The Lone Tryptophan Residue of Human Serum Albumin as Part of the Specific Warfarin Binding Site

The interaction of warfarin and dicoumarol with tryptophan- and tyrosine-modified human serum albumin derivatives was investigated by equilibrium dialysis and circular dichroism measurements. The binding of warfarin to its specific high-affinity binding site is strongly reduced after the modification of the lone tryptophan residue of human serum albumin with three different reagents, while the modification of tyrosine residues has nearly no effect on the binding of warfarin to this site. It is concluded that the lone tryptophan residue is part of the warfarin binding site which is clearly separated from the specific indole and benzodiazepine binding site. Evidence is presented that dicoumarol interacts with the warfarin as well as the indole and benzodiazepine binding site of the human serum albumin molecule. A highly reactive tyrosine residue specifically involved in the indole and benzodiazepine binding site seems to be important for the dicoumarol binding, too. Its nitration with tetranitromethane differently affects the four induced circular dichroism bands of dicoumarol bound to human serum albumin. This effect has been qualitatively and quantitatively characterized by the resolution of the induced circular dichroism spectrum of dicoumarol bound to human serum albumin into the Gaussian component bands using a computer program. The identification of the lone tryptophan residue as a part of the warfarin binding site of human serum albumin is a significant step forward in localizing this important drug binding site.