valid for diffusible hormones such as steroids and L-thyroxine, suggests that only the unbound or weakly bound (to albumin, for instance) molecules would be able to get into the cells and induce their biological effects. If this is true for vitamin D, the DBP polymorphism, with different affinities for 25(OH)D and 1,25(OH)2D, could modify free hormone availability and, consequently, the biological effects on target tissues [33]. Nevertheless, this hypothesis cannot be easily transferred to vitamin D metabolism. In the apical membrane of renal tubular cells the importance of the endocytic megalin/cubilin pathway for the internalization of the entire complex DBP-25(OH)D from the glomerular filtrate has been demonstrated. When inside the cell, 25(OH)D undergoes the 1α-hydroxylation in the mitochondria, resulting in the production of 1,25(OH)2D, which is subsequently released into circulation, while DBP is degraded [6, 34]. Megalin and cubilin knockout mice lose DBP and 25(OH)D in the urine and develop vitamin D deficiency and bone disease [33]. Megalin was found in other tissues including placenta, mammary gland, and parathyroid glands, sites where the 1α-hydroxylase is also present, but the importance of this pathway for the metabolism of vitamin D outside the kidney remains inconclusive.
In addition, in the experimental absence of DBP in knockout mice, the 1,25(OH)2D concentrations in tissues were equivalent to those found in wild-type counterparts, despite its reduced circulating levels, suggesting that the DBP is not necessary for the internalization of vitamin D metabolites in target tissues [35].
Although surrounded by controversies regarding DBP measurements, Powe et al. [36 ]have reported that as much as 9.9% of variation in 25(OH)D levels is explained by genetic polymorphisms. It could be associated with particularities in the production and/or metabolism of each isoform. Besides its influence in DBP absolute levels, the multiple polymorphisms have also been associated with differences in 25(OH)D absolute and bioavailable levels [36]. The greater affinity of GcF1F isoform to vitamin D metabolites makes it a more efficient transporter, but, on the other hand, less likely to leave them free and bioavailable. It is possible that these features could provide a similar concentration of bioavailable vitamin D metabolites with lower 25(OH)D and DBP serum levels in those with this variant [36, 37]. Multiple genome-wide association studies have shown other SNPs that are related to 25(OH)D concentrations, and they are involved in the vitamin D metabolic pathway [18].
Recent data suggest caution in the interpretation of DPB measurements by immunoassays, because some monoclonal antibodies might not recognize all polymorphic forms of the molecule in the same way. This alert was raised after the publication of Powe et al. [36], who found a remarkable difference in the DBP concentrations between black and white Americans. They found much lower DBP values in blacks than in whites, and wrongly concluded that the levels of calculated bioavailable 25(OH)D would be similar in both groups. The authors suggested that these findings could justify the fact that blacks have higher bone mass than whites, despite consistently lower 25(OH)D concentrations [36]. However, later publications have shown that the assay used by Powe et al. was a determinant. The monoclonal assay used reported lower values in blacks because the antibody does not properly recognize the Gc1F form, the most frequent DBP polymorphism in this population, calling the conclusion of this study into question [5]. Subsequently, Henderson et al. [22 ]measured DBP by mass spectrometry and found no differences between races. Recently, Nielson et al. [38 ]evaluated men from different ethnic backgrounds measuring DBP of all samples by 2 different methods, by polyclonal radial immunodiffusion assay and by monoclonal ELISA. They demonstrated that the mean DBP measured by monoclonal assay in subjects of African ancestry was approximately 50% lower than that measured by polyclonal immunoassay. Using the polyclonal antibodies, they did not find any difference between races in the mean of DBP concentrations. The free 25(OH)D was lower in the African descendants living in the United States compared to those still living in Africa, similar to the results for total 25(OH)D. The authors pointed out that the normal range defined by the Institute of Medicine (IOF) should apply to all Americans, no matter what their ethnic background is, and that total 25(OH)D can be used in the general population for the evaluation of vitamin D status.
In a population of Chinese postmenopausal women, the bioavailable and not the total 25(OH)D was an independent predictor factor for bone mineral density. Bioavailable 25(OH)D was calculated based on DBP levels taking into account the correspondent genotype. The authors suggested that the bioavailable vitamin D increases BMD through modulating the bone turnover process, inhibiting PTH excretion and stimulating OPG production [39]. In another study, in Danish premenopausal women, Lauridsen et al. [40 ]found a strong correlation between fractures and Gc phenotypes. They found a 3 times lower fracture risk in women with the Gc2-2 phenotype compared to those with the Gc1-1 phenotypes. The differences were even more striking for fractures caused by low-energy traumas, and they hypothesized that Gc plays a physiological role in osteoclast activity.
It seems that these controversial results may be caused by the different profile of the studied populations, especially because there is a strong linkage between the Gc phenotype and ethnic background. In these two studies (Chinese and Danish), the populations tend to be more homogeneous, and the results may not be transferred to other populations.
Conclusion
The DBP belongs to the albuminoid gene family, initially denominated as a groupspecific component (Gc-globulin). It is a 458-amino acid multifunctional protein with 51- to 58-kDa molecular weight, synthesized by the liver, and secreted into circulation in large concentrations. It is the major transporter of vitamin D metabolites and exhibits different affinity for the multiple compounds. Although vitamin D binding is the main function of DBP, and even responsible for its name, the protein has another important function, which makes its physiology unique and complex. Actin scavenging is a known vital function of DBP, important to avoid actin polymerization and, consequently, tissue damage. Also considered an acute phase reactant, DBP is the precursor of signal MAF (Gc-MAF). Furthermore, DBP is highly polymorphic, with a characteristic distribution among different racial groups. Although its concentration is closely related to the total 25(OH)D, the relevance of the free and bound circulating hormone in human physiology remains unclear.
References
1Cooke NE, Haddad JG: Vitamin D binding protein (Gc-globulin). Endocr Rev 1989;10:294–307.
2Speeckaert M, Huang G, Delanghe JR, Taes YE: Biological and clinical aspects of the vitamin D binding protein (Gc-globulin) and its polymorphism. Clin Chim Acta 2006;372:33–42.
3Benis KA, Schneider GB: The effects of vitamin D binding protein-macrophage activating factor and colony-stimulating factor-1 on hematopoietic cells
