Protein binding of digoxin in human serum

The protein binding of digoxin in human serum was investigated using equilibrium dialysis and tritium-labelled digoxin. A constant protein bound fraction of about 30% was found over a wide range of concentrations of digoxin including therapeutic levels. Interpretation of the findings according to the law of mass-action showed an association constantK = 0.68·10−5l/mol; and, the number of binding sites,n → ∞, indicating an almost infinite apparent maximum binding capacity and a very small affinity of human serum proteins for digoxin.