480 2. ANALOGS OF ENZYME REACTION COMPONENTS 



seems to be synthesized, as shown serologically, but is catalytically inactive. 

 An RNA fraction into which 5-fluorouracil is rapidly incorporated was de- 

 tected and it is possible that this is responsible for the changes in protein 

 synthesis. The halopyrimidines are very potent growth inhibitors but, al- 

 though much is known of their fate and actions (Brockman and Anderson, 

 1963), the over-all effects produced are usually so complex that the primary 

 sites of block have not yet been determined. These analogs can be metabol- 

 ized into such a variety of abnormal substances which can inhibit at many 

 different sites that it is likely no single mechanism for the growth inhibition 

 will be found. More investigations of the changes in the steady-state con- 

 centrations of the intermediates in these pathways, as they are affected by 

 the analogs, would be valuable in determining the important loci attacked. 

 A few examples of enzyme inhibitions which may be involved in feed- 

 back regulation of pyrimidine and purine metabolism will be cited because 

 of the importance of this type of inhibition in the control of nucleic acid 

 and protein synthesis.* Gots and Gollub (1959) described the suppression of 

 the formation of purine precursors in E. coli whenever a purine which sup- 

 ports growth is added, and the accumulation of 5-amino-4-imidazolecar- 

 boxamide is suppressed in certain mutants. Various purine analogs (e.g., 6- 

 mercaptopurine and 2,6-diaminopurine) act like the normal feedback inhibi- 

 tors, only more potently. These analogs can thus act on at least two sites 

 in the biosynthetic sequence, the formation of purine precursors and the 

 eventual utilization of the purines, their therapeutic usefulness possibly be- 

 ing related to this type of sequential inhibition. It may be noted, however, 

 that Rubin et al. (1964) have recently shown that sequential inhibition in 

 pyrimidine biosynthesis is not synergistic. Combinations of 5-azaorotate, 

 which competitively inhibits the conversion of orotate to orotidylate, and 

 6-azauridine, which after its metabolism to 6-azauridylate competitively in- 

 hibits the conversion of orotidylate to uridylate, do not produce greater 

 inhibitions in either isolated enzyme systems or leucocytes than are seen 

 with the individual analogs. Almost every step in the nucleotide synthesis 

 has been shown to be inhibited by more distal intermediates. The PRPP- 

 amidotransf erase, which catalyzes the first irreversible and specific step in 

 purine synthesis, utilizing glutamine as the amino donor, is inhibited by 

 AMP, ADP, ATP, GMP, GTP, other nucleotides, and some analogs (Wyn- 

 gaarden and Ashton, 1959); adenylosuccinate synthetase (Wyngaarden and 

 Greenland, 1963), aspartate transcarbamylase (Bresnick, 1963; Gerhart and 

 Pardee, 1962), phosphoribosylformylglycineamidine synthetase (Henderson, 



* Although an intermediate or product in a metabolic sequence is shown to be 

 an inhibitor of an enzyme catalyzing a previous step, it is perhaps not justified to 

 call it a feedback inhibitor, which implies that inhibition occurs during the in vivo 

 operation of the pathway. For various reasons the substance may not play a role in 

 regulating metabolism, even though it is a reasonably potent inhibitor. 



