Effect of albumin and free calcium concentrations on calcium binding in vitro

In vivo equilibrium dialysis studies were performed to define further the characteristics of calcium binding to bovine albumin. The concentration range for albumin (1 to 9 g/dl) as well as ultrafilterable calcium (0.5 to 2.5 mM) studied encompassed those that might be ordinarily encountered in most clinical situations. Major differences in the regressions of total calcium on ultrafilterable calcium occurred at albumin concentrations of 1, 2, and 9 g/dl but only small differences at albumin concentrations between 3, 5 and 7 g/dl. When albumin concentration was kept constant, the amount of calcium bound to albumin varied directly with ultrafilterable calcium. At any constant ultrafilterable calcium concentration albumin bound calcium varied inversely with the albumin concentrations when albumin was greater than 3 g/dl. Analysis of the data to determine association constants and molar calcium to albumin binding ratios showed that both parameters were dependent on the absolute albumin concentrations. Our results indicate that calcium binding to albumin is a complex process characterised by multiple binding sites whose affinity and binding capacity are variable. These properties suggest that correction of total serum calcium using clinical formulations with fixed calcium to albumin binding ratios may be inappropriate, particularly in hypoalbuminaemic states.