The effect of pressure and the volume of activation on the monovalent cation and glucose permeabilities of liposomes of varying composition.

Measurement of the cation permeability of phospholipid microvesicles as a function of pressure confirmed that a single rate-determining step occurred in each case. The volume of activation was 20 ml-mole minus 1 for Na+ and K+, and about 40 ml-mole minus 1 for valinomycin-mediated K+ permeability. It was virtually independent of membrane composition. The results were explained in terms of Träuble's theory of kink-substrate dissociation at the membrane interface involving possible 2gl and 2g2 kink isomers. The volume of activation for D-glucose was 37 ml-mole minus 1, which was not significantly different from that for any of the valinomycin-mediated K+ permeabilities. However, other data suggest that the rate-limiting steps for the sugar and cation permeabilities are not the same.