Initial Characterization of the Vitamin D Binding Protein (Gc-Globulin) Binding Site on the Neutrophil Plasma Membrane: Evidence for a Chondroitin Sulfate Proteoglycan

Abstract

The vitamin D binding protein (DBP) is a multifunctional plasma protein that can modulate certain immune and inflammatory responses. The diverse cellular functions of DBP appear to require cell surface binding to mediate these processes. Numerous reports have detected DBP bound to the surface of several cell types and would support the concept of a cell surface binding site for DBP. However, direct evidence for such a molecule has been lacking and essentially nothing is known about its basic biochemical properties. In the present study, radioiodinated DBP was used as a probe to characterize biochemically the neutrophil DBP binding site. Radiolabeled DBP binds to and remains associated with the plasma membrane and is not degraded. Quantitation of DBP binding to either intact cells or purified plasma membranes showed nonsaturable (linear) binding with positive cooperativity, possibly suggesting DBP oligomer formation. Solubilization of cell bound 125I-DBP with various nonionic and zwitterionic detergents demonstrated that DBP binds to a membrane macromolecule that partitions to the detergent insoluble fraction. Moreover, this molecule does not associate with the cytoskeleton. Cross-linking of radiolabeled DBP bound to plasma membranes increased the amount of protein that partitioned to the insoluble fraction, and analysis of these complexes by SDS-PAGE revealed that they may be very large since they did not enter the gel. Finally, treatment of plasma membranes with either proteases or chondroitinase ABC completely abrogated membrane binding of DBP, suggesting that the protein binds to a chondroitin sulfate proteoglycan.

Vitamin D binding protein in psychiatric and neurological disorders: Implications for diagnosis and treatment

“…These cells include neutrophils, fibroblasts, monocytes, B and T cells, B lymphoblastoids, placental cytotrophoblasts, human sperms, smooth muscle cells, porcine kidney tubule cells, and rat pancreatic acinar cells. 34 Serum concentration of VDBP ranges from 350 to 500 μg/L, with reduced levels found in other body fluids. 35 …”

Retrospective Analysis of Vitamin D Deficiency in an Adult Population of Arad County, Western Romania (2019–2022)

“…The transport of this vitamin to the organs occurs through vitamin D binding protein (VDBP) [12,13]. VDBP belongs to the albumin superfamily of binding proteins, which includes albumin, alpha-fetoprotein, and alpha-albumin/afamin [14][15][16]. VDBP is synthesized by hepatic parenchymal cells under the influence of estrogen, glucocorticoids, and inflammatory cytokines and plays a crucial role in binding and transporting vitamin D and its metabolites to the target organs [17][18][19].…”