INHIBITIONS BY NUCLEOTIDES 477 



nucleotide is a factor in the binding. However, the binding energy does 

 not always increase with increase in phosphate residues or total negative 

 charge; the potencies of the inhibition of pig kidney phosphodiesterase are 

 AMP > ADP > ATP. Little is known about the topography of enzyme 

 sites for nucleotides but it is clear that multiple binding sites must be in- 

 volved. The relative binding energies for competitive inhibitors of yeast 

 pyridoxal kinase (see tabulation) (Hurwitz, 1953) show that here the purine 



component is of primary importance, addition of ribose or phosphate res- 

 idues having little effect. Yet ITP inhibits while inosine does not, and 

 pyrophosphate is bound fairly tightly to the enzyme. The phosphate resi- 

 dues in ADP must not be oriented for interaction as optimally as free 

 pyrophosphate. 



Azaguanine and Azauracil 



The 8-azapurines are usually inhibitory to growth and this has been gen- 

 erally attributed to the incorporation of these analogs to form abnormal 

 polynucleotides which are nonfunctional or inhibitory. However, it has more 

 recently been found that these analogs or their immediate metabolic prod- 

 ucts are potent inhibitors of certain enzymes involved in purine metab- 

 olism, and it is possible that such actions contribute to the growth de- 

 pression. 8- Azaguanine has been studied the most thoroughly and has been 

 shown to inhibit the growth of many bacteria, fungi, algae, viruses, tissue 

 culture cells, chick embryos, epithelium, and tumors. It is usually antag- 

 onized by guanine or guanidylate, and occasionally by other purines, nu- 

 cleosides, and nucleotides. The intention is not to discuss these azapurines 

 in detail since it is a very large subject but to mention only a few observa- 

 tions bearing on enzyme inhibition. 



Adenosine deaminase is inhibited reversibly by 8-azaguanine {K, = 0.28 

 mM) and the inhibition was stated to be noncompetitive, although the 

 l/f-l/(S) plot appears to indicate uncompetitive or coupling inhibition 

 (Feigelson and Davidson, 1956 b). Xanthine oxidase is also strongly inhi- 

 bited (Table 2-2). Unfortunately, very few enzymes operative in purine 



