The Mechanism of Action of Methyl Xanthines in Mutagenesis 



139 



Although the tracer data dehneate the pathways, they do not define the 

 intermediates. It is, however, possible to conclude from available enzyme data 

 that 'adenine' and 'guanine' pools are made up at least in part of the free 

 bases themselves. This follows from the fact that the known enzymes of purine 

 metabolism which might be involved in the conversion of the hypothetical 

 'purine' precursor to the two types of nucleic acids catalyze reactions involving 

 the free purine base. The purine nucleoside hydrolases, purine nucleoside 

 phosphorylases, purine #-trans-glycosidases, and purine nucleotide pyro- 

 phosphorylases yield the free purine base. These enzymes and the postulated 

 pathway of direct reduction of the riboside to the deoxyriboside constitute 

 the only pathways of interconversion of ribose and deoxyribose purine com- 

 pounds that can be imagined at present. Since the reductive pathway is known 

 not to occur in E. coli (9) (although the interesting work from Volkin's labora- 

 tory may be relevant (10)), it appears quite likely that the free purine base is 

 involved in the 'adenine' and 'guanine' pools. 



In addition to these general considerations, the specific observation of 

 Lampen and Manson (11) that purine deoxyriboside phosphorylase is inhibited 

 by adenine led us to investigate the inhibition of phosphorylases of E. coli 

 by methyl xanthines. 



IV. ENZYMATIC INHIBITION STUDIES 



The main conclusion from these studies (12, 13) was that the organism 

 possesses enzymes, particularly nucleoside phosphorylases of both types 

 (ribose and deoxyribose), that are inhibited by purines generally but specifically 

 by the mutagenic substances. It was also found that even in the presence of 



10 20 



CAFFEINE CONC. (mM) 



30 



5 10 20 



CAFFEINE CONC. (mM) 



Fig. 3. The inhibition of purine nucleoside phosphorylase. 



The effect of caffeine concentration on the arsenolysis of adenine riboside is shown 



at the left, and on adenine deoxyriboside on the right. The systems contain 



arsenate to prevent the complication of back reaction. 



large amounts of inhibitors enzyme action was not completely repressed 

 (Fig. 3). In all cases this suggested the presence of more than one enzyme 

 catalyzing the reaction under study. Studies of the effect of pH and the separa- 

 tion of the bacteria into several chemical fractions supported this notion. 



