Relations between high-affinity binding sites for L-tryptophan, diazepam, salicylate and Phenol Red on human serum albumin.
Binding of L-tryptophan, diazepam, salicylate and Phenol Red to defatted human serum albumin was studied by ultrafiltration at pH 7.0. All ligands bind to one high-affinity binding site with association constants of the order of 10(4)-10(5)M-1. The number of secondary binding sites was found to vary from zero to five, with association constants about 10(3)M-1. Competitive binding studies with different pairs of the ligands were performed. Binding of both ligands was determined simultaneously. L-Tryptophan and diazepam were found to compete for a common high-affinity binding site on albumin. The following combinations of ligands do not bind competitively to albumin: L-tryptophan-Phenol Red, L-tryptophan-salicylate and Phenol Red-salicylate. On the other hand, high-affinity bindings of the three ligands do not take place independently but in such a way that binding of one of the ligands results in a decrease in binding of the other ligands. The decreases in binding are reciprocal and can be accounted for by introducing a coupling constant. The magnitude of the constant is dependent on the ligands being bound. In the present study, the mutual decrease in binding was more pronounced with L-tryptophan-salicylate and Phenol Red-salicylate than with L-tryptophan-Phenol Red.
D001665:Binding Sites; D003975:Diazepam / Q000378:metabolism*; D006801:Humans; D008024:Ligands; D008956:Models, Chemical; D010635:Phenolphthaleins / Q000378:metabolism*; D010637:Phenolsulfonphthalein / Q000378:metabolism*; D011485:Protein Binding; D012459:Salicylates / Q000378:metabolism*; D020156:Salicylic Acid; D012709:Serum Albumin / Q000378:metabolism*; D014364:Tryptophan / Q000378:metabolism*
D008024:Ligands; D010635:Phenolphthaleins; D012459:Salicylates; D012709:Serum Albumin; D014364:Tryptophan; D010637:Phenolsulfonphthalein; D020156:Salicylic Acid; D003975:Diazepam