Binding of colloidal gold granules, coated with bovine factor VIII, to human platelet membranes.

Platelet aggregating factor (PAF) activity is preferentially associated with the larger molecular forms of bovine factor VIII and diminishes considerably after dissociation into smaller oligomers by mild disulphide reduction. PAF activity was regained by the binding of small factor VIII oligomers to colloidal gold granules with a mean diameter of 23 nm. In contrast, adsorption of large factor VIII polymers onto gold particles resulted in partial loss of PAF activity. Thus, an optimum multimeric size of bovine factor VIII appears to be required for the maximal expression of PAF activity. Gold granules, coated with reduced factor VIII, can be used as an electron-dense label. Their interaction with surface receptors for bovine factor VIII on either viable or formalin-fixed human platelets was demonstrated by transmission and scanning electron microscopy.