High pressure-induced changes of biological membrane. Study on the membrane-bound Na(+)/K(+)-ATPase as a model system

In order to study the pressure-induced changes of biological membrane, hydrostatic pressures of from 0.1 to 400 MPa were applied to membrane-bound Na(+)/K(+)-ATPase from pig kidney as a model system of protein and lipid membrane. The activity showed at least a three-step change induced by pressures of 0.1-100 MPa, 100-220 MPa, and 220 MPa or higher. At pressures of 100 MPa or lower a decrease in the fluidity of lipid bilayer and a reversible conformational change in transmembrane protein is induced, leading to the functional disorder of membrane-associated ATPase activity. A pressure of 100-220 MPa causes a reversible phase transition in parts of the lipid bilayer from the liquid crystalline to the gel phase and the dissociation of and/or conformational changes in the protein subunits. These changes could cause a separation of the interface between alpha and beta subunits and between protein and the lipid bilayer to create transmembrane tunnels at the interface. Tunnels would be filled with water from the aqueous environment and take up tritiated water. A pressure of 220 MPa or higher irreversibly destroys and fragments the gross membrane structure, due to protein unfolding and interface separation, which is amplified by the increased pressure. These findings provide an explanation for the high pressure-induced membrane-damage to subcellular organelles.