Direct and indirect determination of binding constants of drug-protein complexes with microparticles.

Albumin can be immobilized in microparticles of poly-acrylamide in such a way that its ligand-binding properties are retained. With radiolabeled salicylic acid, warfarin, and tryptophan, the same characteristics are obtained for binding to albumin in the microparticles as in free solution. The particles can be used conveniently to determine association constants and the number of binding sites directly. The association constant of a competitive displacer can be determined indirectly as well, as shown with diazepam-salicylic acid and tryptophan-salicylic acid.