INHIBITION BY NUCLEOTIDES 481 



1962), and other enzymes are inhibited in similar manner but each exhibits 

 a unique pattern (Table 2-28); while TTP inhibits several steps in its for- 

 mation, including CDP -^ dCDP, deoxyuridine -^ dUMP, and deoxythy- 

 midine -> dTMP (Ives et al., 1963). All of these inhibitions and many more 

 constitute possible feedback situations, but in the cell probably only a few 

 are important, since the concentrations of some intermediates may never 

 rise sufficiently to exert an effect, and compartmentalization may limit the 

 actions of these inhibitors. We have mentioned that certain enzymes ap- 

 pear to contain sites specially evolved for feedback inhibition (Gerhart and 

 Pardee, 1962, 1964), the best documented case being aspartate transcarba- 

 mylase, which is inhibited particularly well by CMP, CDP, and CTP. This 

 enzyme is normally a tetramer and it may be that these inhibitors alter the 

 subunit interactions since the monomer is not inhibited. Another interesting 

 example of this phenomenon is the inhibition of xanthosine-5'-P aminase 

 by psicofuranine (9-D-psicofuranosyl-6-aminopurine), which occurs in two 

 steps, a reversible pyrophosphate-dependent reaction with the enzyme and 

 an irreversible xanthosine-5'-P-dependent reaction (Udaka and Moyed, 

 1963). The first step can be observed in a psicofuranine-resistant bacterial 

 strain and here the inhibition is noncompetitive. It would appear that the 

 inhibitor is bound to a different site than that at which the substrate reacts 

 and this second site could have regulatory function. 



Some interesting results have been obtained in the analysis of the inhi- 

 bitions produced by the metabolites of 6-mercaptopurine, a few of which 

 will be mentioned briefly. One product into which 6-mercaptopurine is con- 

 verted is 6-thio-IMP, a potent competitive inhibitor of IMP dehydrogenase 

 (which is involved in the formation of GMP in certain cells) {K^^ = 0.0036 

 mM) (Atkinson et al., 1963). The inhibition proceeds rather slowly, requir- 

 ing 10-20 min for completion, and the enzyme is then inactivated (Hamp- 

 ton, 1963). Evidence was presented that reaction occurs with an SH group 

 on the enzyme, a stable disulfide bond being formed with the 6-thio-IMP. 

 This is one example where an analog turns out to be an SH reagent. On the 

 other hand, adenylosuccinate lyase is inhibited by 6-thio-IMP only if a 

 metal ion is present and it was postulated that the metal ion forms a bridge 

 between the SH groups (E-S-Me-S-IMP) (Bridger and Cohen, 1963). These 

 inhibitions create new possibilities by which analogs can inactivate enzymes. 

 Another product derived from 6-mercaptopurine is 6-mercaptopurine ribo- 

 side-5'-diphosphate, which inhibits polynucleotide phosphorylase quite po- 

 tently (50% inhibition by around 0.03 mM), rapidly, and competitively 

 (Carbon, 1962). The role this enzyme plays in vivo or the significance of 

 such inhibition is not known. 



These few remarks on the effects of nucleotides and related substances 

 are made only to suggest certain interesting aspects of enzyme inhibition 

 which broaden our concepts of how analogs may act; adequate coverage of 

 this subject, young as it is, would require a volume of this size or more. 



