Immunological and immunoassay studies of the binding protein for vitamin D and its metabolites in human serum

Behind the scenes of vitamin D binding protein: more than vitamin D binding

Although being discovered in 1959, the number of published papers in recent years reveals that vitamin D binding protein (DBP), a member of the albuminoid superfamily, is a hot research topic. Besides the three major phenotypes (DBP1F, DBP1S and DBP2), more than 120 unique variants have been described of this polymorphic protein. The presence of DBP has been demonstrated in different body fluids (serum, urine, breast milk, ascitic fluid, cerebrospinal fluid, saliva and seminal fluid) and organs (brain, heart, lungs, kidneys, placenta, spleen, testes and uterus). Although the major function is binding, solubilization and transport of vitamin D and its metabolites, the name of this glycoprotein hides numerous other important biological functions. In this review, we will focus on the analytical aspects of the determination of DBP and discuss in detail the multifunctional capacity [actin scavenging, binding of fatty acids, chemotaxis, binding of endotoxins, influence on T cell response and influence of vitamin D binding protein-macrophage activating factor (DBP-MAF) on bone metabolism and cancer] of this abundant plasma protein.

Vitamin D binding protein: a multifunctional protein of clinical importance

Since the discovery of group-specific component and its polymorphism by Hirschfeld in 1959, research has put spotlight on this multifunctional transport protein (vitamin D binding protein, DBP). Besides the transport of vitamin D metabolites, DBP is a plasma glycoprotein with many important functions, including sequestration of actin, modulation of immune and inflammatory responses, binding of fatty acids, and control of bone development. A considerable DBP polymorphism has been described with a specific allele distribution in different geographic area. Multiple studies have shed light on the interesting relationship between polymorphisms of the DBP gene and the susceptibility to diseases. In this review, we give an overview of the multifunctional character of DBP and describe the clinical importance of DBP and its polymorphisms. Finally, we discuss the possibilities to use DBP as a novel therapeutic agent.

Gc globulin (vitamin D‐binding protein) levels: an inhibition ELISA assay for determination of the total concentration of Gc globulin in plasma and serum

Gc globulin, also called vitamin D-binding protein, is a plasma protein involved in the actin-scavenger system. In this study, the total Gc globulin concentration in serum or plasma samples was determined using a new, fast, solid-phase inhibition assay. Included in the study were 228 healthy volunteers (131 M, 97 F), 22 pregnant women, 90 cancer patients and 9 patients with chronic liver disease. Moreover, the degree of complexing with actin was determined in selected samples using crossed immunoelectrophoresis. The Gc globulin level in healthy controls was in the range 176-623 mg/L, showing no age dependency. The median level was found to be significantly higher in women than in men. Gc globulin concentrations were raised during pregnancy, showing a median value of 541 mg/L in the first trimester, and slightly raised to 574 mg/L in the second trimester. Cancer patients showed no changes in Gc globulin level, and there was no sign of increased amounts of complexing with actin. Chronic liver patients showed increased levels of Gc globulin following transplantation, but no signs of complexing with actin. This new solid-phase inhibition assay is fast, it is a good complement to the existing quantification methods, and it is especially suitable for determination of the Gc globulin status in acute liver patients before and during treatment.

Polymorphic post-albumin of cattle and horse plasma identified as vitamin D binding protein (Gc protein)

Cattle and horse plasma samples of known post-albumin types were radiolabelled with 14C-vitamin D3. These samples were then analysed by polyacrylamide gel electrophoresis, followed by autoradiography. The patterns observed were identical to those of post-albumin variants. The polymorphic post-albumin protein of cattle and horse was thus identified as the vitamin D binding protein and homologous to the Gc protein of human plasma.

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.

