Human serum binding protein for vitamin D and its metabolites. II. Specific, high affinity association with a protein in nucleated tissue

Significance of the Vitamin D Receptor on Crosstalk with Nuclear Receptors and Regulation of Enzymes and Transporters

The vitamin D receptor (VDR), in addition to other nuclear receptors, the pregnane X receptor (PXR) and constitutive androstane receptor (CAR), is involved in the regulation of enzymes, transporters and receptors, and therefore intimately affects drug disposition, tissue health, and the handling of endogenous and exogenous compounds. This review examines the role of 1α,25-dihydroxyvitamin D₃ or calcitriol, the natural VDR ligand, on activation of the VDR and its crosstalk with other nuclear receptors towards the regulation of enzymes and transporters, notably many of the cytochrome P450s including CYP3A4 and sulfotransferase 2A1 (SULT2A1) as well as cholesterol 7α-hydroxylase (CYP7A1). Moreover, the VDR upregulates the intestinal channel, TRPV6, for calcium absorption, LDL receptor-related protein 1 (LRP1) and receptor for advanced glycation end products (RAGE) in brain for β-amyloid peptide efflux and influx, the sodium phosphate transporters (NaP_i), the apical sodium-dependent bile acid transporter (ASBT) and organic solute transporters (OSTα-OSTβ) for bile acid absorption and efflux, respectively, the renal organic anion transporter 3 (OAT3) and several of the ATP-binding cassette protein transporters-the multidrug resistance protein 1 (MDR1) and the multidrug resistance-associated proteins (MRPs). Hence, the role of the VDR is increasingly being recognized for its therapeutic potential and pharmacologic activity, giving rise to drug-drug interactions (DDI). Therapeutically, ligand-activated VDR shows anti-inflammatory effects towards the suppression of inflammatory mediators, improves cognition by upregulating amyloid-beta (Aβ) peptide clearance in brain, and maintains phosphate, calcium, and parathyroid hormone (PTH) balance and kidney function and bone health, demonstrating the crucial roles of the VDR in disease progression and treatment of diseases.

Immunotherapy with GcMAF revisited - A critical overview of the research of Nobuto Yamamoto

This overview describes the research of Nobutu Yamamoto (Philadelphia) concerning immunotherapy with GcMAF for patients with cancer and for patients infected with pathogenic envelope viruses. GcMAF (Group-specific component Macrophage-Activating Factor) is a mammalian protein with an incredible potency to directly activate macrophages. Since the late 1980s Yamamoto's investigations were published in numerous journals but in order to understand the details of his research, a minute survey of many of his patents was required. But even then, regrettably, a precise description of his experiments was sometimes lacking. This overview tries to summarize all of Yamamoto's research on GcMAF, as well as some selected more recent papers from other investigators, who tried to verify and/or reproduce Yamamoto's reports. In my opinion the most important result of the GcMAF research deserves widespread renewed attention: human GcMAF injections (100 ng per week, intramuscular or intravenous) can help to cure patients with a great variety of cancers as well as patients infected with pathogenic envelope viruses like the human immunodeficiency virus 1 (HIV-1), influenza, measles and rubella (and maybe also SARS-CoV-2). From Yamamoto's data it can be calculated that GcMAF is a near-stoichiometric activator of macrophages. Yamamoto monitored the progress of his immunotherapy via the serum level of an enzyme called nagalase (α-N-acetylgalactosaminidase activity at pH 6). I have extensively discussed the properties and potential catalytic site of this enzyme activity in an Appendix entitled: "Search for the potential active site of the latent α-N-acetylgalactosaminidase activity in the glycoproteins of some envelope viruses".

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.

The Vitamin D Binding Protein and Inflammatory Injury: A Mediator or Sentinel of Tissue Damage?

