478 2. ANALOGS OF ENZYME REACTION COMPONENTS 



metabolism have been examined with respect to 8-azaguanine inhibition, 

 or to inhibition by nucleosides and nucleotides of 8-azaguanine, but this 

 type of mechanism for the growth inhibition must be borne in mind. 8- 

 AzaGTP is formed from 8-azaguanine and can serve as a substrate for 

 adenylosuccinate synthetase; it also inhibits competitively with respect to 

 GTP (Cohen and Parks, 1963). Here one sees the interesting situation in 

 which 8-azaGTP stimulates the rate when GTP is low and inhibits the 

 rate when GTP is high. It was pointed out that in such cases one might 

 obtain selective inhibition of an enzyme in tissues having a relatively high 

 substrate concentration. This behavior is characterized in the double re- 

 ciprocal plot by the curve for the inhibited reaction crossing the curve (or 

 straight line) for the uninhibited reaction and being nonlinear. 8-Azaguanine 

 suppresses the induction of liver glucose-6-phosphatase, fructose-l,6-diphos- 

 phatase, and tryptophan pyrrolase in the rat (Kvam and Parks, 1960) and 

 the formation of catalase in yeast (Bhuvaneswaran et al., 1961). These ef- 

 fects on protein synthesis may relate to an interference with nucleic acid 

 metabolism, but whether it is a general depression or a more specific block 

 is not known. Studies on 8-azaguanine in animals are complicated by the 

 rapid deamination to the relatively noninhibitory 8-azaxanthine so that 

 only a fraction of the quantity fed is available for either inhibition or in- 

 corporation (Mandel, 1955). Thus the usefulness of 8-azaguanine in tumor- 

 istasis is limited. 



6-Azauracil is also generally growth-inhibiting and tumoristatic. It is 

 possible that 6-azauridine-5'-P (6-azaUMP) is the true inhibitor, since it 

 has been shown that orotidylate decarboxylase is strongly inhibited by 

 6-azaUMP, leading to the accumulation of orotidylate (Handschumacher 

 and Pasternak, 1958; Pasternak and Handschumacher, 1959). 6-Azauridine 

 is metabolized to 6-azaUMP and inhibition of the decarboxylase after ad- 

 ministration of 6-azauracil was demonstrated, so it is likely that the block 

 in pyrimidine metabolism is at this point and that this is an important 

 mechanism in the tumoristatic action. The inhibition of the decarboxylase 

 is characterized by a K, of 0.00075 mM (Handschumacher, 1960). It is 

 interesting to speculate that a similar mechanism might be involved in the 

 action of 8-azaguanine. 



Fluoropyrimidines and Feedback Inhibitions in Pyrimidine Pathways 



The fluoropyrimidines are among the most potent inhibitors of nucleic 

 acid biosynthesis yet discovered but the sites of action have not been com- 

 pletely elucidated. The 5-halogen analogs of orotate inhibit the conversion 

 of orotate to the uridine phosphates, the most active being the fluoro com- 

 pound (Stone and Potter, 1957). It was suggested that some of the action 

 could be due to nucleotide analogs formed from these, and it was shown 

 that 5-fluoroorotate is converted to 5-FUMP in yeast (Dahl et al., 1959). 