Vitamin D metabolism in a frugivorous nocturnal mammal, the Egyptian fruit bat (Rousettus aegyptiacus)

The nocturnal, frugivorous Egyptian fruit bat (Rousettus aegyptiacus) has no obvious access to either endogenous or dietary sources of vitamin D. We hypothesized that this species under natural conditions would be vitamin D deficient and that both serum mineral concentrations and vitamin D metabolite concentrations would be low. Both wild populations and captive populations appear to have an impoverished vitamin D status, as concentrations of the principle circulating metabolite, 25-hydroxyvitamin D [25(OH)D] are undetectable (<4 ng/mL) and those of the active metabolite, 1,25-dihydroxyvitamin D [1,25(OH)(2)D] are low. Intraperitoneal administration of labelled 25(OH)D revealed enhanced 1 alpha-hydroxylase activity confirming a natural state of vitamin D deficiency. This may account for the undetectable levels of 25(OH)D; for limited amounts of the prohormone substrate are rapidly converted to the active hormone. Both vitamin D(2) and D(3) metabolites were detected in bat serum, albeit in very small amounts, inferring that in their natural habitat fruit bats may have limited access to both exogenous dietary sources and endogenous sources. Despite the low levels of vitamin D metabolites in wild-caught and captive D-unsupplemented individuals, serum mineral concentrations were well regulated and similar to those of bats receiving D-supplements, with no pathological problems associated with vitamin D deficiency evident.

Measurement of vitamin D-binding protein in pleural fluids and sera by means of a turbidimetric immunoassay measuring system

BACKGROUND

Vitamin D-binding protein (DBP) has been recognized as a multifunctional plasma protein that can modulate certain immune and inflammatory responses. There may be differences between the DBP concentrations in pleural fluids from various diseases involving a variety of possible responses in the pleural cavity.

METHODS

An anti-DBP polyclonal antibody was prepared using commercially available DBP to establish a quantitative measuring system for DBP. With a rabbit antibody, a turbidimetric immunoassay (TIA) was developed for DBP with an automatic analyzer. Using this measuring system, the concentrations of DBP were compared with the protein concentration in pleural fluid and serum specimens from patients with various diseases.

RESULTS

The fluid DBP concentrations in transudative (n=11) and exudative (n=41) effusions were 71.9+/-21.2 and 180.7+/-43.7 mg/l, respectively. Among the exudative effusions, the fluid DBP concentrations in the bacterial (n=10), tuberculous (n=13), and malignant (n=18) effusions were 218.8+/-37.3, 186.7+/-26.2, and 155.1+/-41.3 mg/l, respectively. The DBP fluid/serum ratio and the fluid DBP/protein ratio in bacterial effusions were significantly higher than those in tuberculous (p<0.005, p<0.05, respectively) and malignant effusions (p<0.0005, p<0.005, respectively), although no statistically significant differences in the serum DBP/protein ratio between those effusions were found.

CONCLUSIONS

Using the TIA assay, the DBP concentrations in bacterial pleural effusions were significantly higher than in tuberculous and malignant effusions.

Serum vitamin D metabolites in very low birth weight infants with and without rickets and fractures

Seventy-one very low birth weight (less than or equal to 1500 gm) infants were studied to determine the sequential changes in serum vitamin D metabolite concentrations between infants with and without radiographically documented rickets, fractures, or both (R/F). Usual intake of vitamin D included 20 IU/kg/day from parenteral nutrition or 400 IU/day supplementation with enteral feeding. Radiographs of both forearms and serum samples were obtained at 3, 6, 9, and 12 months. Twenty-two infants had R/F. At 3 months, significantly lower mean (+/- SEM) serum phosphorus levels (4.5 +/- 0.4 vs 6.1 +/- 0.2 mg/dl), higher 1,25-dihydroxyvitamin D (1,25-[OH]2D) concentrations (96 +/- 5 vs 77 +/- 4 pg/ml), and higher free 1,25-(OH)2D index (1,25-[OH]2D:vitamin D binding protein ratio; 5.2 +/- 0.3 x 10(5) vs 4.0 +/- 0.2 x 10(5] were found in the R/F group. These values returned to normal and were similar between groups on subsequent measurements. Serum calcium, magnesium, and 25-hydroxyvitamin D (25-OHD) concentrations were normal and similar between groups. In both groups, serum vitamin D binding concentrations increased initially but remained stable and normal beyond 6 months. We conclude that in very low birth weight infants with R/F, the vitamin D status (as indicated by serum 25-OHD concentrations) is normal, and that lowered serum phosphorus levels, higher serum 1,25-(OH)2D levels, and a higher free 1,25-(OH)2D index support the thesis that mineral deficiency (especially of phosphorus) may be important in the pathogenesis of R/F in small preterm infants.

