as an endocytotic receptor responsible for the reuptake of urinary proteins such as DBP and albumin, was identical to that of the wild-type rats. Excreted albumin appeared to compete for the binding and reabsorption of the DBP-25-hydroxyvitamin D3 [25(OH)D3] complex with megalin, resulting in a loss of 25(OH)D3 into the urine and subsequent reduction of plasma 24,25(OH)2D3. In this prominent rat model of nephritis, supplementation of 25(OH)D3 was effective in preventing bone loss in an early stage of renal insufficiency.

Calcium transport in the kidney

Recent discoveries of calcium-regulating and calcium-transporting proteins have paved the way for a heightened understanding of the mechanisms and control of renal calcium transport. In this review, new findings regarding the multifunctional megalin receptor, chloride channels, a putative calcium entry channel, and the calcium-sensing receptor are discussed.

An endocytic pathway essential for renal uptake and activation of the steroid 25-(OH) vitamin D3

Steroid hormones may enter cells by diffusion through the plasma membrane. However, we demonstrate here that some steroid hormones are taken up by receptor-mediated endocytosis of steroid-carrier complexes. We show that 25-(OH) vitamin D3 in complex with its plasma carrier, the vitamin D-binding protein, is filtered through the glomerulus and reabsorbed in the proximal tubules by the endocytic receptor megalin. Endocytosis is required to preserve 25-(OH) vitamin D3 and to deliver to the cells the precursor for generation of 1,25-(OH)2 vitamin D3, a regulator of the calcium metabolism. Megalin-/- mice are unable to retrieve the steroid from the glomerular filtrate and develop vitamin D deficiency and bone disease.

Osteopathy and resistance to vitamin D toxicity in mice null for vitamin D binding protein

A line of mice deficient in vitamin D binding protein (DBP) was generated by targeted mutagenesis to establish a model for analysis of DBP's biological functions in vitamin D metabolism and action. On vitamin D-replete diets, DBP-/- mice had low levels of total serum vitamin D metabolites but were otherwise normal. When maintained on vitamin D-deficient diets for a brief period, the DBP-/-, but not DBP+/+, mice developed secondary hyperparathyroidism and the accompanying bone changes associated with vitamin D deficiency. DBP markedly prolonged the serum half-life of 25(OH)D and less dramatically prolonged the half-life of vitamin D by slowing its hepatic uptake and increasing the efficiency of its conversion to 25(OH)D in the liver. After an overload of vitamin D, DBP-/- mice were unexpectedly less susceptible to hypercalcemia and its toxic effects. Peak steady-state mRNA levels of the vitamin D-dependent calbindin-D9K gene were induced by 1,25(OH)2D more rapidly in the DBP-/- mice. Thus, the role of DBP is to maintain stable serum stores of vitamin D metabolites and modulate the rates of its bioavailability, activation, and end-organ responsiveness. These properties may have evolved to stabilize and maintain serum levels of vitamin D in environments with variable vitamin D availability.

Plasma vitamin D-binding protein (Gc-globulin): multiple tasks

The transporter of vitamin D and its metabolites in blood has received increasing attention in recent years, and is recognized to be a member of a gene family that includes albumin and alpha-fetoprotein. Identical to the group specific component (Gc-globulin) of serum, the protein is a single-chain polypeptide constitutively synthesized in liver that circulates in amounts in far excess of normal vitamin D metabolite concentrations in blood. It plays the major role in the egress of endogenously synthesized vitamin D, from skin and appears to restrain D-sterols from too rapid/excessive cell entry. Along with plasma gelsolin, it comprises the plasma actin-scavenger system that facilitates removal of actin, liberated from lysed cells, by depolymerization and prevention of polymerization. Recently, the protein has been shown to behave as a co-chemotaxin specific for the complement peptide C5a, and its sialic acid-free form has been reported to play a role in macrophage activation. The latter functions strongly implicate its participation in inflammation responses. A unifying hypothesis might also suggest the protein to provide focal D-sterol delivery to cells that are important to the resolution of tissue injuries.

