g40 ANTIMETABOLITES AS MITOTIC POISONS I3 



0.5 mM adenine sulfate or with about 0.05 mM diaminopurine. The purines in 

 these concentrations individually produce about 30-40% abnormal postmeta- 

 phasic figures after two days exposure. 



Adenine often causes chromosomal "erosion", with occasional segments poorly 

 stained or poorly coiled along the chromosome (Kihlman, 1952). Maleic hydra- 

 zide, which can be construed to be structurally analogous to uracil, also "erodes" 

 chromosomes and causes other cytological effects resembling those of adenine. 

 It is considerably more active than 8-ethoxycaffeine. The question of whether 

 maleic hydrazide acts as an antimetabolite antagonist of pyrimidines is unresolved, 

 however. Damage by maleic hydrazide to vetch chromosomes is not blocked by 

 uracil, thymine, or orotic acid (Loveless, 1953). Uracil blocks neither maleic 

 hvdrazide action on onion chromosomes (Deysson and Deysson, 1953) nor their 

 fragmentation by barbital (Deysson and Rollen, 1951). Uracil itself breaks onion 

 chromosomes (Deysson, 1952), and this effect is not blocked by thymine (Deysson, 

 1954). Although thymine also fails to block chromosomal "erosion" in onion 

 roots by 5-aminouracil (Duncan and Woods, 1953), cytidine sulfate does antagon- 

 ize this effect (Martinez Pico and Duncan, 1953). 



\^arious other analogues of physiological purines have also been tested on 

 chromosomes. 2-Aminopurine at i and 4 mM concentrations causes chromosomal 

 bridging and fragmentation in cultivated mouse cells. Although 7-methyladenine 

 also has some activity, neither 2-methyladenine, 9-methyladenine, nor 2-chloro- 

 adenine appear to have any (Biesele, Berger, Clarke and Weiss, 1952). 



Unsubstituted purine increases the incidence of chromosomal breaks and 

 bridges in sarcoma 1 80 cultures, and is blocked by adenine in low concentrations 

 (Biesele, Slautterback and Margolis, 1955). Thioguanine also causes chromo- 

 somal breakage, and 6-chloropurine causes agglutination and occasional failure 

 of anaphasic separation of the chromosomes in addition, but 6-mercaptopurine 

 causes little obvious damage to the chromosomes unless it is supplemented with 

 insulin or lipoic acid, provided that coenzyme A is not added (Biesele, 1958a). 

 These results have been interpreted to mean that chromosomal breakage occurs 

 in the presence of 6-mercaptopurine when the conditions provoke an increased 

 uptake of exogenous purines into nucleic acid (Biesele, 1958a). However, Kihl- 

 man (1955a) has objected to the hypothesis that chromosome breakage by purines 

 resvdts from an effect on nucleic acid synthesis because of biochemical antagonism 

 between caffeine and such apparently unrelated substances as tryptophan, pro- 

 caine, or aniline and because of the failure of 8-azaguanine to break chromosomes 

 in onions. It may be mentioned, however, that 8-azaguanine incorporation appears 

 to be primarily into RNA, not DNA (Lasnitzki, Matthews and Smith, 1954). 



Chromosome breakage by 8-ethoxycaffeine shows an interesting parallel to that 

 by X-rays (Kihlman, 1955b). 8-Ethoxycaffeine does not break onion root chro- 

 mosomes in the absence of oxygen, air, or hydrogen peroxide, and under aerobic 

 conditions activity is suppressed by cyanide, azide, or dinitrophenol. Hence the 

 conclusion was drawn that compounds such as adenosine triphosphate containing 

 energy-rich phosphate bonds may be involved in the breakage of chromosomes 

 by 8-ethoxycaffeine. Moreover, chromosomal stickiness is affected in the same 

 manner as breakage (Kihlman, 1955c). 



