Gold labeling of urokinase plasminogen activator. Characterization and specific binding in cultured mammalian cells.

A colloidal gold-urokinase plasminogen activator complex (u-PAGC) was prepared and characterized. It was used as an ultrastructural marker to study binding sites for urokinase in human and dermal fibroblasts and bovine adrenal endothelial cells in culture. Both the preparation conditions for 15 nm in diameter gold particles and their labeling with urokinase molecules are reported. The complex was stable for at least 4 weeks and had efficient binding and biological activity. Colloidal gold conjugate was observed as single particles or small clusters scattered on the plasma membrane of the cells at 0 degree C and within vesicles in the cytoplasm after a few minutes at 37 degrees C. These data suggest that urokinase-gold complex is a useful marker for the specific labeling of urokinase binding sites.