KONRAD BLOCH 



recognized that substances which are suspected of being intermediates 

 of either anabolic or catabolic reactions, but which are present in 

 amounts too small for chemical identification, may acquire quantita- 

 tive significance in the light of the rate at which they are regenerated. 

 Although their stationary concentration may be small, they arise and 

 are metabolized in large quantities over an extended period of time. 

 In the breakdown of the major food constituents, certain interme- 

 diates are known to be produced continuously, e. g., the deamination 

 products of amino acids, ornithine, the phosphorylated trioses, and 

 acetic acid. None of these intermediates has actually been isolated 

 as such from animal tissues. Hence, numerous attempts have been 

 made to arrest metabolism at an intermediate stage by inducing an 

 unphysiological state (diabetes, starvation, etc.) or by administration 

 of substances which contain groupings refractory to attack by animal 

 cells. The latter approach, of which the phenyl substituted amino 

 acids and fatty acids employed by Knoop are the most notable 

 examples, represents an early attempt to tag molecules in order to trace 

 them through the animal body. 



A large part of the accepted knowledge of intermediary reac- 

 tions, particularly of carbohydrates and amino acids, has been gained 

 from studies with surviving tissues, tissue preparations, and isolated 

 enzymes. The in vitro techniques deal with reactions which the 

 enzyme systems of cells are able to carry out with an added substrate. 

 The result, although not the primary aim, of this type of experimenta- 

 tion, has been to provide knowledge of the organic chemistry of bio- 

 logical compounds in a "physiological" medium and under the in- 

 fluence of biocatalysts. The use of isolated tissues involves inherent 

 limitations as far as permitting a decision as to whether certain re- 

 actions occur in the living organism. The question as to whether 

 and to what extent an in vitro reaction is part of the normal metabolism 

 must be decided by an independent method. The fact that, for ex- 

 ample, surviving liver and kidney, or enzymes isolated from these 

 tissues, deaminate ^-amino acids at a rapid rate in no way signifies 

 that the cell normally has to deal with the unnatural amino acids. 

 Evidence that the amino acids of ^^-configuration are not normal con- 

 stituents of the animal cell is contained in the following experiment 

 (20). When fl',/-tyrosine or fl',/-glutamic acid labeled with N^^ was 

 administered to rats, the unnatural acids excreted in the urine had an 



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