A method for rapid, continuous monitoring of solute uptake and binding

A method has been developed for dynamically monitoring the free concentration of diffusible, tritiated solutes. The technique utilizes particles of a solid scintillator microencapsulated in gel beads that are permeable to diffusible label. Since tritium beta radioactivity has an effective range in water of only a few micrometers, only label that is free to diffuse through the gel can excite the scintillator, while sequestered label is effectively excluded. Thus, the scintillation light output monitors the freely diffusible concentration of label exclusively. A simple, preliminary encapsulation technique is described and tested, and the theory behind the method is discussed with regard to the time resolution attainable for a given label concentration and type of encapsulation. The feasibility of the method is demonstrated by measuring the uptake of [3H]tetraphenylphosphonium by Escherichia coli membrane vesicles in response to the generation of a membrane potential (interior negative).