Bone disease in chronic childhood cholestasis. I. Vitamin D absorption and metabolism

Metabolic bone disease is common in children and adults with chronic cholestasis. We evaluated baseline vitamin D (vitamin D2 and D3), 25-OH vitamin D2 and D3, 1,25(OH)2 vitamin D, vitamin D-binding protein, bone mineral content and dietary mineral content in six children (mean age: 12.1 years) with cholestasis since infancy. Absorption of 25-OH vitamin D3 and vitamin D2 was evaluated by measuring serial serum concentrations after a test dose. Bone mineral content was reduced by greater than 2 S.D. in five of six subjects compared to age-specific controls; none had radiographic evidence of rickets but all had osteopenia. Dietary Ca and P content in the subjects was comparable to the recommended daily allowance for age-specific children. Baseline serum vitamin D2 concentrations were undetectable in all but one cholestatic subject despite oral supplementation with 2,500 to 50,000 IU per day vitamin D2. Baseline serum 25-OH vitamin D was 33.2 +/- 6.0 ng per ml (mean +/- S.E.) and comparable to our laboratory norms (15 to 50 ng per ml). Serum 1,25(OH)2 vitamin D and "free" 1,25(OH)2 vitamin D were both significantly (p less than 0.05) reduced compared to controls. A significantly blunted rise and reduced area under the absorption curve (both p less than 0.001) after 1,000 IU per kg vitamin D2 was found in cholestatic children (0.8 ng +/- 0.5 ng per ml and 18.0 +/- 14.3 ng hr per ml, respectively) compared to controls (59.5 +/- 10.0 ng per ml and 1,780 +/- 253 ng hr per ml, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Serial ultraviolet B exposure and serum 25 hydroxyvitamin D response in young adult American blacks and whites: no racial differences

We tested the hypothesis that repeated whole body suberythemal ultraviolet B (UVB) exposure would result in less increase of serum 25-hydroxyvitamin D (25OHD) concentrations in black compared with white young adults with no significant change or racial differences in serum calciotropic hormones concentrations. Thirteen white and 7 black adults ranging from 22 to 35 years of age were submitted to sequential total body suberythemal doses of UVB (280-315 nm) biweekly for 6 weeks. Initial UVB dose was 5% below the minimal erythemal dose for the most sensitive skin, followed by 10% increase per exposure for 4 weeks. Blood samples were drawn weekly. Baseline 25OHD concentrations were significantly lower in blacks compared to whites, but the increases in serum 25OHD concentrations were similar in both groups; there were no significant differences by sex or age. Serum 24,25-dihydroxyvitamin D [24,25-(OH)2D] concentrations paralleled the serum 250HD response. Mean serum calcium (total and ionized), magnesium, phosphate, alkaline phosphatase, vitamin D binding protein, C-terminal parathyroid hormone, calcitonin, 1,25-dihydroxyvitamin D [1,25-(OH)2D], and osteocalcin concentrations did not differ between blacks and whites at any time. The ratio of the concentration of 1,25-(OH)2D to 25OHD in their serum was initially higher in blacks compared to whites (p less than 0.0001); the ratios decreased to levels similar to whites by the third UVB exposure. We conclude that, in blacks and whites, sequential suberythemal UVB exposure produces similar elevations of serum 25OHD concentrations and unchanged calciotropic hormones concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D-binding protein synthesized by a human hepatocellular carcinoma cell line

The binding protein for 25-hydroxycholecalciferol was studied in the medium spent for the culture of HuH-7 cells, which were originally derived from a human hepatocellular carcinoma tissue. The binding protein for 25-hydroxycholecalciferol synthesized by HuH-7 cells was immunologically similar to vitamin D-binding protein in human serum and had an inter-alpha mobility. A sedimentation coefficient of 4.1 S was found on sucrose density gradient analyses. The molecular weight was estimated to be approximately 58,000 by gel filtration on a standardized column of Sephadex G-150. When mixed with filamentous actin purified from rabbit skeletal muscle, it depolymerized filamentous actin and bound to monomeric, globular actin to make a 5.5 S 1:1 molar complex with a molecular weight of approximately 100,000. These results support the conclusion that HuH-7 cells produce a functional vitamin D-binding protein.

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.

