Interaction of Prothrombin with Phospholipids Monolayers in Presence of Ca**.

The activation rate of Prothrombin and its conversion into thrombin through a proteolytic degradation is enhanced by phospholipids in the presence of Ca⁺⁺. In this work, we could elucidate stoechiomet rio and structural aspects of the interaction between prothrombin. phospholipids and Ca⁺⁺. We studied the adsorption equilibria of Ca⁺⁺ and of prothrombin un phospholipids monolayers by a new approach which comoines two complementary methods of surface measurement.

Electrochemical investigations on lipid monolayer interacting with prothrombin showed penetration of prothrombin in presence or not of Ca⁺⁺ (by increasing the lipid layer capacitance). with a preferential orientation. The adsorption of Ca⁺⁺ and prothrombin to the lipid layer was measured by counting the surface radioactivity of ⁴⁵Ca and ³H-prothrombin. Synergetic attachment of Ca⁺⁺ and prothrombin to the phospholipid water interface was observed and the number of Ca⁺⁺ per attached prothromoin was determinen. The two methods give comparative results for the proposed ionic-hydrophobic interaction.