Neutrophils are the most abundant type of white blood cell in most mammals including humans. The primary role of these cells is host defense against microbes and clearance of tissue debris in order to facilitate wound healing and tissue regeneration. The recruitment of neutrophils from blood into tissues is a key step in this process and is mediated by numerous different chemoattractants. The neutrophil migratory response is essential for host defense and survival, but excessive tissue accumulation of neutrophils is observed in many inflammatory disorders and strongly correlates with disease pathology. The vitamin D binding protein (DBP) is a circulating multifunctional plasma protein that can significantly enhance the chemotactic activity of neutrophil chemoattractants both in vitro and in vivo. Recent in vivo studies using DBP deficient mice showed that DBP plays a larger and more central role during inflammation since it induces selective recruitment of neutrophils, and this cofactor function is not restricted to C5a, as prior in vitro studies indicated, but can enhance chemotaxis to many chemoattractants. DBP also is an extracellular scavenger for actin released from damaged/dead cells and formation of DBP-actin complexes is an immediate host response to tissue injury. Recent in vitro evidence indicates that DBP bound to G-actin, and not free DBP, functions as an indirect but essential cofactor for neutrophil migration. DBP-actin complexes always will be formed regardless of what initiated an inflammation, since release of actin from damaged cells is a common feature in all types of injury and DBP is abundant and ubiquitous in all extracellular fluids. Indeed, these complexes have been detected in blood and tissue fluids from both humans and experimental animals following various forms of injury. The published data strongly supports the premise that DBP-actin complexes are the functional neutrophil chemotactic cofactor that enhances neutrophil chemotaxis in vitro and augments neutrophilic inflammation in vivo. This review will assess the fundamental role of DBP in neutrophilic inflammation and injury.

Synthesis, characterization and serum albumin binding studies of vitamin K3 derivatives

Synthesis, characterization and bovine serum albumin (BSA) binding properties of three derivatives of vitamin K3 have been described. Results of UV-Vis and fluorescence spectra indicate complexation between BSA and the ligands with conformational changes in protein, which is strongly supported by synchronous and three dimensional fluorescence studies. Addition of the ligands quenches the fluorescence of BSA which is accompanied by reduction in quantum yield (Ф) from 0.1010 to 0.0775-0.0986 range. Thermodynamic investigations reveal that hydrophobic interaction is the major binding force in the spontaneous binding of these ligands with BSA. The binding constants obtained depend on the substituent present in the quinone ring, which correlates linearly with the Taft's field substituent constant (σ_F). The results show that compound with strong electron withdrawing nitro-group forms relatively stronger complex with BSA than amino and thioglycolate substituted ones. Circular dichroism studies show that the α-helical content of the protein, upon complexation with the ligands, decreases in the case of amino and nitro substituted vitamin K3 while increases in thioglycolate substituted compound. Molecular docking studies indicated that the vitamin K3 derivatives are surrounded by hydrophobic residues of the BSA molecule, which is in good agreement with the results of fluorescence spectral and thermodynamic studies.

Gender differences in 1,25 dihydroxyvitamin D3 immunomodulatory effects in multiple sclerosis patients and healthy subjects

Vitamin D(3) is best known as a calcium homeostasis modulator; however, it also has immune-modulating potential. In this study, we demonstrated that immunomodulatory effects of vitamin D(3) are significantly stronger in females than in males in multiple sclerosis patients, as well as in healthy subjects. Inhibition of self-reactive T cell proliferation and reduction in IFN-γ- and IL-17-secreting cell numbers were considerably greater in females. Furthermore, the increase in IL-10-secreting and CD4(+)CD25(+)FoxP3(+) regulatory T cell numbers were also greater in females. In parallel with these findings, female subjects had fewer CYP24A1 transcripts encoding the 1,25-dihydroxyvitamin D(3)-inactivating enzyme, as well as greater binding and internalization of vitamin D(3)-binding protein, a transporter for vitamin D(3) and its metabolites. These gender-based disparities lead to the accumulation of vitamin D(3) and its metabolites in target cells from female subjects and result in a more potent anti-inflammatory effect. Interestingly, 17-β estradiol reproduced these effects on self-reactive T cells and macrophages from male subjects, suggesting a functional synergy between 1,25-dihydroxyvitamin D(3) and 17-β estradiol, mediated through estrogen receptor α. Collectively, these results demonstrate estrogen-promoted differences in vitamin D(3) metabolism, suggesting a greater protective effect of vitamin D(3)-based therapeutic strategies in women.