Vitamin D requirement in infants receiving parenteral nutrition

The adequacy of low dose vitamin D (25 IU/dl) parenteral nutrition (PN) solution was studied in 18 infants. All infants had surgical indications for PN. The birth weights were 2810 +/- 135 g and gestational ages 37.4 +/- 0.5 wk (mean +/- SEM). Duration of study ranged from 5 to 175 days. Thirteen infants were studied for up to 6 weeks and five infants for 71 to 175 days. Results showed that studied infants maintained growth along normal percentiles for weight, length, and head circumference. Vitamin D status as indicated by serum 25 hydroxyvitamin D (25 OHD) rose from 15 +/- 1.9 ng/ml to 26 +/- 2.8 ng/ml, mean +/- SEM (p less than 0.001) after 9 days, and remained normal up to 6 months. Five infants with biochemical liver dysfunction also had normal serum 25 OHD concentrations, indicating the hepatic 25 hydroxylation process was not severely impaired. Serum total and ionized calcium, phosphorus, and vitamin D-binding protein concentrations were normal. Serum magnesium was mildly elevated in five infants (2.6 to 3 mg/dl) on one occasion and resolved spontaneously. Serum alkaline phosphatase (AP) concentrations rose above baseline values in 12 of 17 infants, but remained within normal range (less than 400 IU/liter at 30 degrees C). Another infant with markedly elevated AP values died from liver dysfunction. Radiographs of the forearms were normal except for marked demineralization in one infant in spite of normal 25 OHD concentrations. We conclude that 25 IU vitamin D/dl of nutrient infusate is adequate to maintain normal vitamin D status, as indicated by normal serum 25 OHD concentrations in infants receiving PN for as long as 6 months.

Protective effects of fibronectin in galactosamine-induced liver failure in rats

The effects of supplementation with fibronectin on liver damage and survival in rats with galactosamine-induced liver failure were studied. In rats with acute liver failure induced by a low dose of galactosamine, supplementation with purified plasma fibronectin at 3 hr after the administration of galactosamine provided significant increase of plasma fibronectin levels and augmentation of reticuloendothelial system function at 4 hr, significantly higher plasma fibronectin levels and significant protection of liver damage with shorter prothrombin times, lower AST and less histological damage at 48 hr as compared to control animals. Plasma fibronectin levels were inversely correlated with both plasma prothrombin times and AST. Fibronectin supplementation at 6 hr also resulted in the significant decrease of liver damage at 48 hr as evaluated histologically. When rats with liver failure, induced by a high dose of galactosamine, were supplemented with fibronectin at 3 hr, the survival rate was significantly higher than that of control rats. The results indicate that fibronectin supplementation in the early stages of acute liver failure could reduce liver damage and improve the survival of rats with galactosamine-induced liver failure.

Ethnic variation in vitamin D-binding protein (GC): a review of isoelectric focusing studies in human populations

Since the discovery in 1977 that the GC1 gene could be resolved into two common subcomponents on an isoelectric focusing (IEF) gel, a large number of ethnic groups have been screened to analyze the extent of genetic variation in human populations. Using the IEF technique, approximately 50,000 individuals from 160 different populations have been tested for the GC polymorphism. A marked variation in common GC suballele frequencies in different geographic areas seems to correlate with skin pigmentation and intensity of sun light. Pigmented (black) and keratinized (yellowish) skin type populations have a relatively high frequency of the GC*IF allele as compared to white skin populations. By comparison non-pigmented and non-keratinized white skin populations are generally characterized by having the maximum values of the GC*IS allele. The anthropologic significance of the GC locus has been enhanced further by detecting additional unique GC variants which provide useful information about evolutionary links between different populations. However, the presence of some electrophoretically identical unique variants in genetically and geographically distinct populations demand further investigation of these allelic variants to shed more light on their origins.

