ELECTROSTATIC AND HYDROPHOBIC INTERACTIONS ARE INVOLVED IN FACTOR-VA BINDING TO MEMBRANES CONTAINING ACIDIC PHOSPHOLIPIDS

The interaction of factor Va with phospholipid monolayers was studied using alternating current polarography. For these studies a hanging me rcury drop electrode is positioned in contact with the monolayer at th e air-monolayer interface. Factor Va introduced into the aqueous subph ase beneath the monolayer caused alterations in the capacitance of the circuit. When factor Va was introduced beneath monolayers containing 13-25% mole fraction phosphatidylserine (PS), 87-75% mole fraction pho sphatidylcholine (PC), a significant capacitance increase occurred, wh ich is interpreted to be the result of the penetration of the factor V a molecules through the monolayer. No penetration or associated capaci tance changes were observed with monolayers composed of pure PC or pur e PS. Polarography experiments were also conducted with the electrode placed in the aqueous subphase to measure the binding of factor Va to pure PS monolayers. This approach has allowed detection of factor Va ( at nm concentrations) adsorption to a pure PS monolayer. Thus the memb rane adsorption process can be distinguished from factor Va penetratio n through the monolayer. These results suggest that there are two type s of interaction of the factor Va molecule with PS-containing monolaye rs. The interaction with the PS component of the monolayer is essentia l for binding the factor Va (at nm concentrations) that occurs by surf ace adsorption. For mixed PC/PS monolayers, PS adsorption is followed by penetration bf the protein through the membrane.