39. ANTIMETABOLITES AND NUCLEIC ACID METABOLISM 525 



VII. Concluding Remarks 



Demonstration of the activity of many agents which alter the synthesis 

 of the nucleic acids was accomplished through tests made with living organ- 

 isms ; however, assignment of a direct role in the metabolism of nucleic acids 

 has required further elucidation of their action on isolated systems and ulti- 

 mately with purified enzymes. Such an approach, as empirical and perhaps 

 "therapy oriented" as it may seem, has nonetheless made many contribu- 

 tions to our understanding of the normal pathways involved in the over-all 

 synthesis of the nucleic acids. Pertinent examples are to be found in the ac- 

 cumulation, in the presence of certain antimetabolites of folic acid or of 

 glutamine, of intermediates in the biosynthesis of purine nucleotides. In- 

 deed, detailed study of the sites of action of these and other inhibitors has 

 given impetus to the elucidation of the enzymic reaction mechanisms which 

 these inhibitory compounds influence, and even the chemical nature of the 

 active centers of the enzymes affected, since the bonds between the an- 

 tagonists and the enzymes may be exceedingly firm. Gradually information 

 is being provided which eventually could permit a much more rational de- 

 sign of compounds constructed for the purpose of specific reaction with se- 

 lected enzymes. To attain such ends it is evident that there is need for ex- 

 pansion of our knowledge of the physicochemical properties of analogs and 

 their corresponding metabolites, and particularly of the active centers of 

 enzymes. Thus, the pKa values of many of the effective pyrimidine anti- 

 metabolites are much lower than those of the normal metabolites which they 

 resemble. Unlike the normal compounds, such analogs exist, at physiological 

 pH values, to a significant extent as negatively charged ions, and such modi- 

 fications of ionic structure must alter markedly the affinity of these com- 

 pounds for certain enzymes; yet, in some cases, their extensive anabolism 

 to nucleotide derivatives is permitted; in turn, the affinity of the latter for 

 appropriate enzymic loci is distorted markedly. 



It remains to be established that the action of a number of the agents 

 discussed is directly on the synthesis or function of nucleic acids. Although 

 many cases may be cited in which these compounds cause alterations in 

 nucleic acid metabolism, such observations do not always permit these ef- 

 fects to be distinguished from corresponding changes in other parameters of 

 cellular function. Similarly, it is easy to confuse a given mechanism of re- 

 sistance, deduced by studies either in vivo or in vitro, with the mechanism 

 of action of the compound. This is particularly apparent with such com- 

 pounds as 5-fluorouracil and 6-azauracil, which are relatively inert until 

 converted to nucleotide derivatives, and which, therefore, must experience 

 several enzymic alterations prior to exerting their primary inhibitory ac- 

 tivity. Thus, a major reduction in the efficiency of any one of these enzymic 

 reactions can render the analog inactive, but disclosure of this enzymic de- 



