CHARACTERIZATION OF SITE-I ON HUMAN SERUM-ALBUMIN - CONCEPT ABOUT THESTRUCTURE OF A DRUG-BINDING SITE

Human serum albumin (HSA) possesses at least three sites or areas for high-affinity binding of drugs. Of these sites, site I was investigate d by series of ultrafiltration and equilibrium dialysis experiments. T hree ligands, acenocoumarol, dansyl-L-asparagine (DNSA) and n-butyl p- aminobenzoate (n-butyl p-ABE) were employed as marker ligands. Each li gand binds to a single high-affinity site on HSA, and binding studies with different pairs of the ligands revealed independent high-affinity binding. Preliminary displacement studies performed with the typical site I binding drugs warfarin, phenylbutazone and iodipamide showed di fferent displacement patterns of the three marker ligands. These studi es were followed by stringent competition experiments involving all po ssible combinations of the three test ligands themselves and of these and the three marker ligands. On the basis of the results obtained it seems that the acenocoumarol and DNSA binding regions correspond to th e warfarin and azapropazone binding regions, respectively, of site I r eported by others (Fehske, Schlafer, Wollert and Muller (1982) Mol. Ph armacol. 21, 387-393). The new binding region, represented by n-butyl p-ABE, is probably located adjacent to the acenocoumarol binding regio n but apart from that of DNSA. We have elaborated a model for binding site I in which we propose novel nomenclatures, region Ia, Ib, and Ic for the acenocoumarol, DNSA and n-butyl p-ABE binding regions, respect ively. Furthermore, the relation between these regions and the high-af finity binding sites for other drugs have been discussed.