Intracellular pH of Baker's yeast

Distribution of individual cytoplasmic pH values in a population of the yeast Saccharomyces cerevisiae

Fluorescence ratio imaging microscopy using pH-sensitive fluorescent dyes makes it possible to evaluate statistical distribution of intracellular pH in a population of the yeast S. cerevisiae examined in a thin layer of suspension in a Petri dish. The distribution appears to fit a Gaussian curve with a half-width around the 0.4 pH unit. The curve became slightly narrower after resuspension in a strong buffer; the mean values shifted with the pH of the buffer. The shape of the distribution curves of both resting and growing cells in various phases of growth does not change significantly. Likewise, addition of 1% of glucose, 50 microM suloctidil or 100 microM diethylstilbestrol brings about no alteration. The only value which clearly changes is the average cytoplasmic pH.

Energy efficiency of different mechanistic models for potassium ion uptake in lower eukaryotic cells

Different mechanistic models for potassium ion uptake are analyzed by an equilibrium-thermodynamic formalism in terms of their comparative efficiency in setting chemical potential differences of the potassium ion of different magnitudes across the plasma membrane of lower eukaryotic cells. The possible adaptive advantages for a multimode mechanism(s) operating in alternative modes depending on the physiological and/or environmental conditions of the cells are discussed.

Intracellular pH distribution and transmembrane pH profile of yeast cells

The pH-dependent fluorescence excitation of fluorescein located intracellularly and in the vicinity of cells of the yeast Saccharomyces cerevisiae and Endomyces magnusii was used to obtain local pH values at a linear resolution 0.2 micron. Cells suspended in water or in a diluted (5 mM) acidic buffer had a relatively alkaline interior (about 7.0-7.5) with pH decreasing gradually toward the periphery and further out through the cell wall to the value of the bulk solution. In slightly alkaline weak buffers the cells also showed an alkaline center and a slightly acidic ring-shaped area, but the peripheral region close to the membrane was again alkaline with pH increasing toward the bulk solution. The heterogeneity of intracellular pH was reduced or nearly abolished in starved or antimycin-treated cell. Suspension of cells in strong (200 mM) buffer resulted within 15-20 min in a nearly homogeneous pH pattern throughout the cell, attaining pH values of 5.5-7.5, depending on the pH of the buffer. Addition of glucose with concomitant pH decrease of the extracellular medium did not change appreciably the intracellular pattern for 20-30 min, except with diethylstilbestrol (inhibitor of proton-extruding ATPase) when the cell became more acidic. It appears that the delta pH measurements between the cell as a whole and the bulk solution (as are used for the calculation of the electrochemical potential of protons in proton-driven transports) are not substantiated, the probable pH difference across the plasma membrane being substantially smaller than previously supposed.

Characterization of two trehalases in baker's yeast

Trehalase activities at pH 5 (not inhibited by EDTA) and pH 7 (inhibited by EDTA) were present in the soluble fraction of disintegrated commercial baker's yeast. The pH 5 activity binds strongly to concanavalin A, is only partially salted out by saturated (NH4)2SO4, has an apparent Mr of 215000 (by gel filtration) and is an acidic protein. It has a Km of 1.4 mM, a broad pH optimum (at 40 mM-trehalose) between pH 4 and 5, and is activated by about 30% by 20-300 mM neutral salts such as KCl, NaNO3 and MnCl2. The enzyme is strongly inhibited by acetic acid/acetate buffers, with a Ki of about 15 mM-acetic acid. The pH 7 activity does not bind to concanavalin A, is salted out at 20-32% (w/v) (NH4)2SO4 and has an Mr of 170000 (by gel filtration). It is absolutely dependent on Ca2+ or Mn2+ ions (Mg2+ is ineffective) and strongly inhibited by neutral salts in the 20-100 mM range. It can be activated by treatment with MgATP in the presence of cyclic AMP. Activation decreases, but does not abolish, the Ca2+ requirement, and does not change the Km for trehalose (5.7 mM) or shift the sharp pH optimum at pH 6.7 (at 40 mM-trehalose).

Factors governing substrate-induced generation and extrusion of protons in the yeast Saccharomyces cerevisiae

Experiments with respiration deficient (rho-), ADP/ATP transport deficient (op1) and double (op1 rho-) mutants, with glycolytic and tricarboxylic acid cycle substrates showed that the substrate-induced acidification of yeast suspensions is closely associated with glycolysis. The glucose/proton stoichiometry is 2.5 : 1 to 4 : 1 depending on glucose concentration. The kinetics of the process are complex, the acidification curve having a very fast initial component and two slower exponential components. The first component suggests an initial proton efflux from endogenous sources, triggered by exogenous substrates. The acidification process exhibits two Km values at about 1 and 15 mM D-glucose, indicating two distinct saturable pathways of proton extrusion. The total extent of acidification and thus the final pHout reaches a saturation value with increasing glucose concentration and suspension density. Both the total extent and the rate of acidification are subject to control by extracellular pH which reflects the tendency of the cells to build a fixed [H+]out/[H+]in ratio. When the control is lifted, both quantities are considerably increased. A crucial role in the substrate-induced acidification is thus played by active membrane processes and their control mechanisms.

Toxic effects of fatty acids on yeast cells: possible mechanisms of action

As shown in a previous paper, threshold concentrations of lower and intermediate fatty acids inhibit the uptake of inorganic phosphate, growth, and cell division in yeast cells. This demonstrates that, apart from these effects, the acids cause an increase in the respiration quotient (RQ), inhibition of CO2 fixation, production of ethanol at the expense of anabolic processes, and inhibition of active amino acid transport in the yeast Candida utilis. On the other hand, the threshold concentrations have no effect on intracellular pH. The inhibition of the inorganic phosphate uptake cannot be the sole primary mode of action of fatty acids since the omission of inorganic phosphate in the incubation medium brings about an inhibition of anabolic processes that is lower than that brought about by fatty acids since the omission of inorganic phosphate in the incubation medium brings about an inhibition of anabolic processes that is lower than that brought by fatty acids at concentrations still premitting some phosphate uptake. Although 2,4-dinitrophenol and caproic acid at low concentrations cause an analogous decrease in biomass yield, their combination does not bring about any marked increase in the effect. Considering the physicochemical properties of fatty acids and their preferential action on energy-requiring processes, one of the key sites of action can be assumed to be the mitochondrial membrane. Fatty acids might inhibit the transport of anions, especially phosphate, across the membrane, and disturb the membrane potential by affecting the transport protons. The physiocochemical properties of fatty acids may also give rise to their binding to other intracellular membranes and to a subsequent interference with the function of the corresponding organelles.

Intracellular trehalase of a hybrid yeast

1. The trehalase found in an extract prepared from a yeast strain that cannot ferment trehalose was studied and characterized. The enzyme is highly specific for trehalose with K(m) 1.02x10(-2)m, and an optimum pH of 6.9. 2. It is inhibited by glucose and by trehalose 6-phosphate, and does not facilitate any significant transglucosylations. 3. pK values 7.7 and 5.8 were detected for the groups associated with binding of the non-ionized substrate to the enzyme. 4. The trehalase was found to be highly labile and was inhibited by thiol-binding reagents. 5. The possible role of this enzyme in the trehalose-dissimilation patterns in the yeast cell was evaluated.