ASER ROTHSTEIN 67 



PERMEABILITY OF RESTING YEAST CELLS TO IONS 



Despite the high concentrations of electrolytes in yeast, the cells when sus- 

 pended in distilled water leak electrolytes at only a very slow rate. For ex- 

 ample, over a period of hours aerated cells lose K+ at the rate of about 2 mM/l.hr. 

 of cells. There is some evidence that the leakage of potassium is associated 

 with dissimilation of carbohydrate stores during endogenous metabolism (51, 

 63). Other electrolytes such as bivalent cations, phosphate and bicarbonate do 

 not leak out in appreciable amounts over a period of hours (46). 



Resting yeast cells (no substrate) are also relatively impermeable to the 

 inflow of electrolytes. For example, Hevesy (24) found that in cells fermenting 

 at a very slow rate, there was little exchange of potassium ion from the medium 

 as measured by isotope methods using K"*^. Conway (5) reported that resting 

 yeast in the absence of oxygen showed no measurable exchange of potassium, 

 but when oxygen was admitted an exchange did occur. The resting cell is also 

 impermeable to the inflow bivalent cations such as Ca+^ and Mn++ as shown by 

 isotope techniques. However, small quantities of these ions are reversibly bound 

 by the surface of the cells (54) (see following section). 



In the case of anions, chloride does not freely penetrate into the cell (17, 4). 

 Phosphate compounds have been studied in some detail. There is only a very 

 slow exchange of P^^-labeled inorganic phosphate across the cell membrane 

 in the absence of substrate (23, 35, 18). Organic phosphate esters also fail to 

 penetrate into the cell in measurable amounts as shown by chemical analysis, 

 by isotope technique with P^^^ and by the failure of certain of the compounds 

 to act as substrates even though they are intermediates of metabolism (56, 

 57). However, the permeability of the cell to certain organic phosphates is 

 apparently increased by fluoride (39). It should be pointed out that organic 

 phosphates can be indirectly metabolized by yeast cells. Under certain condi- 

 tions they are hydrolyzed by phosphatases located on the yeast cell surface into 

 metabolizable organic residues (56). 



Organic anions are also unable to penetrate into the cell at an appreciable 

 rate. In contrast, however, many of the undissociated organic acids penetrate 

 rapidly. For example, pyruvic acid is readily respired by yeast. However, if the 

 pyruvic acid is converted to pyruvate ion by raising the pH of the medium, the 

 ion cannot penetrate and is not respired (i). In consequence, the pH curve for 

 the respiration of pyruvate is parallel to its acid base dissociation curve. Simi- 

 larly acetic and lactic acids are respired, but acetate and lactate ions are not 

 (46), presumably because the ions cannot penetrate into the cell. Direct studies 

 of succinate penetration by volume of distribution techniques indicate that the 

 succinate ion does not appreciably distribute in cellular water in an hour (9). 



In summary then, it can be stated that the membrane of the resting yeast 

 cell is relatively impermeable to both anions and cations. 