Proteinuria of B700, a 67 kD albumin-like melanoma-specific antigen

B700 is a murine melanoma antigen that is closely related to, but distinct from, serum albumin. The present study examined the metabolic fate and anatomic distribution of radioiodinated B700 and mouse serum albumin (MSA) administered s.c. to mice. In blood, both proteins were associated with the plasma fraction where the halflife of B700, a glycoprotein, was 0.5 days, compared to 2.7 days for MSA. Of particular interest was the observation that B700, a 67 kD anionic protein, was excreted primarily in urine. The selective B700-proteinuria did not alter urinary volumes or produce hematuria or edema. SDS-polyacrylamide gel electrophoresis and western blot analysis using the H-2-3-3 B700-specific monoclonal antibody revealed that B700 proteinuria occurred in B-16 murine melanoma bearing animals but not in control mice. These studies demonstrate that the tumor-bearing host readily distinguishes between very similar normal protein (MSA) and tumor-associated antigen (B700) molecules and processes them differently.

The uptake and metabolism of 25-hydroxyvitamin D3 and vitamin D binding protein by cultured porcine kidney cells (LLC-PK1)

1. Uptake of 3H-25OHD3, 3H-25OHD3-DBP, 125I-holo-DBP and 125I-apo-DBP by LLC-PK1 cells was linearly related to the concentration of each in the culture media. The presence of DBP in the medium significantly reduced the amount of 3H-25OHD3 taken up by cells. 2. Free 25OHD3 and 25OHD3 bound to DBP were both metabolized by the cells to 24,25(OH)2D3 and an unidentified product of apparent lower polarity than 25OHD3. 3. A significant amount of DBP taken up by the LLC-PK1 cells was metabolized to a TCA-soluble form. 4. Uptake of DBP was similar to horseradish peroxidase, but higher than inulin, indicative of a non-specific endocytic mechanism with an adsorptive component. 5. It is suggested that both free circulating 25OHD3 and that derived from lysosomal degradation of 25OHD3-DBP are available for hydroxylation by the kidney.

Comparison of the metabolic fate and tissue distribution of B700, an albumin-like melanoma-specific antigen with serum albumin in normal and tumor-bearing mice

1. B700, a murine melanoma antigen, is a member of the serum albumin protein family, being closely related to murine serum albumin (MSA). 2. We have studied and compared the metabolic fate and anatomic distribution of radioiodinated B700 and MSA administered to semisyngeneic naive and tumor-bearing mice. 3. Labelled material from both proteins is excreted primarily into urine. 4. The rate of excretion of the two proteins is markedly different, with B700 having a shorter half-life in the body. 5. Despite their similar molecular weights, intact B700 represents approx. 30% of the radioactivity in the urine but only 4% of the MSA in the urine is intact. 6. These studies demonstrate that the host can readily distinguish between very similar normal (MSA) and tumor-associated (B700) molecules and process them differently. 7. Similar findings of differential fate and distribution have been reported in comparing other albuminoid molecules [Dueland S., Blomhoff R. and Pedersen J. I. (1990) Biochem. J. 267, 721-725].

Angiopathic consequences of saturating the plasma scavenger system for actin

Two plasma proteins, vitamin D-binding protein (actin monomer sequestrant) and gelsolin (actin polymer severing), have been found in association with actin in plasma from ill humans and during experimental injury. In vitro, these are the only plasma proteins that display a high affinity for actin. We infused increasing amounts of globular actin intravenously to rats to evaluate its disposition in plasma and tissues. Intravascular filament formation, microthrombi, and endothelial injury were observed, especially in the pulmonary circulation. These pathological changes were not observed when the globular actin in the infusate had been preincubated with the vitamin D-binding protein in vitro. Complexes of actin with both proteins were found in the plasma, suggesting a saturable, plasma actin-binding system in vivo. Our findings suggest that in vivo saturation of these proteins' actin-binding capacities may serve as a paradigm for pulmonary vascular disorders seen during widespread tissue trauma and cell lysis.

