ASER ROTHSTKIN 89 



ENZYMIC NATURE OF EVENTS AT THE CEI.I. SURFACE 



Throughout this paper references hav^e been made to the i)roperties of the 

 outer surface of the cell in regard to the action of cations and to the trans[)ort 

 of ions. It has been indicated that the cell surface contains anionic sites with an 

 affinity for cations, particularly bivalent cations. If uranyl ion combines with 

 these sites, the uptake of sugars is specifically blocked. If Mn++ or Mg"^"*" is 

 combined with the sites, the rate of glucose uptake is enhanced. Monovalent 

 cations, especially K+ and H+, also exert a marked influence on the rate of 

 uptake of glucose by influencing the surface of the cell. What is the nature of 

 the event or events in sugar uptake which occurs at the surface of the cell and 

 which can be so markedly influenced by cations? The active transport of ions 

 must also involve mechanisms operating at the surface of the cell. Is there any 

 connection between the surface reactions in sugar uptake and in ion transport? 

 These questions cannot be answered categorically at the present time. However, 

 adequate information is available concerning the behavior of the surface re- 

 actions to construct a reasonable hypothesis. 



The simplest mechanism by which sugars could pass into the cell is by simple 

 diflfusion, with the rate dictated by the concentration gradient, and by the re- 

 sistance of the permeability barrier of the cell. The inward flow of sugar would 

 be maintained because of its constant removal by metabolic reactions in the 

 interior of the cell. The extracellular cations in this case would act by increasing 

 or decreasing the permeability of the membrane. However, certain of the avail- 

 able data are incompatible with this hypothesis. For example, it is found that 

 certain sugars such as galactose (9) and sorbose (46) cannot diffuse into yeast 

 cells at a measureable rate, even though they have almost identical chemical 

 and physical properties to glucose and fructose. Simple diflfusion through a 

 membrane should not involve such a high degree of specificity. Furthermore, the 

 kinetics of sugar uptake are inconsistent with the diffusion hypothesis. If the 

 surface reaction in sugar uptake is made rate-limiting by adding an inhibiting 

 concentration of UO0++, then it is found that with increasing concentrations of 

 glucose the rate of uptake reaches a maximum. The relationship can be fitted 

 by the Michaelis-Menten equation (27). If diffusion were the limiting factor 

 the kinetics would follow a derivation of the Fick equation for diffusion rather 

 that the ^lichaelis-Menten equation, which is predicated on the assumption 

 that the substrate combines with a constituent present in limited concentration. 



A second possibility would involve the combination of sugar with a constitu- 

 ent of the cell surface as a requisite for its passage into the cell. The reaction 

 would not necessarily be enzymatic, in the sense that the sugar molecule is 

 chemically altered, but might be in the nature of a reversible complex. The 

 cellular constituent in this case would be a 'carrier' with the function of facilitat- 

 ing the passage of sugars into the cell. A 'carrier system' of this kind could pos- 

 sess specificity for different sugars and could also be characterized by Michaelis- 



