SELECTIVE ADSORPTION - A NEW METHOD FOR PURIFICATION OF PROTEIN A-GOLD COMPLEXES

Protein gold complexes are prepared by adding gold colloids to cytoche mically active proteins in solution. The gold particles of the colloid form complexes with the protein spontaneously, but some of the protei n remains uncomplexed. Currently, when protein A-gold complexes are pr epared, the uncomplexed protein A is separated from the complex by ult racentrifugation, which is a lengthy procedure and requires special eq uipment. This report describes a simple and rapid method for removing uncomplexed protein A from freshly-prepared ''crude'' protein A-gold a t the laboratory bench. In this method, larger gold particles of 15-nm diameter are added to a crude protein A-gold preparation made with sm aller particles (e.g., 6-nm diameter). The 15-nm particles adsorb unco mplexed protein A preferentially, but do not form complexes with alrea dy-formed 6-nm protein A-gold. The adsorbed protein A, attached to the 15-nm particles, can then be sedimented in a bench centrifuge, leavin g the purified 6-nm protein A-gold complexes in the supernatant. The s tability, immunocytochemical activity, and degree of aggregation of th e protein A-gold complexes prepared by this method are comparable to p rotein A-gold complexes prepared by ultracentrifugation. The method is simple to perform, avoids lengthy purification procedures, and yields complexes with reproducible labelling characteristics. (C) 1996 Wiley -Liss, Inc.