INHIBITIONS BY NUCLEOTIDES 



479 



However, 5-fluoroorotate blocks earlier in the sequence, an inhibition of 

 dihydroorotase, which forms dihydroorotate by cyclization of carbamylas- 

 partate, having been observed (Smith and Sullivan, 1960). Orotate also 

 inhibits but more weakly, this being an example of negative feedback. 

 Orotidylate decarboxylase which forms UMP is inhibited by UMP, although 

 not by uridine, UDP, or UTP, and it is possible that 5-FUMP would also 

 inhibit at this locus (Blair and Potter, 1961). The conversion of dUMP to 

 TMP by thymidylate synthetase, which is phage-induced in E. coli, is very 

 potently inhibited by 5-F-dUMP, K^ being around 0.00005 mM, and follow- 

 ing a competitive phase there is irreversible reaction with the enzyme (Ma- 

 thews and Cohen, 1963). This illustrates the principle that analogs often 

 simulate feedback inhibition if they are structurally similar to the com- 

 pound normally exerting the inhibition. Some of the inhibitions observed 

 in pyrimidine nucleotide metabolism are shown in the following scheme 

 modified from Smith and SuUivan (1960). 



A + CP 



(FCMP, CTP) 



CA 



(O, FO) 



DHO 



DNA- 



(FdUMP) 

 TMP -« — K dUMP 



)( (FO) 



OMP 



X (UMP) 



UMP 



-UDP 



(FUMP, 

 FUDP) 



X * 



UTP 



CTP 



]<. (FUMP, FUDP) 

 RNA 



(A = aspartate, CP = carbamyl-P, CA = carbamylaspartate, DHO = di- 

 hydroorotate, = orotate, OMP = orotidine-5'-P, and fluorinated analogs 

 are indicated by an inital F). 



The effects of 5-fluorouTacil on protein synthesis in E. coli and B. mega- 

 teriimi are very interesting because total protein synthesis is not altered 

 significantly but the proteins formed have abnormal amino acid compo- 

 sitions (Gros and Naono, 1961). For example, the proteins contain less 

 proline and tyrosine but more arginine. The alkaline phosphatase has nor- 

 mal catalytic activity but is less thermostable, whereas a-galactosidase 



