[74] The Use of Flow Dialysis for Determinations of ΔpH and Active Transport

This chapter discusses that pH gradients (ΔpH) in biological systems that are not amenable to a direct electrophysiological approach can be estimated by measuring the distribution of certain weak acids or bases. Generally, if the internal pH is more alkaline than that of the medium, accumulation of weak acids is observed; conversely, accumulation of weak bases is observed if the internal pH is relatively acidic. The principle on which this approach is based stipulates that the undissociated species of the weak acid or base be freely permeant, whereas the dissociated, ionic species cannot traverse the membrane. Thus, if the internal compartment of the system under consideration is more alkaline than the external medium, the equilibrium between the undissociated and dissociated species of the weak acid will be displaced toward the ionic form, which will accumulate within that compartment. The chapter is concerned with a specific technique that have found to be particularly useful for measuring the distribution of weak organic acids into plasma membrane vesicles isolated from the enteric bacteria Escherichia coli and Salmonella typhimurium. The technique provides a notably accurate, reproducible method for measuring the active transport of a variety of solutes.