The binding of thiopental to human serum albumin at variable pH and temperature

The binding of thiopental was studied in vitro by equilibrium dialysis over the concentration range 0.8-80 micrograms/ml in solutions with 45 g albumin (HSA) per litre. The binding at 37 degrees was from 79% to 67.6% at pH 6 while it was 94.5% to 93.0% at pH 9. At intermediate values of pH the observed values for binding were compatible with the assumption of a gradual transition of an N-form of HSA present exclusively at pH 6 to a B-form present exclusively at pH 9. The variation in binding could not be explained by changes in the balance between ionised and nonionized drug (pKa = 7.6). An increase in r (number of thiopental molecules bound per HSA molecule) was demonstrated when the temperature was lowered to 2 degrees. Graphs showing the relation between drug concentrations and percentual binding all showed a very modest slope and it was concluded that a traditional binding model with calculation of the number of binding sites and association constants was of little use in the evaluation of our binding studies of thiopental. The binding between thiopental and albumin may be characterized best by the solubility of thiopental in hydrophobic areas in the albumin solutions.