ASKR ROTHSTEIN 9 I 



which the enzymes are bound. Much of the available dala are consistent with 

 this hypothesis. Because a detailed description of the evidence supporting the 

 concept of surface enzymes in sugar uptake (45) as well as a general review on 

 the enzymology of the cell surface (44) have appeared elsewhere, only some of 

 the more pertinent observations will be discussed here. 



There is a growing body of evidence to the effect that the cell surface is not 

 simply a ti.xed boundary with mechanical and physical properties, but that it 

 also participates in some of the biochemical activities of the cell, by virtue of 

 enzymes bound in its structure. Enzymes of the cell surface have been im- 

 plicated in functions associated with the interactions of the cell with its en- 

 vironment such as the digestion of extracellular substrates into substances 

 which can be absorbed, the synthesis of extracellular macromolecules such as 

 proteins and carbohydrate polymers, and the active transport of substances 

 into or out of the cell (44). In certain cases, the evidence is unequivocal. For 

 example, in yeast cells a number of phosphatases (57, 56, 14) and polysaccharide 

 splitting enzymes (71, 37, 13) have been localized on the cell surface. 



In the case of the glycolytic enzymes, the evidence is not direct, but is based 

 on parallel behavior of isolated enzymes and of cell-surface reactions in living 

 yeast. For example, the following properties of the interaction of sugar and the 

 cell surface of yeast, are all compatible with the enzyme hypothesis: /) Michae- 

 lis-Menten kinetics (27); 2) high energy of activation (27); j) aerobic-anaerobic 

 differences with respect to U02'^^ inhibition (59) and K+ stimulation (48); 

 4) alteration of end-products of fermentation by extracellular cations (48). 



In general, the kinds of effects of the cations on the living cell reported in this 

 paper have also been observed in isolated systems and in cell-free preparations 

 containing glycolytic enzymes. As early as 19 17 Harden (22) found that the 

 fermentation of glucose by soluble zymase preparations was markedly stimu- 

 lated by K+ and XH4+, but that Na+ had less effect. Dialyzed yeast juice can 

 ferment hexosediphosphate in the absence of K"*" or NH4+, but cannot ferment 

 glucose unless one of these ions is added. Apparently K+ or NH4+ is required 

 for the phosphohexokinase reaction (36). Dried brewer's yeast can ferment 

 glucose at a considerably higher rate if NH4"'" or K+ is added (34). A lyophilized, 

 cell-free preparation from baker's yeast cannot ferment at all if the K+ is re- 

 placed by the organic cation (C2H5)3N+ but can ferment at a rate approaching 

 that of the intact cell if K+ is added in high concentration (50). 



The action of H"*" on the intact cell is also consistent with the enzyme- 

 hypothesis. A lyophilized cell-free insoluble preparation from baker's yeast, 

 in which the general permeability barrier of the cell was broken (as shown 

 by the rapid outward leakage of K"^, phosphates, proteins and enzymes), shows 

 a pattern of effects of H"^ similar to that of the intact cell. Furthermore, the 

 H+-effect was reversible with K+ as in the intact cell (50). Thus the K+ and H+- 

 effects are not associated with the general permeability barrier of the cell. 



In the living cell, there are 2 pH optima for fermentation, 8.5 and 4.5. It is 



