In many ways, vitamin D biology has helped build a solid foundation for targeted mass spectrometry in the clinical laboratory. However, the definition of vitamin D deficiency has been a source of controversy for many years and the appropriate treatment for observed insufficiency is unclear. Further, almost all intervention studies to date have been negative. Significant effort has been invested in trying to understand if there is a better biomarker of deficiency in order to provide more accurate diagnosis and to monitor for the response of potential novel interventions.
To discuss our current understanding of vitamin D binding protein as an analyte and a participant in vitamin D biology.
We developed a trypsin digestion-LC-MS/MS assay for the quantification of vitamin D binding protein. Using protein purified from the plasma of homozygous individuals, we assessed the influence of polymorphisms on our ability to quantify proteins using bottom-up approaches. We applied the assay to different research studies and clinical situations.
Our method can serve as an example of quantitative LC-MS/MS assays of other medium to high abundance proteins in human plasma. It was used to shed light on an important analytical problem with an immunoassay that misled the field. Calibration curves of purified protein spiked into chicken serum or human serum had similar slopes, demonstrating that protein polymorphisms and post-translational modifications can have little influence on the quantitative data provided by bottom-up protein measurements. Data from a large study of cardiovascular disease demonstrated the utility of vitamin D binding protein in predicting outcomes. In addition, vitamin D binding protein concentrations were associated with the clearance of 25-hydroxyvitamin D. Finally, the assay was used to diagnose the first two cases of complete vitamin D binding protein deficiency.
While vitamin D binding protein is not required for life, which surprised many, it may still represent a useful biomarker in understanding vitamin D biology.