Do Isolated Membranes and Purified Vesicles Pump Sodium? A Critical Review and Reinterpretation,
PENNSYLVANIA HOSPITAL PHILADELPHIA DEPT OF MOLECULAR BIOLOGY
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Figure 2 reproduces an old diagram illustrating, according to the association-induction hypothesis, the consequences of the adsorption of ATP onto a protein. Figure 3 incorporates this diagram into the vesicle model. In this model, the passage of Na could occur via the water layers polarized by the surfaces proteins and phospholipods, or via association with, followed by dissociation from, the fixed anionic sites at the surface. Without ATP the water molecules are much closer to a state of normal water with relatively high permeability for solutes such as Na and K as a result more of the labeled Na introduced into the vesicles exchanges with nonlabeled Na in the Sephadex column, after the vesicles have separated from their incubation medium but before they have reached the collection tube. Inclusion of ATP in the incubation mixture during the isotope loading stage causes a conformational change of the ATPase protein, allowing at least a part of the polypeptide chains to become extended.