The Mechanism of Action of Methyl Xanthines in Mutagenesis 141 



mutation probability? We have proposed (12) that this may result from a 

 change in the steady slate concentrations of the intennediates that are to be 

 assembled together to form the macromolecular DNA. This must happen 

 without any change in the flow of intermediates, in accord with the experimental 

 fact that the growth rate of the bacteria is not affected significantly by the 

 mutagens when present at concentrations that give rise to large changes in 

 the mutation rate (1). 



Let us first consider the consequences of lowering of the concentration of 

 whatever adenine deoxyriboside or guanine deoxyriboside derivative is 

 involved in the polymerization reaction leading to macromolecular DNA. The 

 Watson-Crick model for DNA assumes that the specificity lies in the forma- 

 tion of two or three hydrogen bonds between specific pairs of nucleotides: 

 adenine and thymine, and guanine and cytosine. It has been suggested by 

 Watson and Crick (14) that the mutational event is the entry of a heterocylic 

 base which is not complementary. This would yield a double helix which 

 is energetically less stable. Upon subsequent duplication this yields two stable 

 molecules, one of the parental type and one of a new mutant type. 



guanine thymine 



Fig. 4. 



It is to be recognized that the mutational event is an improbable one, and 

 therefore quite improbable structures may be involved. Two options for the 

 unfavorable pairing are available. First, two pyrimidines or two purines may 

 become situated opposite each other. This gives structures that should be 

 capable of forming hydrogen bonds, but are either too long or too short. 

 Alternatively, a purine and a pyrimidine may pair, but the purine may occur 

 in the uncommon tautomeric form and consequently pairing will occur abnor- 

 mally. Watson and Crick (14) suggested adenine in the lactim form binding 

 with cytosine, more probable is the pairing of guanine with thymine (Fig. 4). 

 This pair has the proper dimensions; there are no steric difliculties. In this 

 structure guanine is written with the oxygen in the 6- position in an enol form. 

 X-ray-diffraction workers have concluded that guanine is ordinarily found in the 

 keto form, but the evidence is not strong that the keto form is even dominant 

 (15), and considerations of the resonance possibilities indicate a considerable 

 stabilization of the enol fonn because the latter allows aromaticity of the 

 heterocyclic ring. 



Thus, guanine-thymine pairing might well be of likely occurrence. With 

 this in mind, we have attempted in our enzyme studies to find differences of 

 the effects of mutagens on the inhibition of reactions of the adenine compounds, 

 as opposed to the guanine ones, that would be implied if this structure were to 



