122 INTERMEDIARY METABOLISM AND GROWTH I 



cumulation of precursors can be prevented by adding to the culture medium an 

 excess of the required metabolite. Apparently, a later product of a biosynthetic 

 sequence tends to inhibit one or more steps in the formation of earlier members 

 of the same sequence or metabolites of parallel chains. Examples of this negative 

 feedback mechanism have been observed in connection with amino acid, purine, and 

 pyrimidine synthesis (Yates and Pardee, 1956). 



The experiments of Ochoa, Kornberg, Roseman, and Zamecnik and their 

 associates are prototypes of studies which will enhance our understanding of 

 nucleoprotein and mucoprotein synthesis. In the past, heterogeneous mixtvires 

 of these macromolecular substances have been investigated. As time goes on, there 

 will be an increasing tendency fruitfully to investigate the regulation of the 

 biosynthesis o^ specific proteins having catalytic functions. 



The elucidation of metabolic pathways and the purification of enzymes will 

 continue; however, we may anticipate an accelerated application of the facts of 

 intermediary metabolism to rational chemotherapeutics and to problems of 

 hereditary metabolic diseases, including cancer. 