Effects of calcitriol on the immune system: new possibilities in the treatment of glomerulonephritis

1. 1,25-Dihydroxyvitamin D3 (1,25(OH)2D3), the hormonal form of vitamin D, is widely appreciated to play a central role in calcium and phosphate homeostasis. However, it is becoming increasingly clear that the sterol also play an important role in the regulation of cellular growth, haematopoietic tissues and the immune system, as well as in the modulation of hormone secretion by several endocrine glands. 2. In the present review, some of the mechanisms by which 1,25(OH)2D3 regulates immune function are highlighted. Moreover, a number of studies on the effects of calcitriol in several experimental animal models of renal disease are reported, suggesting new possibilities in the therapy of glomerulonephritis.

Endocytosis of three serum proteins of a multigene family and of arachidonic acid in human lectin-stimulated T lymphocytes

Alpha-fetoprotein (AFP), serum albumin (SA), and vitamin D binding protein (DBP) are members of a multigene family of proteins showing high structural homology. AFP and SA exhibit a reciprocal relation during development and carry mostly fatty acids, while DBP carries vitamin D and its metabolites in the plasma. Covalent conjugates of these proteins with horseradish peroxidase (HRP) were used to follow by cytochemistry, at the electron microscope level, the protein uptake and intracellular pathways in peripheral blood human lymphocytes stimulated to blast formation by phytohemagglutinin (PHA). Transferrin (Tf), an iron-binding plasma protein, was used as a control. Combined with the results of competition and saturability experiments reported elsewhere, the ultrastructural observations are in favour of a specific endocytosis of the four proteins through cell surface receptors. Tf and AFP enter the cells via small vesicles and endosomes and move to multivesicular bodies (MVBs) and tubular vesicular elements located in the Golgi-centrosphere region to be finally recycled back into the medium. A noncovalent conjugate of AFP-HRP with 3H arachidonic acid [3H-(20:4)] is strongly internalized at 37 degrees C in PHA-stimulated lymphocytes; the autoradiographic labelling, localized in cellular membranes and mostly in lipid droplets, was only occasionally associated with organelles where the presence of AFP-HRP was cytochemically detected. SA, which competes with AFP for a common binding site on the surface of activated T cells, is endocytosed through small vesicles, endosomes, and MVBs before being released in a degraded form from the cells, in agreement with the localization of SA-HRP in lysosome-like organelles. DBP-HRP is poorly internalized through noncoated vesicles, endosomes, and MVBs and is finally routed to lysosomes. The physiological role of AFP and SA would be to mediate the transfer of fatty acids into cells, while that of DBP would be to facilitate the intracellular delivery of vitamin D.

Comparative affinity of the major genetic variants of human group-specific component (vitamin D-binding protein) for 25-(OH) vitamin D

Binding studies with [3H] 25-(OH) D3 were performed under a variety of conditions using Gc (Vitamin D-binding protein) purified from individuals displaying different phenotypes. No significant differences in affinity of binding were found between Gc1f, Gc1s and Gc2 allelle products in either homozygous or heterozygous individuals, nor between Gc1 anodal and Gc1 cathodal isotypes. Affinity was not significantly affected by different reaction temperatures (4, 22 or 37 degrees C), the presence or absence of Ca2+ ions, and binary and ternary interactions with G-actin and G-actin-DNase complexes respectively. However, reduction of pH caused a progressive decrease in binding with virtual abolition at pH less than or equal to 5.0. The latter might promote dissociation of D3 metabolites from Gc carrier protein in acidic compartments of cells.