Depression of gelsolin levels and detection of gelsolin-actin complexes in plasma of patients with acute lung injury

Actin is the major protein of muscle and nonmuscle cells and is one of the most abundant body proteins. Physiologic or pathologic cell death may therefore result in the liberation of large amounts of this fibrous protein into the extravascular space. The potential for long actin filaments to increase plasma viscosity and change the rheology of the microvasculature are potentially obviated by the presence of 2 recently recognized plasma actin-binding proteins, vitamin-D-binding protein, and plasma gelsolin. As part of our initial evaluation of this newly recognized physiologic system in humans, we measured levels of gelsolin in plasma samples from patients with extensive lung injury. Gelsolin levels were depressed in 25 of 25 patients with the adult respiratory distress syndrome (ARDS), a disease characterized by massive cellular injury, as determined by either of 2 functional assays for gelsolin. Mean total gelsolin concentration of 20 patients with ARDS was 89.2 +/- 33 micrograms/ml (normal levels, approximately 240 micrograms/ml; p less than 0.001) and the mean free gelsolin concentration 69.6 +/- 29 micrograms/ml (normal levels, approximately 240 micrograms/ml; p less than 0.001). Gelsolin concentrations of 6 patients with bacterial pneumonias were also depressed, but to a lesser degree (mean total level, 117 +/- 21 micrograms/ml). Direct demonstration of the presence of actin in these plasmas (but not in normal plasmas) was performed by precipitating actin directly with DNase-Sepharose beads, or indirectly with antigelsolin-Sepharose beads, as confirmed with immunoblotting. Actin was found in 18 of 19 patients using DNase beads and in 7 of 19 using antigelsolin beads.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of group-specific component (vitamin D binding protein) in clearance of actin from the circulation in the rabbit

The possible role of group specific component (Gc) (vitamin D-binding protein) in the clearance of cellular actin entering the circulation was examined with 125I-labeled Gc and actin injected into a rabbit model. Although filamentous F-actin is depolymerized primarily by plasma gelsolin, greater than or equal to 90% 125I-actin injected in either monomeric G- or F-form became complexed eventually with Gc (1:1 molar ratio). Clearance of Gc complexes was much faster (greater than 90% within 5 h) than that of native Gc (t1/2 = 17.2 h). Nephrectomy did not significantly alter the clearance of either Gc or actin. Since Gc complexes are dramatically increased in situations of tissue necrosis such as in fulminant hepatic failure, the current results suggest a crucial role for Gc in sequestration and clearance of released cellular actin.

The effects of chronic vitamin D deficiency on the skeleton in the adult rabbit

Albino rabbits were fed a 1.0% Ca, 0.5% P, vitamin D-deficient diet for 11.7 to 31.3 mo. Control rabbits were fed either this diet with the addition of 2.2 units/gm of vitamin D3 or a standard laboratory rabbit ration. Serum levels of 25-OH-D and 1,25-(OH)2D were both undetectable in all vitamin D-deficient rabbits but were present at levels typically found in other species in the control rabbits. Vitamin D deficiency resulted in elevated serum PTH values but did not produce significant changes in serum Ca levels, femur length, femur ash weight to body weight ratio, or tibial breaking strength. The vitamin D-deficient rabbits could be readily separated into two distinct subgroups. Four of these rabbits were normophosphatemic (P = 3.7 +/- 0.4 mg/dl) whereas the other five were severely hypophosphatemic (P = 0.8 +/- 0.2 mg/dl). During the last 10 days of the study the control and normophosphatemic vitamin D-deficient rabbits were in positive Ca and zero P balance. The hypophosphatemic vitamin D-deficient rabbits were in zero Ca and negative P balance. This negative P balance resulted from a net intestinal secretion, as urinary P excretion was negligible. Femur ash weight as a percentage of dry weight was decreased in hypophosphatemic but not the normophosphatemic vitamin D-deficient rabbits. Histomorphometric analyses indicated the bones from the normophosphatemic vitamin D-deficient rabbits were normal. In contrast, vertebral trabecular bone from the hypophosphatemic rabbits contained large amounts of osteoid that was not mineralizing, as indicated by a failure to take up the fluorescent label calcein.(ABSTRACT TRUNCATED AT 250 WORDS)

Altered conformation of Gc (vitamin D-binding protein) upon complexing with cellular actin

