92 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



perhaps more than coincidence that the pH optimum for crystalHne yeast 

 hexokinase, the first enzymic step in the fermentation scheme, is also 8.5 (61), 

 and that a cell-free soluble enzyme preparation with hexokinase activity has 

 been prepared from yeast which has a pH optimum in the range 4.0 to 5.0 

 (46). The latter is distinct from the crystalHne enzyme, but unfortunately is so 

 unstable that its exact properties are difficult to assess. As in the intact cell, 

 K"*" can stimulate the activity of crystalline hexokinase, not at the pH optimum, 

 but at pH on the acid side of the pH optimum. In the case of the enzyme, 

 however, the effects of K+ are small compared to the effects in the intact cell 

 and in the lyophilized cell-free preparation. Perhaps, in part the K+ effect in 

 the cells is related to the structure in which the enzymes are bound. 



The bivalent cations serve as cofactors in the phosphorylation reactions of 

 glycolysis. It might be expected therefore that they would influence any 

 enzyme reactions occurring at the cell surface. However, no comparisons are 

 possible at a quantitative level. It is of interest that Ca++ in high concentrations 

 inhibits fermentation at pn 8.5, whereas AIg++ and Mn++ do not (table 9). 

 Crystalline hexokinase is also inhibited by Ca++ and activated by Mg^ and 

 Mn++ (25). However, the inhibition in the case of intact cells was never more 

 than 30%, whereas in the case of the pure enzyme, the inhibition may be much 

 higher. Unfortunately, the bivalent cations cannot be completely removed from 

 cellular structures in order to test the requirements for those ions. 



The inhibitory action of UO2++ ion is of some interest. It has been indicated 

 previously that this ion inhibits glucose uptake by combining with groups of 

 the cell surface (55). Uranyl ion will also inhibit the hexokinase reaction. In 

 both cases Mg++ competes with UO2++ and reverses the inhibition. In both cases 

 1102"'^ inhibits by combining with a polyphosphate structure. In the case of 

 the enzyme system the 1702"^ has a special affinity for the hexokinase-ATP 

 complex (25). 



Admittedly, comparisons of enzymes in solution with enzymes in cellular 

 structure are somewhat hazardous because of properties of the latter which are 

 difficult to assess. Enzymes associated with structural elements may possess 

 different kinetic properties because probability factors in regard to substrate- 

 enzyme collisions are not the same as in solution. Furthermore, there may be 

 local conditions within the structure which are different from those within the 

 surrounding medium, particularly in regard to the concentrations of electro- 

 lytes, because of Donnan effects or membrane phenomena. However, there are 

 other kinds of evidence which are not subject to the same criticism. Thus the 

 yeast cell is ordinarily capable of taking up three monosaccharides, glucose, 

 mannose and fructose, but its membrane is essentially impermeable to non- 

 fermentable sugars such as galactose, sorbose and ribose (9, 46). The membrane 

 of the yeast cell posesses a permeability which peculiarly enough has the same 

 specificity as the enzyme, hexokinase (2, 30). Not only is the specificity the 



