Enantioselective binding of disopyramide to alpha(1)-acid glycoprotein andits variants

Objective: alpha (1)-Acid glycoprotein (AAG) has three main genetic variant s, Fl, S, and A variants. There are few reports on the correlation between AAG variants and binding activity of drug enantiomers. We studied the diffe rences between the binding characteristics of enantiomers of disopyramide ( DP), which is a basic drug. The aim of this study was to elucidate the caus e of the differences between the binding characteristics of DP enantiomers. Methods: The variants in human AAG were separated by hydroxyapatite chromat ography. Binding of DP enantiomers to AAG variants was studied by the ultra filtration method. The characteristics of the binding of DP enantiomers to total variants and each variant were examined by Scatchard analysis within a range of concentrations from 0.5 to 50.0 mug/ml. Results: The binding capacity of S-DP was significantly higher than that of R-DP in variant 3, although the binding capacities of DP enantiomers were almost the same in variant 2. On the other hand, the binding capacities for both S-DP and R-DP in variant 3 were significantly higher than those in va riant 2. Furthermore, there was an almost 2.4-fold difference in the dissoc iation constant (K-d) between S-DP and R DP in variant 3, although no signi ficant difference was observed in the number of binding sites (N). In varia nt 2 no significant differences between DP enantiomers were observed in eit her the dissociation constant or number of binding sites per molecule of AA G. On the other hand, significant differences between variants 2 and 3 in t he dissociation constant for both S-DP and R-DP were observed. The differen ces in dissociation constant between variants 2 and 3 were 4.0-fold in S-DP and 1.7-fold in R-DP. Conclusion: The difference between the binding capacities of S-DP and R-DP is due to differences in the association of DP to variants 3-6, and the rol e of the variants 1 and 2 in the binding of drugs to AAG is minor.