Chromatofocusing in the purification and separation of apo- and holo-(vitamin D-binding protein)

Chromatofocusing was used to purify the vitamin D-binding protein (DBP) from pig plasma in a procedure that consisted of an initial DEAE-cellulose chromatography followed by DEAE-Sephadex chromatography, with final purification by chromatofocusing. The protein was purified 184-fold over its concentration in plasma. When the plasma was labelled with a tracer concentration of [3H]calcidiol, it was apparent that holo- and apo-DBP did not co-chromatograph on chromatofocusing. The separation of these two forms of DBP on chromatofocusing was verified by using purified apo-DBP mixed with either a tracer or a saturating concentration of calcidiol. This separation was consistent with differences observed in their isoelectric points. The ability to separate apo and holo forms of DBP should permit the study of their specific interactions with other binding proteins and help determine the physiological relevance of these interactions.

Free 1,25-dihydroxyvitamin D levels in serum from normal subjects, pregnant subjects, and subjects with liver disease

We measured the free concentration of 1,25-dihydroxyvitamin D (1,25[OH]2D) using centrifugal ultrafiltration, and the level of vitamin D-binding protein (DBP) in 24 normal subjects, 17 pregnant subjects, and 25 alcoholic subjects with liver disease. Our objective was to determine whether the increase in total 1,25(OH)2D levels in pregnant women and the reduction in total 1,25(OH)2D levels in subjects with liver disease reflected a true difference in free 1,25(OH)2D levels or whether such differences were due solely to the variations in DBP levels (and thus, the amount of 1,25[OH]2D bound) in these groups. In subjects with liver disease the mean total 1,25(OH)2D concentration (22.6 +/- 12.5 pg/ml) and the mean DBP concentration (188 +/- 105 micrograms/dl) were nearly half the normal values (41.5 +/- 11.5 pg/ml and 404 +/- 124 micrograms/dl, respectively, P less than 0.001), whereas the mean free 1,25(OH)2D level was similar to normal values (209 +/- 91 fg/ml and 174 +/- 46 fg/ml, respectively). In contrast, in pregnant subjects the mean total 1,25(OH)2D level (82 +/- 21 pg/ml) and mean DBP level (576 +/- 128 micrograms/dl) were significantly higher than normal (P less than 0.001). Although the mean percent free 1,25(OH)2D level in pregnant subjects was below normal (0.359 +/- 0.07% vs. 0.424 +/- 0.07%, P less than 0.001), the mean free 1,25(OH)2D level was 69% higher than normal (294 +/- 98 fg/ml vs. 174 +/- 46 fg/ml, P less than 0.001). When data from all three groups were combined, there was a linear correlation between total 1,25(OH)2D and DBP levels but not between DBP and percent free 1,25(OH)2D levels; the increased DBP levels in the pregnant subjects were associated with less of an effect on percent free 1,25(OH)2D than were the reduced DBP levels in the subjects with liver disease. Our data suggest that (a) free 1,25(OH)2D levels appear to be well maintained even in subjects with liver disease and reduced DBP levels, (b) free 1,25(OH)2D levels are increased during pregnancy despite the increase in DBP levels, and (c) free 1,25(OH)2D levels cannot be inferred accurately from measurements of total 1,25(OH)2D and DBP levels alone in subjects with various physiologic and pathophysiologic conditions.

Phosphorus administration in patients with profound hypophosphatemia

The influence of severe hypophosphatemia (less than or equal to 1.0 mg/dl) on vitamin D metabolism was prospectively determined in 11 patients before and after intravenous phosphorus administration. Evidence of liver dysfunction was present in ten patients. The mean (+/- SE) plasma 25 hydroxycholecalciferol [25(OH)D] was significantly decreased before phosphorus therapy when compared to control subjects (9.4 +/- 1.3 vs. 17.8 +/- 1.3 ng/ml, P less than 0.001). With phosphorus administration, serum phosphorus increased from 0.59 +/- 0.07 to 2.58 +/- 0.09 mg/dl while 1,25 dihydroxycholecalciferol [1,25(OH)2D] decreased from 34.6 +/- 4.3 to 14.3 +/- 2.9 pg/ml (P less than 0.001). Plasma 25(OH)D, plasma immunoreactive PTH (both amino and carboxyterminal) and serum calcium did not change after phosphorus administration, suggesting that phosphorus alone was responsible for the change in plasma 1,25(OH)2D concentration. An inverse correlation was found between serum phosphorus and plasma 1,25(OH)2D (r = -0.62, P less than 0.005). In addition, a direct correlation was observed between plasma 25(OH)D and 1,25(OH)2D both before (r = 0.66, P less than 0.005) and after (r = 0.74, P less than 0.005) phosphorus administration. Thus, the decrease in 1,25(OH)2D levels with phosphorus therapy suggests a role of serum phosphate in the regulation of this sterol, and hypophosphatemia or phosphorus depletion may change the relationship of substrate [25(OH)D] to product [1,25(OH)2D].

