Octanoate binding to the indole- and benzodiazepine-binding region of human serum albumin

Binding of L-tryptophan, diazepam and octanoate to defatted human serum albumin was studied at pH 7.0 by equilibrium dialysis at low ligand/protein molar ratios. L-Tryptophan binding takes place at only one site of the protein with an association constant of 4.4 x 10(4) M-1. Under the present experimental conditions, binding of diazepam and octanoate could be accounted for by high-affinity binding alone with primary association constants of 3.8 x 10(5) M-1 and 1.6 x 10(6) M-1 respectively. During the simultaneous presence of L-tryptophan plus octanoate or diazepam plus octanoate, pronounced mutual reductions in binding were observed. Analysis of the data suggests that the reductions in binding represent competition for a common high-affinity binding site. Thus a region seems to exist that is capable of binding one molecule of these diverse ligands with a high affinity. The location of this region within the albumin molecule is discussed.