The Mechanism of Action of Methyl Xanthines in Mutagenesis 145 



removal of adenine deoxyriboside is to be maintained constant and determined 

 solely by the process of removal, then a steady-state will quickly ensue in which 

 (S) oc (/), and in which the rate of formation of / is dependent only on the rate 

 of utilization. The concentration of / will become adjusted to estabHsh such a 

 condition. 



In the presence of the mutagen, the total inhibitor is effectively derived from 

 three sources; deoxyribosides, free normal bases, and the mutagen. While 

 maintaining constant synthesis of DNA, the effect of the mutagen will then be 

 to decrease the level of the normal reaction product, adenine. Similar relations 

 will hold for guanine deoxyriboside. 



It should be noted that in this case, although not in the case considered 

 above, any number of intermediates may occur between the step under considera- 

 tion and the polymerization step, if these reactions are rapidly reversible. Then 

 a change in adenine deoxyribose concentration will lead to a proportional 

 change in the precursor immediately used for the formation of the macro- 

 molecule. 



This model can then utilize the enzymatic finding, and the biological facts. 

 There is, however, one additional fact that should be introduced, viz. certain 

 specific substances, the purine ribosides (26), are anti-mutagens. That is, these 

 substances will prevent the action of caffeine and related compounds in causing 

 mutations. Moreover, they will decrease the so-called 'spontaneous mutation' 

 rate. 



This can be tentatively explained on the basis that these substances are 

 substrates or immediate precursors of the substrates of the key step, and that 

 their increase simply affects the system so as to cause an increase in the concen- 

 tration of purine deoxyribosides and thus a decrease in the mutation rate. 



VII. ALTERNATIVE HYPOTHESES 



In concluding, I should like to list various hypotheses that one should consider 

 in this type of chemical mutagenesis. They will be considered in order of the 

 intimacy of the mutagen with the duplication process. 



1. The mutagen is incorporated into the nucleic acid. This is tentatively 

 rejected as indicated above, from the tracer evidence, and the argument that 

 methylation in the imidazole ring prevents A^-glycoside formation. It should 

 be noted that production of a self-duplicating 'methylated gene' can be rejected 

 because the mutants cannot metabolize methyl purines and certainly do not 

 require them (3). 



2. The mutagens inhibit enzymes of nucleic acid biosynthesis, and this causes a 

 change in the concentration of intermediates. This latter effect changes the 

 probability of mutation. This is the hypothesis we favor, but it is clear that a 

 great deal of work will be required to establish it or some variant thereof. It is 

 also clear from what has been said above that special circumstances must occur 

 in order that the proposed mechanism can work. 



3. The mutagen causes some change in the general metabolism of the organism 

 and this leads to a change in the mutation probability. It is certainly true that 

 the mutation probability is dependent on a great many factors. Kihlman (27, 