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.

A simple and sensitive assay for 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D and 1,25-dihydroxyvitamin D in human serum

An improved method is described which permits the simultaneous determination of 25-hydroxyvitamin D [25-(OH)D], 24,25-dihydroxyvitamin D [24,25-(OH)2D] and 1,25-dihydroxyvitamin D [1,25-(OH)2D] in milliliters of human serum. Methodological improvements enabled a rapid and almost complete extraction of the three metabolites from serum and omission of adding labeled internal standards to each serum sample for the calculation of individual recoveries. Commercially available stable chick embryo intestinal mucosa cytosol preparation made the troublesome preparation of cytosol receptor for 1,25-(OH)2D unnecessary. The procedure involves saturation of serum with ammonium carbonate and extraction with methanol/ethyl acetate, followed by separation of 25-(OH) D from the dihydroxy metabolites of vitamin D by Sephadex LH-20 column chromatography and further separation of the dihydroxy metabolites into 24,25-(OH)2D and 1,25-(OH)2D by high-pressure liquid chromatography. This is followed by individual determination of each metabolite by competitive protein-binding assay or radioreceptor assay.

Vitamin A toxicity and hypercalcemia

A patient hospitalized with hypercalcemia and a history of chronic vitamin A ingestion was studied in order to investigate the rarely reported association between elevated serum calcium and vitamin A toxicity. The clinical presentation marked by profound weight loss, a psychiatric disturbance, total body alopecia, erosive dermatitis, and liver disease, was compatible with hypervitaminosis A. The diagnosis of vitamin A toxicity was established by elevated total vitamin A levels and the component due to retinyl esters. Other etiologies for hypercalcemia were excluded. In view of these results and the well-known effects of vitamin A on bone metabolism, it is concluded that the most likely etiology of the hypercalcemia in this patient was vitamin A toxicity.

Serum concentrations of vitamin D-binding protein (group-specific component) in cystic fibrosis

Vitamin D-binding protein (DBP) concentrations were determined in the sera of 90 cystic fibrosis homozygotes, 57 obligate heterozygotes, and 46 normal controls. Very significantly lower mean concentrations were found in the sera of CF homozygotes compared with both heterozygotes and controls (P less than 0.01, Wilcoxon Rank Sums Test). Subdivision of the samples by Gc phenotype showed that this relationship held true both in the Gc1 and Gc2-1 phenotypes. The small sample size of the Gc2 genotype makes the significance levels of limited usefulness, but the pattern of variation of DBP levels among CF homozygotes, heterozygotes, and controls was consistent with that observed for the Gc1 and Gc2-1 classes. Haptoglobin levels showed high coefficients of variation when compared among CF homozygotes, obligate heterozygotes, and controls, presumably because of nonspecific elevation in the acute-phase response. Alpha 2-macroglobulin levels were, if anything, slightly elevated in CF homozygotes compared with controls, while albumin levels showed no significant mean differences between these groups. Since the DBP concentration does not vary with age nor with levels of vitamin D and its metabolites, we interpret our results to mean that DBP levels are specifically decreased in cystic fibrosis, perhaps as the result of impaired glycosylation of the protein.

