ENZYMES AS REAGENTS 



due to the peculiarity of the aconitase enzyme, which dehydrates 

 the oxaloacetate-end of citrate rather than its acetate-end. 



PHYSIOLOGICAL SIGNIFICANCE OF EXCHANGE REACTIONS 



It is apparent from the above that one must differentiate 

 between exchange reactions and incorporation due to the net 

 synthesis of AB*: 



A + B* > AB* 



Carbon 14 from carboxyl-labeled acetate will appear in oxalo- 

 acetate which on decarboxylation can yield pyruvate or phos- 

 phoenol pyruvate. From these three-carbon compounds, glyco- 

 gen containing the carbons of acetate will be formed. It has been 

 known to physiologists for many years that acetate cannot be 

 used for the biosynthesis of glycogen. We are dealing here with 

 an exchange reaction. It is also apparent that it is impossible 

 to draw conclusions about quantitative aspects of biosynthetic 

 pathways from incorporation studies with isotopes only, unless 

 exchange reactions can be ruled out. 



Although no net synthesis of a peptide bond, for example, 

 can be achieved by transfer exchange reactions, these reactions 

 may participate in biosynthetic pathways. It is very probable 

 that transaminases and transpeptidases contribute to the bio- 

 synthesis of amino acids and peptides. It is nature's privilege 

 to delegate the specific function of reductive amination to glu- 

 tamic dehydrogenase. The glutamate formed by this enzyme 

 from ammonia and a-ketoglutarzte acts as the key middleman 

 for numerous transfer reactions to other keto acids which lead to 

 new amino acids. 



The physiological role of the metabolic exchange reactions 

 is not as apparent. Since in the course of these curious reactions 

 a spatial redistribution of certain groups (e.g., acetyl or phos- 

 phate) takes place, it might be conceived that reactions of this 

 type play a role in the transportation of groups and compounds 

 across the cell membrane or from one intracellular structure to 

 another. If work is to be accomplished, the exchange must be 

 catalyzed by two different kinds of reactions as shown in Figure 



235 



