Measurement by a Novel LC MS/MS Methodology Reveals Similar Serum Concentrations of Vitamin D Binding Protein in Blacks and Whites.
BACKGROUND Vitamin D deficiency is associated with poor bone health and other adverse health outcomes; however, the associations are greatly attenuated in black vs white individuals. One possible explanation for this attenuation is different concentrations of bioavailable vitamin D metabolites in plasma, which are estimated with equations that include the total concentration of vitamin D binding globulin (VDBG) and haplotype specific dissociation constants. METHODS We developed a method to quantify VDBG with LC MS/MS that could also identify the haplotypes/isoforms of VDBG present. We validated the method according to recent recommendations for publications of biomarker studies. We determined serum VDBG concentrations in samples from the Atherosclerosis Risk in Communities cohort and compared the results with a widely used monoclonal immunoassay. RESULTS With 10 mL of serum or plasma, the lower limit of quantification for the assay <20% CV) was 71 mg/mL. The assay was linear from 62 to 434 mg/mL, with total imprecision of 7.3 9.0% CV at approximately 250 mg/mL. Significant hemolysis interfered with quantification. The identification of isoforms was 97% concordant with genotyping (k coefficient) Method comparison with immunoassay revealed significant isoform specific effects in the immunoassay. Mean concentrations (SD) of VDBG by mass spectrometry were similar in whites and blacks [262 (25) vs 266 (35) mg/mL, respectively; P 0.43] CONCLUSIONS Validated mass spectrometric methods for the quantification of proteins in human samples can provide additional information beyond immunoassay. Counter to prior observations by immunoassay, VDBG concentrations did not vary by race.
Clinical chemistry