On the role of vitamin D binding globulin in glucose homeostasis: results from the San Luis Valley Diabetes Study

Several studies have reported association between noninsulin-dependent diabetes mellitus and GC, the vitamin D binding protein of human plasma, with the GC 1 allele in significant excess among diabetics. Additionally, there is a considerable body of animal data suggesting that vitamin D has a significant impact on insulin secretion. Examination of the insulin levels in Dogrib Indians showed that the lowest levels of fasting insulin were associated with the GC IF-IF genotype. The present study examined levels of glucose, C-peptide, and insulin at fasting and 1 hr and 2 hr following a 75 g oral glucose challenge, in a population of Hispanic-Americans and Anglos in the San Luis Valley of southern Colorado. The sample comprised a total of 468 individuals with normal glucose tolerance. Of these, 289 were Anglos and 179 were Hispanic-Americans. An analysis of covariance was performed to determine the effect of the GC genotypes on mean levels of the primary variables--glucose, C-peptide, and insulin--and a secondary variable--insulinogenic index adjusting for the covariates age, body mass index (BMI), gender, and ethnicity. The analyses revealed that there is a significant difference in mean levels of glucose at fasting (F value = 2.46; P = 0.033) among the GC genotypes in the sample. Additionally, the differences in mean levels of 1 hr postprandial glucose among the GC genotypes although not significant at a 5% level, were significant at the 10% level. No other significant phenotypic effects were observed. These analyses are not in concordance with the results of an earlier study, where lower fasting insulin was associated with the GC 1F-1F genotype.

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.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

Purification of human serum vitamin D-binding protein by 25-hydroxyvitamin D3-Sepharose chromatography

25-Hydroxyvitamin D3-Sepharose was prepared by coupling 25-hydroxyvitamin D3-3 beta-(1,2-epoxypropyl)-ether to thio-activated Sepharose CL-6B, forming a protease-resistant linkage between the sterol and the matrix. Vitamin D-binding protein from human plasma was obtained 85-92% pure after ligand affinity chromatography. Subsequent hydroxylapatite chromatography provided homogeneous protein. The purified vitamin D-binding protein was fully active in regard to 25-hydroxyvitamin D3 and actin binding capabilities.

Direct regional assignment of the gene for vitamin D binding protein (Gc-globulin) to human chromosome 4q11-q13 and identification of an associated DNA polymorphism

Using a characterized human vitamin D binding protein (DBP) cDNA probe and a panel of rodent X human somatic cell hybrids, we established the chromosomal location of the structural gene for DBP on human chromosome 4. In situ hybridization of 3H-labeled DBP cDNA to human metaphase chromosomes confirmed this assignment and allowed regional localization to bands 4q11-4q13. A restriction fragment length polymorphism associated with the DBP gene that should prove useful in future linkage studies was identified with the enzyme BamHI.

The amino acid sequence of the NH2-terminal portion of rat and human vitamin D binding protein: evidence for a high degree of homology between rat and human vitamin D binding protein

We determined the amino terminal sequence of rat and human vitamin D binding protein (VDBP). The sequences of the two proteins are: Rat VDBP: LeuGluArgGlyArgAspTyrGluLysAspLysValCysGlnGluLeuSerThrLeuGlyLys Human VDBP: LeuGluArgGlyArgAspTyrGluLysAsnLysValCysLysGluPheSerHisLeuGlyLys AspAspPhe GluAspPhe There are 19 matches out of a total of 24 residues sequenced giving a percent match/length of 79.2%. Differences in the composition of the two proteins at residue 10, 14, 16, and 22 can be accounted for by single base changes in the the gene for the proteins. The difference (Thr----His) at residue 18 requires a change in two bases in the respective genes. We conclude that the sequence of the amino terminus of rat and human VDBP is similar with a high degree of homology between the two proteins. Vitamin D sterols, such as 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D3, and 25,26-dihydroxyvitamin D3, are bound with high affinity by a plasma alpha-globulin - VDBP, also known as group-specific component (Gc). Other vitamin D sterols, such as 1,25-dihydroxyvitamin D3 and vitamin D3 itself, are bound to this protein with a lesser affinity. VDBP also binds actin with high affinity. Its role in vitamin D physiology is unclear, although it may play a role in the bioavailability of different D sterols. Svasti et al. have shown that human group specific component (Gc) exists as different isoforms that have rapid or slow mobility on gel electrophoresis. The different human Gc isoforms have similar NH2-terminal and COOH-terminal amino acid sequences. The difference in the mobility of the various isoforms is due to post-translational modification of the protein by various carbohydrate residues; treatment of the protein with neuraminidase results in the conversion of the different isoforms to a single isoform. The amino acid sequence of the amino terminus of rat VDBP is not known. Recently, Cooke reported preliminary data from the analysis of cDNA clones showing that rat and human VDBP are partially homologous and that rat and human VDBP exhibit homology with rat and human albumin and alpha-fetoprotein. The NH2-terminal sequence of the rat VDBP, however, has not been reported. In order to learn more about the nature of the NH2-terminal sequence of human and rat VDBP, we isolated these proteins in relatively pure form and determined the NH2-terminal amino acid sequence of both of them.(ABSTRACT TRUNCATED AT 400 WORDS)

