STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 153 



uridylic acid (uracil ■ ribose - 5' - P) -j- pyrophosphate (PP) ?=^ 



uracil -f 1 - P - P - ribose - 5- P 

 P - P - ribose - P + adenine ^ PP + adenylic acid (adenme - ribose - 5' - P) 



In a variant of this scavenging reaction, if uridylic acid has been hydro- 

 lyzed to the nucleoside, uridine, the ribose portion can also be shuttled about 

 by a phosphorylytic mechanism which can conserve the special properties of 

 the ribosyl linkage. Thus: 



uridine (uracil - ribose) + phosphate (P) ^ uracil + 1 - P - ribose 

 1 - P - ribose + adenine ^ P + adenosine (adenine - ribose) 



In this case, ATP is necessary in the conversion of the nucleoside, adenosine, 

 to the nucleotide via an adenosine kinase: 



adenosine + adenosine triphosphate (ATP) > adenosine monophosphate (AMP) + 



adenosine diphosphate (ADP) 



It is of the utmost interest that many compounds, e.g., the tri- and 

 diphosphates of the ribose nucleosides, important in the activation of other 

 molecules, are themselves the activated intermediates of polymer synthesis. 

 It follows, therefore, that by affecting the utihzation of one of these com- 

 pounds it is sometimes very difficult to separate relatively distant areas of 

 metabohsm. For example, the synthesis of cerebroside in brain tissue 

 requires galactose derivatives, the activation of which requires uridine 

 triphosphate and other uridine derivatives. Inliibition of ENA synthesis by 

 the use of some uracil analog may well affect cerebroside formation, as well 

 as polysaccharide synthesis and protein synthesis. Furthermore, since uracil 

 is the precursor for cytosine, whose nucleotide derivatives are active in 

 phosphohpid biosynthesis, an effect may also be obtained in this area of 

 metabohsm. It is evident, then, that problems of polymer biosynthesis 

 cannot be too readily compartmentahzed as a consequence of the existence 

 of exclusive systems of reactions. Quite the contrary, we are confronted by 

 an interpenetratmg network of related reaction systems whose existence 

 compels the development of the most detailed information, if the phenomena 

 of cellular biosyntheses are to be imderstood and are to be controlled. If such 

 information is available, it may become possible to make use of the special 

 properties of these reaction systems which relate to specific aspects of 

 metabohsm. 



A. Phosphate Transfer 



By far the major part of activation reactions mvolves the utihzation of 

 phosphate compounds. The role of inorganic phosphate was di.scovered by 

 Harden and Young (1905) in the fermentation of glucose in yeast extracts. 

 Since that finding, it has been learned that almost aU, if not aU metabohtes, 

 are formed or function as phosjihate derivatives or in association with such 



