Metal-binding properties of the isolated glomerular basement membrane

The nature of binding of metal cations to the glomerular basement membrane has been investigated using isolated bovine glomerular basement membrane. Highest-affinity binding for a number of ions is attributable to the glycosaminoglycans (mostly heparan sulfate) of the membrane. Some ions, such as divalent Mn, Ca and Ni, have specific binding sites on these polymers, while for others the ion-polyelectrolyte interaction is of a non-specific nature. Both structural and binding data indicate a linear charge density of close to unity for the heparan sulfate of the glomerular basement membrane, which at the ionic composition of the plasma filtrate corresponds to a polymer surface potential of about -45 mV. Several independent observations are better explained by a model of counter-ion condensation about the glycosaminoglycans than by conventional double layer theories. These include the valence dependence of ion binding, the sharp ejection of divalent ions at a critical concentration of La3+, and the relative insensitivity of 63Ni2+ binding to NaCl concentration in the neighbourhood of physiological ionic strength. In its interactions with metal ions, the glomerular basement membrane behaves like a dilute solution of polyelectrolytes. This conclusion has important consequences for the extent of charge reduction of the filtration barrier of the kidney, bathed as it is in an electrolyte solution of mainly monovalent salts.