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.

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

This study reports the development of a specific and sensitive radioimmunoassay and a simple and accurate radial immunodiffusion (RID) assay for the human serum-binding protein for vitamin D and its metabolites (DBP). These immunoassays employed a monospecific antiserum that was prepared in rabbits against human DBP. The radioimmunoassay effectively measured DBP in amounts of 1-10 ng, whereas the RID assay measured DBP accurately in amounts of 0.2-0.8 mug. The results obtained with the two immunoassays on the same samples of serum agreed well with each other. Using the RID assay, the mean (+/- SD) serum DBP concentration observed in 35 normal persons was 422 +/- 27 micrograms/ml. Generally similar levels were observed in 66 hyperlipidemic subjects. In molar terms, the mean DBP concentration (approximately 8 microgramsM) was of the order of 50 times the usual serum level of 25-hydroxyvitamin D (25-OH-D) plus vitamin D. Thus, most of plasma DBP circulates as apo-DBP, not containing a bound molecule of 25-OH-D or of vitamin D. DBP and 25-OH-D concentrations were measured in a limited number of patients with hypercalcemia, mild hypocalcemia, and markedly elevated serum 25-OH-D levels due to oral vitamin D supplementation. It was found that major changes can occur in the serum levels of 25-OH-D and of calcium with very little or no associated changes occurring in the serum concentration of DBP, The results suggest that neither serum 25-OH-D nor serum calcium plays an important role in the regulation of the metabolism of DBP. Data were obtained that confirmed and extended an earlier report on the identity of the group-specific component (Gc) protein in plasma with the plasma vitamin D-binding protein. On immunodiffusion against whole serum, the line formed with the anti-DBP antiserum showed a complete reaction-of-identity with the line formed with commercial antiserum against Gc protein. Furthermore, serum that had been depleted of DBP by treatment with Sepharose containing covalently coupled antibodies against DBP was found to be depleted also of immunoreactivity against anti-GC protein antiserum. In addition, the properties of the purified DBP preparation agreed closely with those previously reported by others for Gc protein. Finally, a comparative immunology study showed that sera from several different mammalian orders showed some immunoreactivity against the antihuman DBP antiserum. Thus, proteins immunologically similar to human DBP are present in sera from a number of mammalian species and orders.

The transport of vitamin D and its 25-hydroxy metabolite in human plasma. Isolation and partial characterization of vitamin D and 25-hydroxyvitamin D binding protein

This study reports the isolation and partial characterization of vitamin D and 25-hydroxyvitamin D binding protein (DBP), the specific transport protein for vitamin D and its 25-hydroxy metabolite in human plasma. DBP was labeled by the addition of a tracer amount of 3H-labeled 25-OH-D3 to the original plasma used for protein fractionation. Previous experiments have shown that such 25-OH-D3 added in vitro binds to the same protein normally responsible for the transport of endogenous 25-OH-D and of vitamin D. The isolation of human DBP was achieved by an extensive sequence of procedures which resulted in a final yield of only approximately 4 mg of purified DBP from a starting volume of 34 liters of plasma. Purified DBP was homogeneous in the analytical ultracentrifuge and showed a single band of protein on analytical polyacrylamide gel electrophoresis. DBP had a sedimentation constant of 3.49s and a mol wt of approximately 52,000. The molecular weight was assessed by sedimentation equilibrium analysis and also by sodium dodecyl sulfate-disc-gel electrophoresis and by gel filtration on a standardized column of Sephadex G-150. The amino acid composition of DBP was determined and was generally consistent with the estimated extinction coefficient (E1cm1% at 280 nm) of about 9.1. The isoelectric point of DBP was estimated as 4.8 from isoelectric focusing experiments. Direct study of the binding capacity of the purified DBP for added 25-OH-D3 showed that the isolated DBP had a high affinity for 25-OH-D3, with an apparent maximum binding capacity of one molecule of 25-OH-D3 per molecule of protein.