as the kidney, liver, skeletal muscle, bone, lung, intestine, and heart. Multiple proteases are involved in its degradation [2]. Physiological and pathological conditions that influence DBP levels are listed in Table 1 [2].
DBP was initially characterized by the isoelectric focusing model. Although more than 120 variants were described, 3 major forms accounted for the majority of the findings and were of greater interest. These alleles were named according to their electrophoretic migration pattern such as Gc2 (the slowest), Gc1S (slow), and Gc1F (fast). Interestingly, they exhibit different affinities for 25(OH)D and 1,25-dihydroxyvitamin D (1,25[OH]2D) with Gc2 showing the lowest affinity, Gc1F the highest, and Gc1S an intermediate pattern [16, 17]. As each individual has 2 alleles, multiple combinations may influence DBP and may also affect vitamin D metabolite release at target tissues [16].
Table 1. Conditions that influence DBP levels
| Increase DBP | Decrease DBP |
| High estrogen statesPregnancyEstrogen therapy | Severe hepatic diseaseNephrotic syndromeMalnutritionSmoking |
Table 2. Structural differences between polymorphisms and associated characteristics
The variants also have an interesting distribution among ethnic groups. Gc1F is more frequent in those of African ancestry, while Gc1S is more common in Europeans [16]. In Asian populations, both isoforms are found with intermediate frequency. On the other hand, Gc2 is rare in black ethnic groups and is reported in similar frequencies in Asians and Europeans [16]. In the past, these genetic variances were used by population geneticists as tools for tracing migration patterns and the relatedness of groups around the world [17]. The advent of genome sequencing technology showed that these variants correspond to polymorphisms. The 3 major polymorphic forms differ only by the amino acids in positions 416 and 420 as described in Table 2 [16, 17].
DBP Functions
Vitamin D Metabolite Transport
The main role of DBP is the transport of vitamin D metabolites [17]. Steroid hormones are lipophilic and need to be carried by a protein to become soluble in the bloodstream. Therefore, after cutaneous synthesis, vitamin D is transported bound to DBP. In the liver, it is converted into 25(OH)D by the action of vitamin D 25-hydroxylase (CYP21R) and re-enters the bloodstream where it circulates once more, mainly bound to DBP. This is the metabolite measured to establish vitamin D status; however, it is not the active form. To be converted into 1,25(OH)2D in the kidney, the DBP-bound 25(OH)D needs to undergo endocytosis by the proximal tubular cells. The process is mediated by megalin, a large transmembrane protein, and facilitated by two others, cubilin and disabled-2. In the kidney, 1,25(OH)2D is synthesized by the action of CYP27B1. The active form is transported bound to DBP.
Fig. 1. Bioavailability of 25(OH)D. About 85% of Vitamin D metabolites circulate bound to DPB, and less than 1% of 25(OH)D is free. Albumin concentrations in blood are 130 times higher than DBP, and a significant amount of Vitamin D (10 to 15%) circulates bound to it, but because this binding is not so tight, both free and Albumin-bound Vitamin D are considered bioavailable.
Other tissues such as the placenta, and the mammary and parathyroid glands also express megalin; however, the role of megalin in DBP transport outside the kidney is less clear [17]. DBP-bound vitamin D may also be internalized by other tissues in a process that is megalin-independent, as is observed, for example, in lymphocytes [18]. Besides the bound forms, small amounts of free vitamin D metabolites also circulate and may enter the cells via diffusion.
Both 25(OH)D and 1,25(OH)2D circulate bound to DBP (85–90%) or to albumin (10–15%) or in the free form (<1%). DBP’s affinity for vitamin D metabolites is much greater than that of albumin [16]. DBP’s measured affinity constant for 25(OH)D is 7 × 108M–1 and for 1,25(OH)2D it is 4 × 107M–1, while ALB is 6 × 105 and 5.4 × 105M–1, respectively [17]. DBP binding reduces hepatic degradation of vitamin D metabolites, increasing the circulating half-life, and it limits the access of target cells to them [2].
Although the affinity for DBP is much greater than for ALB, ALB is much more abundant in serum (approximately 130-fold) and a significant amount of vitamin D metabolites (10–15%) binds to this protein. However, as the binding is not so tight, ALB-bound vitamin D is considered bioavailable. Therefore, although less than 1% of vitamin D metabolites are found free in the serum, 10–15% is considered bioavailable once it is not bound to DBP (Fig. 1) [17].
Absence of DBP has never been described in humans [1], suggesting that the protein is essential for human viability. Surprisingly, DBP knockout mice are healthy and fertile despite lower circulating levels of 25(OH)D and 1,25(OH)2D while on a vitamin D-replete diet [19]. However, on vitamin D-deficient diets, the mice briefly developed low phosphate, high PTH, and high alkaline phosphatase levels, signs of vitamin D deficiency [19]. In the animal model, vitamin D metabolites are more likely to bind to albumin but the mice are less effective in preventing urinary loss of vitamin D and more sensitive to vitamin D deficiency [19]. Although the absence of DBP in knockout mice decreases the circulating 1,25(OH)2D levels, this does not seem to influence the ability of the hormone to enter the cells and the biological actions in vivo [3]. Animal studies have also shown that DBP slows vitamin D actions in the intestines and other target tissues and protects vitamin D from degradation [16]. Therefore, DBP acts as a vitamin D modulator, protecting it from degradation, facilitating renal uptake, and reducing renal loss, and also limiting tissue bioavailability.
