The Nature of the Binding Site for Prothrombinase at the Platelet Surface as Revealed by Lipolytic Enzymes

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Abstract
The nature of the receptor for the prothrombinase complex at the surface of non‐activated platelets was investigated by measuring the platelet prothrombin‐converting activity with a chromogenic substrate assay, after treatment of the platelets with various phospholipases or three different proteolytic enzymes. Platelet pro‐thrombin‐converting activity only decreased after treatment with those phospholipases which are able to hydrolyse phospholipids in the intact platelet and also have the ability to degrade negatively charged phospho‐lipids, phosphatidylserine and phosphatidylinositol. Those phospholipases which do hydrolyse phospholipids in the intact platelet but have no activity towards phosphatidylserine (and phosphatidylinositol) produce an in‐ crease in the platelet prothrombin‐converting activity. Proteolytic treatment of platelets with trypsin, chymotrypsin or papain did not result in a decrease of pro‐thrombin‐converting activity. It is concluded that negatively charged phosphatidylserine and possibly phos‐phatidylinositol are involved in the prothrombin‐converting activity of non‐activated platelets. We could not demonstrate the involvement of platelet membrane proteins in a receptor for the components of the pro‐thrombinase complex.

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