Role of Platelet Fibrinogen in the Reactions of Platelets to Thrombin*

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Abstract
Washed human blood platelets contain a thrombin clottable protein (platelet fibrinogen), but the role of this protein in various functions of the platelet is not completely understood. Thrombin, trypsin, and papain inactivated the clot-table protein of human blood platelets and released serotonin and ATP from the platelets. Platelets that did not contain clottable protein after treatment with these enzymes did not aggregate when subsequently incubated with thrombin and calcium, but retained their ability to support clot retraction when suspended in a medium containing fibrinogen. Plasmin and [alpha]- chymotrypsin, in concentrations equivalent to trypsin in proteolytic activity, did not inactivate the clottable protein or release significant amounts of serotonin or ATP from intact platelets, although, they rendered the clottable protein incoagulable in platelet lysates. Platelets treated with plasmin and [alpha]-chymotrypsin in these concentrations did not aggregate but did release serotonin and ATP when subsequently incubated with thrombin and calcium. The results are compatible with the hypothesis that platelet fibrinogen is a functional part of the structure of normal platelets.