A study on the allosteric interaction between the major binding sites of human serum albumin using microcalorimetry

The binding of warfarin and oxyphenbutazone to albumin has been studied at pH 6.8 and pH 9.2 by measuring the heat of binding of these ligands to their high-affinity binding sites on albumin (delta Ho'1). The -delta Ho'1 values for the binding of warfarin at pH 6.8 and 9.2 and oxyphenbutazone at pH 6.8 and 9.2 were found to be 16.9(+/- 0.6), 28.8(+/- 0.6), 10.5(+/- 0.4) and 17.4(+/- 0.6) kJmol-1, respectively. The Gibbs energies (delta Go'1) corresponding to these delta Ho'1 values cover a much smaller range. The pH dependences of delta Go'1 and delta Ho'1 are explained in terms of pK shifts in the albumin upon binding warfarin or oxyphenbutazone. Diazepam, which binds to a site on albumin which is different from the warfarin-oxyphenbutazone binding site, increases - delta Ho'1 for the binding of warfarin and oxyphenbutazone to albumin at pH 6.8, but it does not influence the -delta Ho'1 at pH 9.2. This phenomenon may be attributed to an allosteric interaction between the diazepam binding site and the warfarin binding site. This allosteric interaction must have its origin in a phenomenon other than the N-B transition.