Investigating the Role of Sulfate Groups for the Binding of Gd Ions to Glycosaminoglycans with NMR Relaxometry.

Glycosaminoglycans (GAGs) are highly negatively charged macromolecules with a large cation binding capacity, but their interaction potential with exogeneous Gd ions is under-investigated. These might be released from chelates used as Gadolinium-based contrast agents (GBCAs) for clinical MR imaging due to transmetallation with endogenous cations like Zn . Recent studies have quantified how an endogenous GAG sequesters released Gd ions and impacts the thermodynamic and kinetic stability of some GBCAs. In this study, we investigate and compare the chelation ability of two important GAGs (heparin and chondroitin sulfate), as well as the homopolysaccharides dextran and dextran sulfate that are used as models for alternative macromolecular chelators. Our combined approach of MRI-based relaxometry and isothermal titration calorimetry shows that the chelation process of Gd into GAGs is not just a long-range electrostatic interaction as proposed for the Manning model, but presumably a site-specific binding. Furthermore, our results highlight the crucial role of sulfate groups in this process and indicate that the potential of a specific GAG to engage in this mechanism increases with its degree of sulfation. The transchelation of Gd ions from GBCAs to sulfated GAGs should thus be considered as one possible explanation for the observed long-term deposition of Gd in vivo and related observations of long-term signal enhancements on T -weighted MR images.