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SOME ASPECTS 



OF INTERMEDIARY 



METABOLISM 



KONRAD BLOCH, associate in biochemistry, college of 



PHYSICIANS AND SURGEONS, COLUMBIA UNIVERSITY 



/: 



'NTERMEDIARY metabolism is a branch of biochemistry 

 which attempts to trace the pathways of food components 

 in the animal body, with the ultimate aim of describing the sequence 

 of the chemical reactions involved, and of providing a quantitative 

 account of food utilization. Ideally, the study of normal intermediary 

 metabolism should be carried out under conditions which do not of 

 themselves aflfect the physiological state of the animal. In order 

 to maintain the "normal" state, it is necessary to employ a diet on 

 which the weight of the animal and the composition of the excretion 

 products remain substantially constant. Because the animal cell is 

 unable to distinguish between the various isotopes of an element, 

 substitution of a normal dietary constituent by one which contains an 

 excess of an isotopic element, but which is otherwise the same, does not 

 disturb its physiological state. In order to illustrate the type of in- 

 formation which can be gained with the aid of isotopically labeled 

 substances, it will be useful to consider briefly the scope and the limita- 

 tions of other methods which have been applied to the study of inter- 

 mediary metabolic reactions. 



Chemical analysis of animal tissues can, of necessity, contribute 

 only limited information about intermediary reactions. It has been 



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