Mapping of drug binding sites on al-acid glycoprotein

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There are at least two genetic variants of human al-acid glycoprotein (AGP) (the A and F1*S variants)that are encoded by two different genes. AGP is a major carrier of basic drugs in circulation and the variants of AGP have different drug-binding properties. The purpose of this study was to identify the amino acid residues that are responsible for the selectivity of drug binding to genetic variants of AGP using site-directed mutagenesis. First, we screened amino acid residues in the region proximal to position 100 that are involved in binding of warfarin and dipyridamole, which are F1*S-specific ligands, and of propafenone, which is an A-specific ligand, using ultrafiltration. In the F1*S variant, His97 and His100 were involved in warfarin- and dipyridamole-binding, respectively; Trp122 also contributed to binding of both ligands. G1u92, His100 and Trp 122 participated in the binding of propafenone in the A variant. Exchange of the residue at position 92 between AGP variants reversed the relative strength of propafenone binding to the two variants but had a markedly di8erent effect on binding of warfarin and dipyridamole. The V92E mutation decreased warfarin binding to the Fl*S variant, while the E92V mutation increased dipyridamole binding to the A variant; although, both drugs had greater binding affinities for the wild-type F1*S variant than for either mutant. These findings indicate that the amino acid residue at position 92 plays a significant role in drug-binding selectivity in AGP variants, especially for drugs that preferentially bind to the A variant.