Vitamin D-binding protein. Structure and pattern of expression

Health implication of vitamin D on the cardiovascular and the renal system

Vitamin D regulates the calcium and phosphorus balance in the body. The activated form of vitamin D (1 α,25-dihydroxyvitamin D) binds to vitamin D receptor which regulates genes that control cell proliferation, differentiation and apoptosis. In the cardiovascular system, the vitamin D receptor is present in cardiomyocytes and the arterial wall. A clear correlation between vitamin D level and cardiovascular diseases is established. Vitamin D deficiency affects the renin-angiotensin system leading to ventricular hypertrophy and eventually to stroke. While clinical trials highlighted the positive effects of vitamin D supplements on cardiovascular disease these still need to be confirmed. This review outlines the association between vitamin D and cardiovascular and renal disease summarising the experimental data of selective cardiovascular disorders.

Influence of obesity on vitamin D-binding protein and 25-hydroxy vitamin D levels in African American and white women

25-Hydroxy vitamin D (25OHD) is lipophilic and highly bound to vitamin D-binding protein (VDBP) in plasma. In the present study, we examined VDBP and 25OHD levels by race and body mass index (BMI) in young adult women to determine whether circulating VDBP plays a role in the low levels of 25OHD with obesity and among African Americans. In agreement with previous studies, mean 25OHD levels were lower in African American women than in whites (P < .01). In a hierarchical multiple regression model, BMI was associated with 25OHD after adjustment for age in white women (P = .02, R(2) = .10) but not in African American women. The VDBP levels, by contrast, were similar in African Americans and whites, and were unrelated to BMI in either racial group. Furthermore, VDBP was unrelated to the plasma level of 25OHD. These data confirm an interaction between race and obesity in vitamin D metabolism, and imply that the carrier protein is not an important determinant of circulating 25OHD in women, nor is it affected by race or adiposity.

Widespread expression of the mRNA encoding a novel vitamin D/thyroxine dual binding protein in the turtle Trachemys scripta

Vitamin D-binding protein (DBP) is the major transport protein for the vitamin D sterols in a variety of mammalian and avian species. The DBP found in the emydid family of turtles is unique in that it exhibits high-affinity binding of both vitamin D and thyroxine (D/TBP). Sequence analysis has revealed that the emydid D/TBP is homologous to mammalian DBP and shares no homology with mammalian thyroxine-binding globulins. Northern blot analysis was used to examine the spatial profile of D/TBP transcription in the turtle Trachemys scripta. In both adults and hatchlings, two transcripts--the expected 1.5-kb full-length transcript and a second 0.8-kb transcript--were present in nearly all of the tissues examined. In adults, highest expression of the 1.5-kb transcript was seen in the kidney, gonad, and spleen, with lower levels in the liver and lung and no transcripts in skeletal muscle. In hatchlings, the full-length transcript was detected in a variety of tissues at similar levels. Injection of hatchlings with thyroxine increased levels of circulating D/TBP and transcript levels. These data are in marked contrast to observations in mammals in which transcription of DBP is confined predominantly to the liver. Further, the increase in circulating D/TBP associated with increased thyroidal activity may result from a direct or indirect activation of D/TBP transcription by thyroxine.

Specificity of polyclonal antibodies raised against a novel 24,25-dihydroxyvitamin D3-bovine serum albumin conjugant linked through the C-11alpha or C-3 position

Novel hapten-carrier conjugants were prepared by coupling 11 alpha-hemiglutaryloxy-(24R)-24,25-dihydroxyvitamin D3 or (24R)-24,25-dihydroxyvitamin D3 [24,25(OH)2D3] 3-hemiglutarate with bovine serum albumin (BSA), to obtain an antibody with high specificity and affinity for use in 24,25(OH)2D3 immunoassay. The polyclonal antibodies showing high titre were each elicited in three or four rabbits against these two conjugants; the antibodies obtained from the former and the latter conjugants were expressed as Ab11 and Ab3, respectively. These had a much higher affinity for 24,25(OH)2D3 than that of the vitamin D binding protein (DBP). Specificity of the antibodies was investigated by crossreactivities with 11 related compounds in a radioimmunoassay (RIA) system. The Abll well discriminated the 1 alpha-hydroxylated metabolites such as 1,24,25(OH)3D3 (< or = 0.69%) and 1,25(OH)2D3 (< or = 0.25%), but significantly crossreacted with some side chain modified compounds such as (24S)-24,25-dihydroxyvitamin D3 [24S,25(OH)2D3] (> or = 67%), 25(OH)D3 (> or = 14%) and 25,26(OH)2D3 (> or = 23%). On the other hand, the Ab3 showed only negligible crossreactivities with the compounds having a different side chain structure such as 24S,25(OH)2D3 (< or = 3.0%), 25(OH)D3 (< 0.3%) and 25,26(OH)2D3 (< or = 0.53%). A significant crossreaction was found only with 1,24,25(OH)3D3 (> or = 68%). These results demonstrated that the Ab3 are promising for developing an immunoassay system which is much more specific and sensitive than conventional competitive protein binding assays based on DBP.

Genetic variation in two bone-related proteins: is there an association with bone mineral density or skeletal size in postmenopausal women?

In an effort to determine if structural variation in proteins important in bone metabolism might influence bone mineral density (BMD) and thus be a determinant of susceptibility to osteoporosis in older women, the authors typed a group of 258 non-Black women (age 65-90) participating in the Study of Osteoporotic Fractures (SOF) for two polymorphic bone-related proteins, group specific component (Gc), also known as vitamin D-binding protein, and alpha 2HS glycoprotein (AHSG). These two proteins exhibit common structural variation in populations that can be detected by isoelectric focusing/immunoblotting of serum. An important function of Gc is the binding, solubilization, and transport of vitamin D sterols in the bloodstream while AHSG is a glycoprotein constituent of calcified cortical bone matrix. There are six common phenotypes of Gc and four of AHSG. Using Gc or AHSG phenotypes as categorical variables, statistical analyses were done to determine if bone mineral density of the proximal or distal radius or calcaneus differed by phenotype. Neither Gc nor AHSG phenotype demonstrated a statistically significant relationship with BMD at any site. Adjustments for age and degree of obesity did not substantively affect these results. Subsequent analyses to determine if phenotype of either of these proteins was associated with variables related to skeletal size showed an association of AHSG with height (P less than .02). This may indicate that AHSG phenotype is related to postmenopausal loss of height, or it may be a chance statistical finding.