Soluble transcobalamin receptor, sCD320, is present in human serum and relates to serum cobalamin – establishment and validation of an ELISA

Abstract Background: The cell-surface receptor, TCblR/CD320, binds and internalizes transcobalamin-bound cobalamin. So far, no assay has been developed in order to quantify the soluble form of the receptor. Methods: This study developed an ELISA employing a monoclonal antibody that recognized the extracellular domain of CD320 as capture antibody and a biotinylated polyclonal antibody for detection. Serum samples were analyzed from blood donors (n=153) and samples from patients referred for analysis of serum cobalamin [n=757, serum cobalamin 75–2000 pg/mL (55–1476 pmol/L)]. Results: The ELISA specifically measured soluble CD320 with a quantification limit of 1.6 arbitrary units and a total variation of 4%–8%. Size exclusion chromatography was used which showed that the immunoreactive protein observed in serum eluted in the same position as soluble CD320 in the calibrator. Male donors (n=76) showed significantly higher median concentration [18 arbitrary units (arb.u.)] than female donors (n=77, median 16 ar...