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

The metabolic disposition of the plasma binding protein (DBP) for vitamin D and its metabolites was studied in adult rabbits. Apo-DBP was purified from rabbit plasma and enzymatically labeled with radioiodine. The radioiodine-labeled protein retained its ability to bind vitamin D sterols and its physicochemical properties. When 125I-labeled DBP and 131I-labeled rabbit albumin were simultaneously injected intravenously, the 125I was cleared from plasma at a faster rate (t 1/2 = 1.7 d) than 131I (t 1/2 = 5 d) and 125I was present in excess of 131I in kidney, liver, skeletal muscle, heart, lung, intestine, testis, and bone 1 h after injection. In contrast to DBP, 25(OH)D3 was cleared more slowly (t 1/2 = 10.7 d). Compared to albumin, DBP radioactivity appeared earlier and in greater quantity in the urine of catheterized rabbits. Gel filtration analyses of plasma revealed most of the 125I to elute in the position of DBP, with only small amounts in the less than 1,000-dalton region. In contrast, almost all of the urine 125I eluted in this small molecular weight fraction. The molar ratio of DBP to 25(OH)D3 in normal rabbit plasma was 138/1. The extravascular pool of DBP was calculated to be 1.5-2.4 times larger than the intravascular DBP pool, and the molar replacement rate of DBP was 1,350-fold higher than that of 25(OH)D3. The plasma disappearance curves of holo-DBP, prepared either by saturating with 25(OH)D3 or by covalently linking 3 beta-bromoacetoxy-25(OH)D3, were very similar to that of apo-DBP. Neuraminidase treatment of DBP did not alter its plasma survival. These studies indicate that DBP or DBP-25(OH)D3 complex is removed from plasma by a variety of tissues, that the DBP moiety is degraded during this process, and that a significant recirculation of 25(OH)D3 probably occurs. The molar excess of DBP to 25(OH)D3 in plasma, and the relatively rapid turnover of DBP indicate that a high capacity, high affinity, and dynamic transport mechanism for vitamin D sterols exists in rabbit plasma.

Influence of the vitamin D-binding protein on the serum concentration of 1,25-dihydroxyvitamin D3. Significance of the free 1,25-dihydroxyvitamin D3 concentration

The influence of the serum binding protein (DBP) for vitamin D and its metabolites on the concentration of its main ligands, 25-hydroxyvitamin D(3) (25-OHD(3)) and 1,25-dihydroxyvitamin D(3) (1,25-[OH](2)D(3)) was studied. The concentration of both 1,25-(OH)(2)D(3) and DBP in normal female subjects (45+/-14 ng/liter and 333+/-58 mg/liter, mean+/-SD, respectively; n = 58) increased during the intake of estro-progestogens (69+/-27 ng/liter and 488+/-90 mg/liter, respectively; n = 29), whereas the 25-OHD(3) concentration remained unchanged. A positive correlation was found between the concentrations of 1,25-(OH)(2)D(3) and DBP in these women. At the end of pregnancy, the total concentrations of 1,25-(OH)(2)D(3) (97+/-26 ng/liter, n = 40) and DBP (616+/-84 mg/liter) are both significantly higher than in nonpregnant females and paired cord serum samples (48+/-11 ng/liter and 266+/-41 mg/liter, respectively). A marked seasonal variation of 25-OHD(3) was observed in pregnant females and their infants, whereas in the same samples the concentrations of both DBP and 1,25-(OH)(2)D(3) remained constant throughout the year. The free 1,25-(OH)(2)D(3) index, calculated as the molar ratio of this steroid and DBP, remains normal in women taking estro-progestogens, however, and this might explain their normal intestinal calcium absorption despite a high total 1,25-(OH)(2)D(3) concentration. In pregnancy the free 1,25-(OH)(2)D(3) index remains normal up to 35 wk of gestation, but during the last weeks of gestation, the free 1,25-(OH)(2)D(3) index increases in both circulations. A highly significant correlation exists between the (total and free) 25-OHD(3) and 1,25-(OH)(2)D(3) concentrations in maternal and cord serum both at 35 and 40 wk of gestation.

The vitamin D status of man at 70 degrees north

The overall vitamin D status of man at 70 degrees north has been evaluated by measuring the total 25-hydroxy-vitamin D (25-OHD) in serum samples from seventeen healthy adults from September 1977 to September 1978. The lowest 25-OHD-concentration was found in March (mean 51.5 nmol/l, SD = 19.5). This was significantly lower than in July and September 1977 and 1978 (P less than or equal to 0.0006). Of particular interest was the highly significant rise in the 25-OHD-concentration from March to July, most probably reflecting sun-induced endogenous synthesis in the skin. Despite the sun being below the horizon for two of the winter months, the 25-OHD-concentration remained at a constant and fairly high level (50-55 nmol/l).

