Intracellular pH topography of Penicillium cyclopium protoplasts. Maintenance of delta pH by both passive and active mechanisms.

The intracellular pH distribution in protoplasts of Penicillium cyclopium has been studied using the recently developed fluorescent probe microscopic technique. The technique gives detailed pH maps of the interior of the protoplasts with the exception of vacuoles (no fluorescence signal from vacuoles was observed). In the cytoplasm two separate layers were distinguished: a thinner outer layer with acidic pH (around 5) and the larger core region with near neutral pH. The pH of the core region is decreased by the addition of uncouplers, inhibitors of respiration and during the uptake of L-phenylalanine. These compounds do not change the pH of the surface layer, which is, however, acidified by addition of vanadate, an inhibitor of the proton pump of the plasmalemma. We suggest that the pH of the surface layer is maintained by the combined effects of a Donnan distribution of protons (bound to postulated anion binding proteins) and the proton extrusion via the plasmalemma proton pump. This mechanism explains the protection of the cytoplasmic core of acidophilic eukaryotes from the influence of the usually acidic environment.