Interactions between protein-gold complexes and cell surfaces: a method for precise quantitation.

We have developed a rapid and precise electron microscope technique for the quantitation of gold particles in suspension using latex microspheres as a reference (EM latex technique). This technique allowed us to determine the specific absorption of colloidal gold at its absorption maximum (520 nm) and the average number of ligands ([125I]IgG) bound to one gold particle. On the basis of these values important binding characteristics of protein-gold complexes to cell surfaces were analyzed in a model system consisting of Staphylococcus aureus with protein A on the cell wall as a specific binding site for IgG-Au. Our observations showed that the number of binding sites represented by one IgG-gold complex depended primarily on the particle size, with one 20-nm IgG-Au corresponding to 15 and one 6-nm IgG-Au to 2.5 binding sites. Hence, the efficiency of binding of IgG-Au complexes increased with decreasing gold particle size. Saturation of binding sites, however, was not achieved. The technique also made possible the determination of the affinity between IgG-Au complexes and the cell surface; this affinity can either be regarded as a characteristic of the ligand IgG or of the gold particle. We observed that the affinity of IgG decreased with the size of the gold particles to which IgG was bound, whereas the affinity of the entire gold particle increased with particle size. The EM latex technique for quantitation of gold particles extends the general use of protein-gold complexes to the quantitative characterization of their interaction with cell surface constituents.