SYNTHESIS OF PHENYLALANINE AND 

 TYROSINE IN YEAST 



KONRAD BLOCK 



The advances of biochemical knowledge which are attribu- 

 table to the tracer technique are nowhere as evident as in the 

 area which deals with the origin of organic molecules in 

 biological systems. The use of labelled molecules must 

 surely be credited for whatever limited information is at 

 hand on subjects such as the biosynthesis of steroids, por- 

 phyrins, purines, etc. The success which has been attained 

 should however not obscure the fact that most of the findings 

 in this field were due to accidental observations and not to 

 rationally designed experiments. 



The experiments which I would like to discuss today illus- 

 trate the power of the tracer technique, but also underscore 

 the highly empirical nature of present-day biochemical 

 research. 



During recent years the role of acetic acid as a building 

 block for heterocyclic and alicyclic systems has been well 

 established (Bloch, 1947). In particular, good evidence 

 exists that the cell uses acetate as the principal carbon source 

 for the polynuclear as well as for the aliphatic moiety of the 

 sterols (Bloch and Rittenberg, 1942). Working on the 

 assumption that the study of simple monocyclic compounds 

 might aid in clarifying the mechanism of synthesis for the 

 more complex polycyclic steroids. Dr. Gilvarg in our labora- 

 tory has investigated the biosynthesis of the amino-acids 

 phenylalanine and tyrosine, two readily accessible benzene 

 derivatives. I may anticipate here the final conclusions of 

 this research by saying that the biosynthesis of benzene rings 

 proved to be totally unrelated to that of the ci/c/o-pentano- 

 phenanthrene derivatives. In this sense these experiments 



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