In vitro effect of Cl2Ca and Cl2Mg on warfarin-human serum albumin (HSA) binding

The binding of warfarin to 10 mumol/L human serum albumin (HSA) was studied in vitro by equilibrium dialysis in presence of two different calcium concentrations. At 2.3 and 23 mmol/L calcium concentrations, the percentage binding decreased. The number of saturable sites of warfarin-HSA at these two calcium concentrations were 1.02 and 1.23 respectively; the number of saturables sites observed with warfarin alone was higher (n1 = 1.32). The association constant for saturable sites (K1 = 0.187 mumol-1) and the affinity coefficient for the nonsaturable sites (n2K2 = 0.0955) remained unchanged. The binding to HSA was 98 per cent at therapeutic levels, that is a physiological concentrations of protein and therapeutic levels of warfarin, in the presence of several concentrations of calcium (2.5; 5; 25; 50 and 100 mmol/L) or magnesium (1;2;10;50 and 100 mmol/L) as chloride compounds, the free fraction of warfarin remained constant until calcium or magnesium reached concentrations of 50 mmol/L, in which cases the percentage binding fell to 92 per cent. In conclusion a displacement of bound warfarin can be expected only if the plasma concentrations of calcium or magnesium and as well chloride concentration are raised by much.