932 ANTIMETABOLITES AS MITOTIC POISONS I3 



if the guanine residues in RNA, by analogy with the Watson and Crick (1953) 

 double-heUx model of DNA structure, are hydrogen-bonded to cytosine residues. 

 Greaser (1955a, 1955b) found that the inhibition of enzyme formation by 8-azagu- 

 anine in certain bacteria can be reversed with guanine, xanthine, or hypoxanthine. 

 A similar effect of 2,6-diaminopurine is partly reversed by hypoxanthine. Although 

 these effects may be related to inadequate function of RNA containing the 

 analogues, another mechanism may be suggested for the inhibition of formate 

 incorporation into the nucleic acids of mouse viscera by 8-azaguanine (Skipper, 

 Mitchell, Bennett, Newton, Simpson and Eidson, 1951), because this guanine 

 analogue partly inhibits transformation of folic acid into citrovorum factor (Doc- 

 tor and Trunnell, 1955). It is of interest that 8-azaguanine inhibits respiration 

 of sarcoma 37 in mice (Finkelstein, Winters, Thomas, Davidson and Smith, 1951) 

 over the same period in which it depresses mitosis (Finkelstein and Thomas, 1951). 



2,6-Diaminopurine has shown little selectivity toward neoplastic cells with 

 respect to mitotic inhibition, although it appears to be grossly rather more toxic 

 to sarcoma 180 cells than to skin fibroblasts in tissue culture of mouse embryo 

 (Biesele, Berger, Wilson, Hitchings and Elion, 1951). Mitosis is inhibited in both 

 cell types by about o.i mA/ 2,6-diaminopurine (Biesele, 1954b). The agent is too 

 toxic to the host animal to be therapeutically effective against mouse sarcoma 

 180 (Stock, 1950). The mitotic inhibition of cultured mouse cells caused by 2,6- 

 diaminopurine is readily reversed by adenine sulfate at the same or one-fourth 

 the concentration (Biesele, Berger, Clarke and Weiss, 1952). 



There are several examples of adenine-reversed inhibition of growth in plants 

 by 2,6-diaminopurine {e.g., Nickell, 1955). The inhibition of cell division in to- 

 bacco callus cultures by diaminopurine is reversed by adenine and slightly by 

 guanine and hypoxanthine (Miller, 1953). Vicia faba root cells are prevented 

 from entry into prophase by 0.096 raM 2,6-diaminopurine, but a competitive 

 reversal is achieved by adenine with an inhibition index of 0.5, perhaps because 

 there is a need for free adenine to initiate mitosis in this plant (Setterfield and 

 Duncan, 1955). This may be related to the observation that adenine sulfate pro- 

 motes growth of pea epicotyls and reverses inhibition by the plant hormone, in- 

 dole acetic acid (Howell and Skoog, 1955). 



2,6-Diaminopurine also appears to prevent the onset of DNA synthesis while 

 permitting that underway to continue, for treated Vicia meristem contains nuclei 

 with both the diploid and the doubled amounts of DNA, but not with intermediate 

 quantities (Setterfield and Duncan, 1955). The inhibition of DNA synthesis is 

 reversed by adenine, adenosine, or deoxyadenosine, indicating that the interference 

 may proceed by way of nucleoside or nucleotide metabolism. 



2,6-Diaminopurine is a precursor of nucleic acid purines and is utilized in a 

 manner paralleling the utilization of guanine (Bendich and Brown, 1948; Brown, 

 1955). A small portion of it may be incorporated as such into mouse nucleic acids 

 (Skipper, 1953). If 2,6-diaminopurine is a normal metabolite, it is possible that 

 physiologically excessive concentrations are inhibitory (Brown, 1955). However, 

 diaminopurine probably competes with adenine because they are incorporated 

 by the same pathways into similar but antagonistic derivatives, which are more 

 likely to be simpler nucleosides or nucleotide coenzymes than to be nucleic acids. 



