II PREPROPHASIC INHIBITION 935 



tentiated by insulin and by a-lipoic acid (Biesele, 1958a). Insulin is known to 

 increase the consumption of glucose and oxygen and the synthesis of nucleic acid 

 and protein in tissue cultures (Paul and Leslie, 1954), as well as the incorporation 

 of exogenous labeled adenine into nucleic acids (Lu and Winnick, 1954). Hence, 

 insulin perhaps increases incorporation of 6-mercaptopurine into nucleic acids, 

 and the resultant defective nucleic acids could account for some of the toxicity 

 and mitotic inhibition. An additional indication of the uptake of 6-mercaptopurine 

 into chromosomal nucleic acid is furnished by the increased frequency of chromo- 

 somal breakage when insulin or lipoic acid is added with 6-mercaptopurine to the 

 medium (Biesele, 1958a). 



(d) Nucleosides 



The more toxic among 28 purine nucleosides studied in cultures of mouse 

 tissues corresponded closely in structure to adenosine (Biesele, Berger and Clarke, 

 1952). Ribofuranosyl compounds are more likely to show activity than are 

 glycopyranosyl compounds. 



Adenosine itself may influence mitosis adversely. Thus it has been found to in- 

 hibit cultivated chick cells from entering prophase (Hughes, 1952c). Adenosine 

 at 1.5 vaM progressively decreases the number of mitoses in mouse sarcoma 180 

 cultures over two days (Biesele, Berger, Clarke and Weiss, 1952). 



Several substituted adenosines have interesting effects on mitosis in mouse 

 tissue cultures (Biesele, Berger, Clarke and Weiss, 1952). Crotonoside, or 2- 

 hydroxyadenosine, at 0.2 raM selectively inhibits mitosis in embryonic skin cul- 

 tures, but 2-methyladenosine is more inhibitory to divisions of sarcoma 180 cells 

 than to embryonic skin cells at 4 and i mM. Two days exposure to 2 mM 2-ami- 

 noadenosine eliminates mitoses from both embryonic skin cultures and sarcoma 

 180 cultures. This agent also inhibits Ophiostoma growth at 0.003 to 0.03 mM 

 concentrations, but adenosine at 0.5 or 2.0 mM blocks this inhibition (Fries, 1955) 



The nucleoside of unsubstituted purine, or 9-[3-D-ribofuranosylpurine, is highly 

 active (Biesele, Slautterback and Margolis, 1955). Two days exposure to as little 

 as 0.0025 rnM results in a mitotic incidence only one third of normal in mouse 

 tissue cultures, with sarcoma 180 cells showing greatest sensitivity and embryonic 

 skin epithelial cells showing greatest resistance. By contrast, 9-p-D-ribopyranosyl- 

 purine is almost innocuous (Biesele, 1957). 



In antagonism studies, the mitotic inhibition and nuclear degeneration caused 

 by 0.0 1 mAf 9-[3-D-ribofuranosylpurine in mouse tissue cultures were found to be 

 blocked in part by concentrations about one hundred times greater of adenosine 

 or physiological substances containing the adenosine-5'-phosphate structure 

 (Biesele, Slautterback and Margolis, 1955). This may be related to the formation 

 of mono-, di-, and triphosphates of this nucleoside after its injection into rats 

 (Gordon and Brown, 1956). These authors suggested that the toxicity of 9-[3-d- 

 ribofuranosylpurine in vivo probably results from the formation of coenzyme ana- 

 logues containing it. Gordon and Brown (1956) also found this nucleoside gives 

 rise to polynucleotide adenine and guanine when injected into rats but is not 

 incorporated unchanged into nucleic acid. 



The 6-substituted purine nucleosides are in general more toxic than their free 



Literature p. Q4y 