Gc (vitamin D-binding protein) binds the 33.5 K tryptic fragment of actin

Limited proteolysis of G-actin was performed with trypsin and chymotrypsin to compare the binding sites for Gc and DNase. DNase I bound to the N-terminal area corresponding to the major cleavage site on G-actin (residues 62-68) and inhibited proteolysis, but did not bind the 33.5K C-terminal fragment (G-actin33.5) generated. In contrast, Gc did not exert any inhibitory effect upon proteolysis of the intact native G-actin42.0 molecule, although its presence protected G-actin33.5 from further proteolysis. This was shown by gel filtration to be due to the formation of complexes between Gc and G-actin33.5.

Accurate quantitation of native Gc in serum and estimation of endogenous Gc: G-actin complexes by rocket immunoelectrophoresis

Complex formation between purified Gc and G-actin caused increased rocket height on immunoelectrophoresis with monospecific Gc antiserum, and artifactually high calculated Gc levels. The increase in rocket height varied in log: linear fashion with the amount of G-actin present, up to a plateau attained at equimolarity. The raw Gc values could therefore be corrected to within +/- 10% of known levels by addition of excess G-actin and use of standard plots obtained with Gc after saturation with G-actin. This also allowed quantitation of the percentage of Gc complexed with G-actin. In subsequent studies of whole human sera, comparison of normal controls with pregnant subjects and patients with liver disease showed evidence of differences both in absolute quantities of Gc and the relative proportion circulating as complex with G-actin. This appeared to be due to increased release of cellular actin into the extracellular space. These results show that rocket immunoelectrophoresis can be modified to provide accurate Gc levels, and also information concerning different molecular forms of this protein.

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.

Gc (vitamin D binding protein) binds to cytoplasm of all human lymphocytes and is expressed on B-cell membranes

Peripheral blood lymphocytes were examined immunohistologically for evidence of interactions with Gc protein, a major vitamin D binding protein in serum. In the cytoplasm, binding sites for purified Gc were readily detectable in all cells, and these sites were only partially occupied by Gc. In contrast, on the membrane of viable cells, there was negligible evidence of binding of either the apo- or holoform of Gc protein, but substantial quantities of firmly bound immunoreactive endogenous Gc were detected. Separation experiments and double-label fluorescence with antisera recognizing defined phenotypic markers showed immunoreactive membrane Gc on 30-40% of unfractionated mononuclear cells and greater than 95% of monocytes or B cells. Only 5-8% of T cells were similarly reactive; these were not apparently confined to any given subset. Extraction of unfractionated cells with 6 M urea or solubilization in Nonidet P-40 released immunoreactive Gc protein, with physicochemical properties indistinguishable from those of Gc purified from serum (apparent MW 56K; pI 4.8-5.1). These findings indicate that membrane Gc may represent another surface immunofluorescence marker for B cells, and may play a role in immunocyte function.

