Viscosimetric affinity assay.

Affinity ligands and/or affinity receptors may be quantified by a viscosimetric assay which can be carried out with a simple technique and has the potential of broad applications. The viscosimetric affinity assay is based on the high contribution of affinity bonds to the viscosity of an aqueous dispersion of a hydrocolloid that is bearing affinity ligands. In dispersions of such sensitive hydrocolloids at a concentration above the overlapping point, agglutination is not possible and the modulation of viscosity by the formation or dissociation of intercolloidal affinity bonds may be several orders of magnitude larger than the basic viscosity measurable in the absence of intercolloidal affinity bonds. If dispersions (30 g liter-1) of branched dextran with high molecular weight were used as reagent for concanavalin A (Con A), the Con A concentration necessary for a significant rise in viscosity was decreased with increasing colloid size. The viscosity of dispersions containing both a ligand-bearing high-molecular-weight dextran and an appropriate polyvalent receptor protein (lectin or antibody) showed a dependence on the concentration of free ligands (sugars or insulin) according to the law of mass action. In this competitive mode the viscosimetric affinity assay seems to be well adaptable to many analytical problems.