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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.

DBP circulates in the bloodstream at higher levels than vitamin D metabolites and less than 5% of the protein’s sterol binding sites are occupied [1, 12]. The molar excess suggests that DBP could act as a protective mechanism against vitamin D toxicity, but also that the protein is involved in other physiological functions.