The measurement of internal pH in microorganisms, in yeast cells and in cells in general, has been studied for many years. Several mechanisms are involved in the regulation of the internal pH of the cell, many cellular processes are regulated by the internal pH, and many transport processes depend on the H ϩ cycle. In yeast cells, very crude procedures were initially used, with disruption of the cells by boiling or freezing and thawing, after which the pH of the resulting sap was measured. In 1950, the group of Conway used the distribution of weak acids, such as carbonic or propionic acid, to measure the internal pH of yeast cells (5,6). By these methods, the pH was probably obtained as an average of that of the entire cell interior, including all internal compartments. More recently, other methods have been used; among them, the shift of the P i peak in nuclear magnetic resonance spectra has been useful but complicated and expensive (1, 3).In yeast cells, the use of ionizable fluorescent probes capable of crossing the membrane and distributing between the cells, organelles, or vesicles depending on the internal and external pH (16) has proven useless. Slavík (17) first introduced indicators into yeast cells whose fluorescence depends on the surrounding pH; some of these, which are available commercially, can be introduced into the cells as acetoxymethyl esters (the permeant form), which are cleaved inside by esterases, transforming them into an impermeant form and preventing their efflux. A recent report on the use of one of these dyes to measure the internal pH of yeast cells has appeared (9). However, Slayman et al. (18) have pointed out some of the drawbacks of these dyes; because of the concentration of esterases in some intracellular compartments, they are preferentially hydrolyzed and accumulated in vacuoles or other internal compartments which accumulate hydrolytic enzymes.Kano and Fendler (10) introduced the use of pyranine (8-hydroxy-1,3,6-pyrene-trisulfonic acid), a fluorescent dye with a ionizable -OH group, which shows remarkable pH dependence in fluorescence. The dye could be trapped in liposomes and used to estimate their internal pH (see also reference 4), the most important advantage being the relatively low interaction with the bilayer or proteins, because of its hydrophilic nature.Unfortunately, this property makes its use with whole cells difficult, because of problems with entry. However, a controlled electric shock of high intensity and short duration appears to produce transient openings of the cell membrane that are closed in a short time after the treatment (11). This phenomenon has been used to introduce even substances with high molecular weights into cells by a procedure known as electroporation. This method has been successfully used in yeast cells, and great improvements have been made recently (2,8).The present communication deals with the use of pyranine to measure the internal pH of yeast cells.
MATERIALS AND METHODSCells from an isolated colony of commercial yeast cells (La Azt...