Rickets with alopecia: an inborn error of vitamin D metabolism

Rickets with alopecia, an inborn error of vitamin D metabolism, is described in two sisters. The rachitic disorder began during the first year of life and was refractory to 50,000 IU of vitamin D2/day. Surprisingly, both children had marked elevations in serum concentrations of 1,25-(OH)2D. Although the molecular basis for this disorder is not evident to date, intestinal end-organ unresponsiveness to exceedingly high levels of 1,25-(OH)2D was present, in addition to hyporesponsiveness of bone to these high levels of the hormone, since normocalcemia was maintained despite elevated serum levels of PTH. Therapy with oral 1,25-(OH)2D3 failed to reverse the disorder, but oral phosphorus supplements resulted in significant radiographic and clinical improvement.

Quantitative studies of the interaction of cholecalciferol ((vitamin D3) and its metabolites with different genetic variants of the serum binding protein for these sterols

Cholecalciferol (vitamin D3) and its 25-hydroxy metabolite are transported in plasma bound to a specific protein, the binding protein for cholecalciferol and its metabolites (DBP). DBP is identical with the group-specific component (Gc) proteins, which are known to display genetic polymorphism. Studies were conducted to explore whether or not major differences in the transport of cholecalciferol and its biological metabolites might exist among persons with different Gc phenotypes. Detailed quantitative studies were first carried out on the interaction of 25(OH)D3 with DBP in 21 different samples of serum, representing eight different Gc phenotypes. The studies used a filter disc assay method that provided highly reproducible quantitative results with cholecalciferol-related sterols. The Gc phenotypes studied included the three common types (Gc 1-1, 2-1, and 2-2) and several uncommon genetic variants (Gc Ab-Ab, Ab-1, Ab-2, Chip-1, and Chip-2). The binding affinities for 25(OH)D3 observed with these different sera were all fairly similar to each other. More extensive studies were then conducted to compare the binding of four cholecalciferol-related sterols to each of three genetic variants of DBP, by using sera from homozygous persons with the Gc 1-1, Gc 2-2 and Gc Ab-Ab phenotypes. The ligands tested included cholecalciferol, 25(OH)D3, 1,25(OH)2D3, and 24(R) 25(OH)2D3. The affinities of the three genetic types of DBP/Gc protein were found to be similar for each of the four cholecalciferol-related sterols. The apparent association constants for 25(OH)D3 and 24,25(OH)2D3 were similar (approx. 1--2 x 10(8) M-1); lesser affinities were observed for 1,25(OH)2D3 (kA approx. 1 x 10(7) M-1) and for cholecalciferol (kA approx. 3--4 x 10(5) M-1). Thus the common genetic variants of DBP/Gc protein, and the uncommon genetic variants studied here, all appear to have similar binding properties for cholecalciferol and its several metabolites.

Plasma binding capacity for 25-hydroxy-vitamin D in the elderly

Plasma 25-hydroxy-vitamin D (25-OHD) concentrations and 25-OHD binding capacities were measured in 14 elderly patients and in 14 young controls. Both 25-OHD concentrations and 25-OHD binding capacities were reduced in the elderly. In neither group, however, was there a significant correlation between 25-OHD binding capacities and 25-OHD concentrations. Thus, although 25-OHD binding capacities are reduced in sick old people this does not account for the low 25-OHD concentrations often found in this group.

Detection of genetic variation with radioactive ligands. II. Genetic variants of vitamin D-labeled group-specific component (Gc) proteins

A novel technique for detecting electrophoretic and quantitative variants of group-specific component (Gc) proteins is described. The technique, in vitro labeling with radioactive vitamin D followed by polyacrylamide gel electrophoresis and autoradiography (PAGE autoradiography), permits sensitive, high resolution detection of Gc variants by virtue of a physiologically significant property: the ability of Gc to bind vitamin D and 25-hydroxyvitamin D. Using this procedure, anodal Gc variants, with mobility similar to Gc Aborigine and Gc Eskimo, were observed in Chinese, Japanese, African Pygmies, and American Blacks. The gene frequency of these variants ranges from 2.6% to 15%; they were not previously known to be polymorphic in these populations. In addition to qualitative variants, individual variation in Gc band density ratios is documented and discussed. These studies not only illustrate the utility of PAGE autoradiography in screening Gc, but also confirm that a major functional role of Gc in man and other animals is the transport of vitamin D and vitamin D metabolites.