Evidence for the Contribution of Surface Potential to the Transtonoplast Potential Difference Measured on Isolated Vacuoles with Microelectrodes

Evidence for a uridine-5'-diphosphate-glucose-protectedp-chloromercuribenzene sulfonic acid-binding site in sugarcane vacuoles

The uptake of uridine-5'-diphosphate (UDP) glucose into vacuoles isolated fromSaccharum sp. cells was fully inhibited by pretreatment with 50 μMp-chloromercuribenzenesulfonic acid (PCMBS) and was not affected by N-ethylmaleimide up to a concentration of 5 mM. The addition of 10 mM UDP-glucose during the pretreatment partially protected the uptake mechanism from PCMBS inhibition, while the presence of adenosine-5'-diphosphate (ADP) glucose or of various hexose-phosphates had no protective effect. Parallel experiments on the binding of [(203)Hg]PCMBS to the vacuoles showed that UDP-glucose and UDP added at 10 mM concentrations caused a 40% decrease in the binding of PCMBS while ADP-glucose did not inhibit the binding. The results indicate the presence in a previously proposed group translocator of at least one site that can bind UDP-glucose. This site, which is blocked by PCMBS, interacts with the nucleotide moiety of UDP-glucose.

Model for the electrolytic environment and electrostatic properties of biomembranes

Physical and chemical interactions of ions with biomembranes are described by a model originating from the Stern theory. Equations of the model have analytical solutions only for very simple, often unrealistic situations. The numerical resolution adopted permits a much wider application of the model: Potentials and concentrations can be calculated anywhere from the surface and in any electrolytic environment. The model is applied to biomembranes. Simulations are presented in three-dimensional figures which allow one to use the model as a practical research tool. In particular, the simulations reveal that, in practice, it is possible to induce an increase of the surface charge density simultaneously with a decrease of the surface potential, and, theoretically, that the potential at the exclusion distance (which estimates the diffuse layer thickness) exhibits a remarkably constant value as the composition of the free solution is varied.