Measurement of 25-hydroxyvitamin D-1 alpha-hydroxylase activity in mammalian kidney

Until recently measurement of 25-OH-D3-1 alpha-hydroxylase activity in mammalian kidney has not been possible due to the presence of a protein which inhibits the enzyme by reducing available substrate. However, utilization of sufficient unlabeled 25-OH-D3 (80 nmol/ml renal homogenate) to overcome the effect of the inhibitor while maintaining optimal concentration for 1-hydroxylation has made quantitation of enzyme activity possible. We have modified this existing technique in order to increase the sensitivity and to permit detailed study of 1 alpha-hydroxylate regulation in mouse kidney. The modifications that we have incorporated include (i) simplifying the purification scheme for obtaining measurable 1,25-(OH)2D3 by reducing to one the necessary number of high-performance liquid chromatography steps and (ii) quantifying 1,25-(OH)2D3 by radioligand assay. The sensitivity of the assay is 10 pg, which, corrected for fractionation and recovery (50-60%), allows the measurement of 0.5 fmol 1,25-(OH)2D3 produced per milligram kidney per minute. Moreover, reliability and precision of the assay have been confirmed by demonstrating that samples from carefully matched, identically treated mice have reproducible enzyme activity (interassay coefficient of variation = 9.1%, n = 5) and show appropriate dilution characteristics. We have also demonstrated appropriate modulation of enzyme activity by known effectors of 1-hydroxylation. Kidneys from D-deficient mice exhibit significantly higher enzyme activity (15.28 +/- 1.17, n = 21) than do normal mouse kidneys (5.14 +/- 0.26, n = 33). In contrast, enzyme activity is suppressed significantly in kidneys obtained from calcium-loaded (1.20 +/- 0.04, n = 5) and parathyroidectomized animals (2.94 +/- 0.29, n = 5). Our assay now permits the indepth study of 1 alpha-hydroxylase regulation in mammalian (mouse) kidneys.

1,25(OH)2D3 is not the only D metabolite involved in the pathogenesis of osteomalacia

Three patients are described in whom there was no simple correlation between plasma 1,25(OH)2D3 concentration and the occurrence of osteomalacia. One patient had severe osteomalacia with high plasma 1,25(OH)2D3 and normal mineral ion product; the second had a normal mineral ion product and no evidence of osteomalacia even though plasma 1,25(OH)2D3 was undetectable; and the third had osteomalacia, low plasma 1,25(OH)2D3 and a reduced mineral ion product. In considering these data in the light of presently available information, it is concluded that osteomalacia can occur as a consequence of a lack of a vitamin D metabolite other than 1,25(OH)2D3, or a consequence of a reduced mineral ion product, but not as a consequence of 1,25(OH)2D3 lack if the mineral ion product is normally maintained and other D metabolites are present. However, a deficiency of 1,25(OH)2D3 normally leads to a reduction in the mineral ion product hence 1,25(OH)2D3 deficiency may play a role in the development of certain forms of osteomalacia.

Comparison of the effects of vitamin D metabolites on collagen synthesis and resportion of fetal rat bone in organ culture

We compared the effects of four vitamin D metabolites, 1 alpha,25 dihydroxy vitamin D3 (1 alpha,25(OH)2D3), 1 alpha hydroxy vitamin D3 (1 alpha OH D3), 25 hydroxy vitamin D3 (25 OH D3), and 24R,25 dihydroxy vitamin D (24R,25(OH)sD3) on resorption and collagen synthesis in fetal rat bone maintained in organ culture. Resorption was quantitated by measuring the release of previously incorporated 45Ca from long bone shafts of 19-day fetal rats, and collagen synthesis was assessed by measuring the incorporation of 3H-proline into collagenase digestible protein (CDP) in calvaria from 21-day fetal rats. All four compounds stimulated bone resorption and inhibited collagen synthesis, but 1 alpha,25(OH)2D3 was approximately 1000 times more potent in both organ culture systems. Although the differences were small among the other three compounds, the order of potency was 1 alpha OH d3 > 25 OH D3 greater than or equal to 24R,25(OH)2D3. These results suggest that the receptor for 1 alpha 25(OH)2D3 in both bone resorbing and bone forming cells has similar affinities for several vitamin D metabolites.