32 ELECTROLYTES IN BIOLOGICAL SYSTEMS 



to the medium. According to this hypothesis the 'R' group must be large 

 enough to prevent the outward diffusion of the metabolic intermediates. 



Finally, the introduction of the 'R' group provides an additional reaction 

 and hence an additional free parameter in the quantitative interpretation of 

 competition results. In the reaction series below, compounds A and C might 

 compete well in the formation of the final product D if the reactions A + R — > 

 AR and C + R — > CR proceed with rapidity. At the same time compound B 

 might compete poorly or not at all if the reaction B + R ^ BR proceeds 

 slowly. 



Endogenous A + R^AR^BR->CR->DR->D + R 



t T R^ 



Exogenous A B C 



Some definite examples of such reactions have been observed. Acetate is 

 believed to be used as acetyl-CoA (Ac — R = Ac — CoA) ; however, exogenous 

 acetate enters the metabolic cycle without difficulty showing that acetate is 

 readily attached to CoA even though this reaction is endothermic. This is 

 similar to the first reaction A + R -^ AR. On the other hand, external pyruvate 

 does not enter the Krebs cycle presumably because the active intermediate is 

 not pyruvate but phosphorylenolpyruvate. This is similar to the reaction 

 where B (pyruvate) is not readily converted to BR (phosphorylenolpyruvate). 

 Intermediate cases are also known where the external compound competes 

 incompletely and the degree of competition does not depend in any simple 

 fashion on the concentration of the exogenous compounds. Incomplete com- 

 petition could be due either to a slow rate of association of the external mate- 

 rial with the carrier 'R' or to a slow rate of dissociation of the endogenous 

 complex. 



In summary, the concept of altered intermediates is entirely adequate and 

 often necessary to interpret the experimental results found in studies of perme- 

 ability, extracellular products, and isotopic competition. 



Unfortunately, there are very few hints as to the nature of the 'R' groups 

 involved. In some cases studies of separated enzymes have identified the group. 

 Thus, CoA is seemingly the 'R' group involved in acetate condensation, and 

 in the reactions of succinate. Coenzyme A is large enough so that it could quite 

 reasonably hold the metabolic intermediates within the cell membrane. 



On the other hand, while phosphate can be considered as a part of the 'R' 

 group which is attached to glucose and fructose, it is not large enough to be 

 the entire 'R' group. The cell wall is permeable to the hexose phosphates. Per- 

 haps the phosphate serves as a handle by which the carbohydrates are attached 

 to larger carriers. 



There is no direct evidence whatsoever as to the nature of any 'R' groups 

 concerned in amino acid synthesis. Most of the amino acids are good competitors 

 indicating a rapid exchange between the free and the bound forms. In bacteria 



