asi:r kothstein 93 



same, bul the relative rale of uptake of niannose and glucose by the cells at 

 pH 8.5 is about the same as that of the crystalline enzyme at pH 8.5. In the case 

 of galactose, although yeast cells are normally impermeable to this sugar, they 

 can be adapted to itafter prolonged exposure. During the process of adaptation, 

 the enzyme galactokinase appears in the yeast (66). The abiUty of galactose to 

 pass into the cell seems to depend on the presence of the enzyme, galactokinase. 



Another kind of evidence pointing to fermentation reactions in the periphery 

 of the cell is given by studies of phosphate uptake. Phosphate is taken up against 

 an apparent concentration gradient by yeast only during active metabolism of 

 sugars, aerobic or anaerobic. Yet the cell isrelatively impermeable to phosphate 

 even as it is taken up, as shown by the failure of P^' labeled phosphate in the 

 medium to exchange with that in the cell (18, 19). Thus the mechanism by 

 which phosphate is transported into the cell allows for no direct communication 

 between intracellular and extracellular orthophosphate. It has been suggested 

 therefore that phosphate must be esteritied at the periphery of the cell. If this 

 be so, the only reaction in fermentation of glucose capable of esterifying ortho- 

 phosphate is the 3-phosphoglyceraldehyde dehydrogenase reaction. The action 

 of azide in low concentrations is compatible with this concept. This inhibitor 

 prevents esterification in the dehydrogenase reaction without blocking the 

 oxidative reaction (67). Similarly, in intact yeast cells azide blocks PO4 up- 

 take without slowing the rate of fermentation (29). Recent studies with red 

 blood cells also suggest that phosphate is esterified by the dehydrogenase 

 reaction in the periphery of the cell, the stroma (20, 41). 



How much of the glycolytic machinery is present in the periphery of the 

 cell? From a purely theoretical point of view it is necessary that all of the 

 enzymes down to the glyceraldehyde dehydrogenase reaction be located there, 

 in order that the ATP used in the initial phosphorylation of glucose, be re- 

 plenished. Actually there is evidence that a complete fermentation scheme 

 may be associated with the cell-surface structure. An insoluble cell-free prepa- 

 ration has been prepared which metabolizes glucose to alcohol and glycogen at 

 a rate comparable to that in the intact cell. It also esterifies inorganic phos- 

 phates. Like the intact cell, it is impermeable to anions such as the phos- 

 phorylated intermediates of metabolism and pyruvate. In addition, it requires 

 K+ and is influenced by H"^ in somewhat the same way as the intact cell. By 

 volume of distribution techniques, using sugar phosphates (which cannot pene- 

 trate into this element), it has been shown that it occupies less than 6% of 

 the total cell volume (50). It seems therefore that a large proportion of the 

 glycolytic machinery is not distributed at random in the cytoplasm, but is 

 associated with a specific part of the cellular structure possessing its own 

 permeability properties. Thus, if a few of the specific enzymes are located in 

 the periphery of the cell, then in all probability the whole glycolytic structure 

 is also located in the periphery. Based on this assumption, it can be calculated 