Complexing of serum Gc (Vitamin D-binding protein) with cellular actin can occur in the extracellular space as a result of cell turnover, and particularly cell necrosis. The clearance of such complexes is significantly more rapid than that of Gc alone, and several tissues are involved in their uptake, but the mechanisms involved are unknown. We present evidence here that interaction with actin results in alteration of certain physicochemical properties of Gc. Fluorescence of the hydrophobic probe 2-p-toluidinylnaphthylene-6-sulfonate was abolished by complex formation with actin. In addition, isoelectric focusing of complexes between Gc, and actin from different tissues, revealed that complexes were generally more acidic than either protein individually. These findings indicate that complexing of Gc with actin results in altered conformation.

Vitamin D binding protein sequesters monomeric actin in the circulation of the rat

Plasma vitamin D binding protein (DBP) may scavenge actin released during cell lysis. We examined the plasma disappearance and tissue appearance of 125I-DBP, 125I-G-actin, and the DBP-G-actin complex after their intravenous administration to rats. The plasma disappearance of DBP and DBP-actin were indistinguishable, with rapid initial (t1/2 = 2.6 h) and slower second (t1/2 = 7 h) slopes. After 125I-G-actin (nanomole) injection, plasma disappearance paralleled that of DBP and DBP-actin. All injected actin was associated with DBP, without evidence of free actin, actin-gelsolin complexes or actin oligomers. Tissue appearances of 125I-apo-DBP (apo) or holo-DBP were similar, with highest accumulations in perfused liver, kidney, and skeletal muscle. Although more complex phenomena (plasma entry of F-actin and intracellular actin binding proteins) would occur in vivo after cell lysis, our results suggest a role for DBP in the sequestration and disposition of actin monomers in the circulation.

Role of plasma gelsolin and the vitamin D-binding protein in clearing actin from the circulation

We determined the plasma kinetics of both actin and complexes of actin with the two high affinity actin-binding proteins of plasma, gelsolin, and vitamin D-binding protein (DBP). Actin is cleared rapidly from the plasma by the liver (half-disappearance time, 0.5 h). Using radiolabeled actin-binding proteins, we found that actin accelerated the clearance of both plasma gelsolin and the vitamin D-binding protein. In separate experiments we found that DBP-actin complexes were cleared more quickly than gelsolin-actin complexes, at a rate comparable to the clearance of actin from the blood. A low affinity interaction (dissociation constant, 2.9 X 10(-4) M) between actin and fibronectin was found, suggesting that little actin will bind to fibronectin in plasma. We conclude that while plasma gelsolin and DBP may both clear actin from the circulation, DBP appears to play a more important role. By so doing, DBP may conserve the filament-severing activity of plasma gelsolin.

Binding protein for vitamin D and its metabolites in rat mesenteric lymph

A protein with high affinity for vitamin D3 and 25-hydroxyvitamin D3 in rat mesenteric lymph has been studied. Mesenteric lymph was collected after duodenal instillation of radiolabeled vitamin D3 and 25-hydroxyvitamin D3. As previously described, approximately 10% of vitamin D3 (S. Dueland, J.I. Pedersen, P. Helgerud, and C.A. Drevon, J. Biol. Chem. 257: 146-150, 1982) and 95% of 25-hydroxyvitamin D3 (S. Dueland, J.I. Pedersen, P. Helgerud, and C.A. Drevon, Am. J. Physiol. 245 (Endocrinol. Metab. 8): E463-E467, 1983) recovered in mesenteric lymph were associated with the alpha-globulin fractions. The radioactive vitamin D3 recovered in the lymph fraction with d greater than 1.006 (free of chylomicrons) coeluted with purified rat serum binding protein for vitamin D and its metabolites (DBP) from an antirat DBP column. The results obtained by immunoblotting after sodium dodecyl sulfate polyacrylamide gel electrophoresis showed that this protein in mesenteric lymph had molecular weight and immunological properties identical with purified serum DBP. Purified serum DBP labeled with 125I was injected intravenously and mesenteric lymph was collected. in lymph, suggesting that DBP may be transferred from blood to mesenteric lymph and that plasma and lymph DBP may have a similar origin.

