Binding of 6-propyl-2-thiouracil to human serum albumin destabilized by chemical denaturants.

We compared the binding affinity of 6-propyl-2-thiouracil (PTU) with native and destabilized human serum albumin (HSA) as a model to assess the binding ability of albumin in patients suffering from chronic liver or renal diseases. Urea (U) and guanidine hydrochloride (Gu.HCl) at a concentration of 3.0M were used as denaturation agents. Increasing the concentration of PTU from 0.8x10(-5) to 1.20x10(-4)M in the systems with HSA causes a decrease in fluorescence intensity of the protein excited with both 280 and 295nm wavelengths. The results indicate that urea and Gu.HCl bind to the carbonyl group and then to the NH-group. To determine binding constants we used the Scatchard plots. The presence of two classes of HSA-PTU binding sites was observed. The binding constants (K(b)) are equal to 1.99x10(4)M(-1) and 1.50x10(4)M(-1) at lambda(ex)=280nm, 5.20x10(4)M(-1) and 1.65x10(4)M(-1) at lambda(ex)=295nm. At lambda(ex)=280nm the number of drug molecules per protein molecule is a(I)=1.45 and a(II)=1.32 for I and II binding sites, respectively. At lambda(ex)=295nm they are a(I)=0.63 and a(II)=1.54 for the I and II binding sites. The estimation of the binding ability of changed albumin in the uremic and diabetic patients suffering from chronic liver or renal diseases is very important for safety and effective therapy.