Characterization of drug binding sites on alpha 1-acid glycoprotein.

The classification of drug binding sites on alpha 1-acid glycoprotein (AGP) was studied by displacement experiments using fluorescent probes. Basic drugs not only displaced basic probes strongly but also acidic probes as well. Acidic probes, on the other hand, were displaced by some acidic drugs such as phenylbutazone and sulfadimethoxine which had no effect on most of the basic probes. This contradiction suggests that the basic drugs do not completely share a binding site with the acidic drugs. The polarity of the basic drug binding site was higher than that of the acidic drug binding site. The negative charges were probably located in or near the former, different from the latter. The basic drug binding site was more sensitive to the conformational change of AGP. It seems that there are particular drug binding sites on the AGP molecule for acidic and basic drugs. However, all the displacement data do not fully support the possibility of two independent drug binding sites. Therefore, it is rather reasonable to consider that these sites are not completely separated but are significantly overlapped and influenced by each other. Accordingly, AGP seems to have one wide and flexible drug binding area.