© 2018 S. Karger AG, Basel
Introduction
Vitamin D-binding protein (DBP) is known as the main transporter of vitamin D metabolites in the blood. DBP actions, however, go beyond vitamin D physiology. Peculiar characteristics and several functions of DBP make it an interesting molecule. The circulating concentration (5 × 10–6M) is remarkably higher than the main ligand, 25-hydroxyvitamin D (25[OH]D; 5 × 10–8M), suggesting that it might have other functions [1]. Besides acting as a vitamin D carrier, DPB has a major role in the clearance of G-actin monomers from plasma [2] and seems to directly regulate neutrophil, macrophage, and osteoclast activity [3, 4]. DBP deletion in humans has so far not been reported, suggesting that its absence may be lethal [1]. It is a highly polymorphic protein and multiple assays have been used to evaluate the concentration of DBP. The findings of discrepant results have given rise to many debates [5].
Recently, DBP has returned to the spotlight due to the discussion about its role in vitamin D physiology, which will be discussed in this chapter.
Structural Characteristics
Initially described in 1959 as a “group-specific component” (Gc-globulin) [6], DBP was renamed due to its role in vitamin D transport [7]. It is a multifunctional plasma protein that belongs to the albumin superfamily of binding proteins, which includes albumin (ALB), α-albumin, and α-fetoprotein (AFP) [8]. The DBP gene is located in the long arm of chromosome 4, at 4q11-q13, close to the ALB and AFP genes [9]. It is composed of 13 exons and 12 introns and results in a protein rich in cysteine residues, such as ALB and AFP [8]. These residues, in unique arrangements, allow the formation of disulfide bonds that gather the molecule into 3 distinct functional domains: 2 repeated homologous domains of 186 amino acids and a shorter one of 86 residues at the C-terminus [10]. The first domain includes the vitamin D-binding site (amino acids 35–49) and the cell-binding site while the second mediates a large number of other interactions. The third domain is similar to the first one, but it does not interact directly with vitamin D [11]. Another important region is the actin binding site, located between residues 373 and 403 [12]. The 3 domains form a 458-amino acid protein. Posttranslational modifications such as cleavage and glycosylation produce a protein with a molecular weight of approximately 58 kDa [10]. Different domain orientations and O-linked carbohydrate chains are important to establish the physiological functions of DBP [13].
DBP is produced mainly in the liver; however, the polymerase chain reaction has detected lower levels of the protein mRNA in the kidney, yolk sac, testis, and abdominal fat [14, 15]. DBP production is relatively stable through life and in vivo half-life is 2.5–3 days [13]. The protein has also been detected in the cerebrospinal fluid, seminal fluid, saliva, and breast milk.
DBP, free or bound, can be removed from plasma by different tissues such 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].