Characterization of binding sites for sulfadimethoxine and its major metabolite, N4-acetylsulfadimethoxine, on human and rabbit serum albumin.

In order to gain an understanding of protein binding of sulfadimethoxine (SDM) and its major metabolite, N4-acetylsulfadimethoxine (N4-AcSDM), the binding of SDM and N4-AcSDM to human and rabbit serum albumin (HSA and RSA) was investigated using circular dichroism (CD), fluorescence and dialysis techniques. The CD spectral characteristics of the compounds bound to the albumins suggested that the drug-binding sites on the HSA and RSA had somewhat different asymmetries. The binding constants for SDM-HSA and -RSA interaction were smaller than those for N4-AcSDM. Two specific drug-binding sites were found on RSA, similarly to HSA, from the results of competitive displacement using fluorescence probes. Moreover, SDM and N4-AcSDM were found to share the same first binding site on the albumins. It can be presumed from the displacement data with a series of p-aminobenzoates that the characteristics of the binding sites (such as depth and width of the hydrophobic cleft) for SDM and N4-AcSDM on RSA may be almost the same, but the characteristics of these drug-binding sites on HSA may be somewhat different.