Selective, rapid removal of the vitamin D-binding protein and its sterol ligands from human and bovine plasma

Rapid and selective removal of plasma vitamin D-binding protein was effected by the serial passage of plasma over four columns of agarose containing covalently linked skeletal muscle G-actin. By maintaining an actin-to-binding protein molar ratio of at least 4 to 1 throughout, greater than 99% of the binding protein was removed from the fourth column's eluate. In contrast, 87% of the total plasma or serum protein applied was recovered, and electrophoretic analyses of human and bovine sera that had undergone these affinity chromatography steps revealed no major alterations in protein distribution. The procedure also removes vitamin D sterols selectively, with preference for 25-hydroxycalciferol (90% removal) over 1,25-dihydroxycalciferol (65-70% removal) and calciferol (70% removal), in accordance with the known affinity displayed by the binding protein for these sterol ligands. Recovery of other serum constituents (cortisol, proteins, peptide hormones, calcium and alkaline phosphatase) was excellent, further confirming the selectivity of the technique. Utilizing vitamin D-deficient serum, serum depleted of the vitamin D-binding protein was not distinguishable from control serum in supporting the growth of human fibroblasts in vitro. In comparison with other methods to remove serum-binding protein or sterols, the present technique is more selective and can be used for mammalian and avian sera. Material so prepared could prove useful for studies of the cellular access, metabolism, and effects of vitamin D sterols in vitro.

Actin affinity chromatography in the purification of human, avian and other mammalian plasma proteins binding vitamin D and its metabolites (Gc globulins)

The human plasma protein binding vitamin D and its metabolites (Gc globulin; group-specific component) has been isolated from human plasma by column affinity chromatography on gels to which monomeric actin was covalently attached. Rabbit skeletal-muscle G-actin was covalently coupled to amino-agarose gels before the application of human plasma. At actin/protein molar ratios of 4-8:1, excellent recovery (approximately 58%) of purified binding protein was achieved. After 0.75 M-NaCl washes, the binding protein was eluted from the columns in 3 M-guanidinium chloride, dialysed and analysed. These eluates contained the binding protein as 34-100% of the total protein, reflecting a 130-fold average purification in this single step. In the presence of Ca2+, gelsolin (another plasma protein that binds actin) was apparently retained by the affinity column, but this was prevented by chelation of plasma Ca2+. The actin affinity step also was effective in the isolation of the binding protein from rat, rabbit and chicken plasma, as indicated by autoradiographs of purified fractions analysed by gel electrophoresis after incubation with 25-hydroxy[26,27-3H]cholecalciferol. Further isolation by hydroxyapatite chromatography yielded a purified binding protein which displayed characteristic binding activity toward vitamin D metabolites and G-actin, and retained its physicochemical features. This brief purification sequence is relatively simple and efficient, and should prove to be useful to investigators studying this interesting plasma protein.

The relationship of adequate and excessive intake of vitamin D to health and disease

Vitamin D is required for intestinal calcium and phosphorus absorption, kidney functions, bone synthesis, and possible vital functions in several other target tissues. Active forms of vitamin D are generated by hydroxylations, first in the liver and second in the kidney. The requirement for vitamin D is normally met by its synthesis in the skin. Inadequate skin exposure to sunlight, dark-pigmented skin, and a northern geographical latitude make some individuals susceptible to a deficiency. In the United States, various foods are fortified with vitamin D to ensure that such deficiencies do not occur. As a result, most individuals consume and synthesize more vitamin D than they require. Vitamin D is a toxic compound, and excessive amounts can cause soft-tissue calcification. We have suggested a mechanism by which this calcification might occur. There is a narrow leeway between the amount required and that initiating tissue damage. As most individuals appear to be at risk of obtaining too much vitamin D rather than too little, we suggest that fortification of foods with vitamin D should be curtailed, preferably abolished, that excessive fortification of animal foods be reduced to the level required, and that the use of dietary supplements be restricted. Populations at risk could be monitored closely and counseled to prevent vitamin D deficiency.

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.