Characteristics of Protein Binding of Thiobarbiturates and 6-n-Propyl-2-thiouracil

To investigate the protein binding characteristics of thiamylal (TA), thiopental (TP) and 6-n-propyl-2-thiouracil (PTU), the effects of pH, ionic strength and temperature on the binding of these three drugs were compared by equilibrium dialysis. The bindings between each of the three drugs and bovine serum albumin (BSA) were similarly dependent on pH. However, the binding constants of TA and TP at pH 7.4 were scarcely influenced by changes of the ionic strength made by the addition of KCl or K2SO4, while that of PTU decreased with increase of the ionic strength. On the basis of these results and the considerable changes in the pKa of bound PTU (previous paper), it is possible that the binding of PTU involves a larger contribution from the interaction with the charged region on BSA than those of TA and TP. A thermodynamic study showed major contributions of ΔS° in the cases of TA and TP and of ΔH° in the case of PTU to ΔG°, suggesting that the binding sites of TA and TP might be different from those of PTU. The pH dependency of binding was also studied by fluorometric titration employing 8-anilinonaphthalene-1-sulfonate (ANS) as an indicator. The displacement ratios of TA and TP increased with increase of pH, in accord with the results of equilibrium dialysis, but that of PTU was only slightly dependent on pH.