Gold-labelled Low Density Lipoproteins Bind to Washed Human Platelets

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Abstract
There is still disagreement about the presence of receptors for low density lipoproteins (LDL) in human platelets. Therefore, washed human platelets in suspension were incubated with gold-labelled LDL, and the binding sites for LDL were revealed by transmission electron microscopy on ultrathin sections and on surface replicas. The LDL-gold complexes bound to the platelet membrane and appeared in the open canalicular system. Gold particles were also found within the coated vesicles. The predominant labelling pattern, showed by the surface replicas, was that of scattered single gold particles randomly distributed over the platelet surface. Some small clusters of gold particles were also observed, uniformly distributed between the individual particles. Competitive experiments using an excess of unlabelled LDL suggested that the binding was due to specific binding sites for LDL on the platelet membrane, possibly the LDL receptor. However, the binding sites are not expected to be the classical apolipoprotein B/E receptor, since the binding was inhibited by preincubating the platelets with anti-glycoprotein IIb-IIIa.