944 ANTIMETABOLITES AS MITOTIC POISONS I3 



this effect of aminopterin, but only if it is applied 24 h. beforehand. Hence the 

 antagonism may depend on conversion of the citrovorum factor to an active agent. 

 The antagonism succeeds with chick osteoblasts and acute leukemic cells of the 

 mouse, but not with normal bone marrow of the mouse. It was concluded that 

 the citrovorum factor or a derivative of it contributes significantly to the early 

 anaphasic separation of sister chromosomes. The antifolics may be held to dis- 

 place this active factor, which participates in the synthesis oftheRNA-like material 

 that is given off from the chromosomes at anaphase and that may be concerned in 

 contraction of the chromosomal fibers, elongation of the spindle-body, polar 

 bubbling, and equatorial cleavage. 



(b) Nucleic acid components and analogues 



The methylated xanthines were the first purines investigated for their effects on 

 the second half of cell division. Gosselin (1940) treated rootlets of peas and oats 

 with 1 : 1,000 caffeine or theophylline for 1-6 days and observed that binucleate 

 cells were sometimes produced because cell walls were not formed. Some of the 

 polyploid cells resulting from nuclear fusion had non-functional giant spindles. 

 Mangenot and Carpentier (1944) also found that caffeine and theophylline at 2 

 and 10 xixM concentrations do not arrest mitosis in the manner of colchicine in 

 plant root tips, but that a single irregular mitotic figure may fonn from the two 

 nuclei of a binucleate cell if the nuclei were joined by chromatin bridges from the 

 previous mitosis. Nakahara (1952) observed that caffeine tends to inhibit separa- 

 tion of chromosomes in early anaphase but not to affect elongation of the spindle 

 in grasshopper germ cells. 



The suppression of plant cell wall formation by caffeine and theophylline was 

 confirmed by Kihlman and Levan (1949). The N-alkylated oxypurines also do 

 this (Kihlman, 1949). In addition, clear tendencies to a colchicine-like metaphase 

 arrest were found with caffeine and theophylline in such strong concentrations as 

 I % (Kihlman and Levan, 1949). These tendencies were pronounced in sub- 

 stituted xanthines having the methyl groups replaced by larger alkyl groups, which 

 decreased the water-solubility. The results were therefore in agreement with the 

 Levan and Ostergren (1943) theory of narcosis. 



Adenine also causes metaphase arrest at 20 and 40 mM concentrations in onion 

 root tips (Kihlman, 1950a). However, the low relative lipoid-solubility of ad enine 

 suggests that this effect may be more specific than a simple narcosis (Kihlman, 



1952). 



With less extreme concentrations on chick tissue cultures, Hughes (1952c) 

 found that adenine, 2,6-diaminopurine, and benzimidazole prevent cleavage and 

 tend to inhibit anaphasic movement of chromosomes. Concentrations as low as 10 

 [j,g/ml of 6-[^-indolylethylaminopurine have a similar effect (Lettre and Endo, 

 1956). The adenine analogue, 4-aminopyrazolo-(3,4-d) pyrimidine, at o.i mM 

 and up causes abortive metaphases in cultures of the human carcinoma, HeLa 

 fHsu, Robins and Cheng, 1956). 



Adenine produces this effect in Vicia root tips at concentrations of 2 and 5mA/ 

 (Woll, 1953). Metaphase arrest also results from treatment with i and 4% solu- 

 tions of sodium ribose nucleate. Von Olah (1952) also observed this in onion roots. 



