934 ANTIMETABOLITES AS MITOTIC POISONS 13 



centration of lo mM produces only moderate inhibition of mitosis in sarcoma i8o 

 cells but not in cultivated mouse skin fibroblasts. 



Much study has been devoted to the mechanism of action of 6-mercaptopurine 

 as an anti-cancer agent. Some of its actions can be explained on the basis of inter- 

 ference in nucleic acid metabolism. Various physiological purines, nucleosides, 

 and nucleotides block its toxic effects (Elion, Singer and Hitchings, 1954; Goldin, 

 Venditti, Humphreys, Dennis, Mantel and Greenhouse, 1954). 6-Mercaptopurine 

 inhibits de novo synthesis of nucleic acid (Skipper, 1954) and is itself incorporated 

 into nucleic acid in mice (Elion, Bieber and Hitchings, 1954). 6-Mercaptopurine 

 decreases the synthesis of purines in E. coli (Mandel, Inscoe, Maling and Smith, 

 1956), bvit does not affect the incorporation of exogenous adenine into the nucleic 

 acid of mouse tissues (Fernandes, LePage and Lindner, 1956). 



The partial mitotic inhibition caused in mouse tissue cultures by i.o vsxM 

 6-mercaptopurine is relieved to different extents by equimolar concentrations of 

 various physiological purines, nucleosides, and nucleotides (Biesele, 1954c, 1958). 

 These substances protect mouse embryonic skin cells more readily than sarcoma 

 180 cells against 6-mercaptopurine. 



Coenzyme A is a nucleotide that produces the opposite effect (Biesele, 1954c, 

 1955). This coenzyme at both 0.2 and 0.02 mM concentrations prevents the mitotic 

 inhibition of sarcoma 180 cells caused by i.o mM 6- mercaptopurine, but in the 

 case of embryonic skin cells higher concentrations are needed. 6-Mercaptopurine 

 and coenzyme A also have antagonistic effects on lipogenesis and mitochondrial 

 morphology in tissue culture (Biesele, 1955). Therefore mitotic suppression by 6- 

 mercaptopurine may result, in part, from its action as an antagonist of coenzyme 

 A, possibly with respect to functions of coenzyme A in the tricarboxylic acid cycle 

 and the oxidative phosphorylation (formation of high-energy phosphoric bond)'. 

 Perhaps 6-mercaptopurine interferes in a coenzyme A-regulated synthesis of a 

 hypothetical lipid essential for mitosis. 6-Mercaptopurine and thioguanine both 

 block acetylation of sulfanilamide in a coenzyme A system (Garattini, Morpurgo, 

 and Passerini, 1955). 6-Mercaptopurine acts as a preprophase poison in inhibiting 

 liver regeneration for the first day after partial hepatectomy of rats (Garattini, 

 Pacini and Settimi, 1955), and it also decreases the rate of cholesterol increase 

 in the regenerating liver (Garattini, Mor and Pacilli, 1955). 6-Mercaptopurine 

 inhibits the incorporation of acetate into lipid at lower concentrations than it 

 interferes in nucleic acid synthesis in E. coli (Mandel and Bolton, 1956). 



There are other indications that 6-mercaptopurine might influence purine 

 coenzymes. This conclusion was drawn, for instance, from the depression by 6- 

 mercaptopurine of the incorporation of glycine-2-''*C into acid-soluble purines 

 but not into nucleic acid purines of the susceptible mouse tumors TA3 and sar- 

 coma 180 (Fernandes, LePage and Lindner, 1956). 6-Mercaptopurine also in- 

 hibits the increased synthesis of diphosphopyridine nucleotide that normally fol- 

 lows injection of nicotinamide in mice (Kaplan, Goldin, Humphreys, Ciotti and 

 Stolzenbach, 1956). Adenylic acid partly reverses this inhibition. 



The mitotic inhibition of mouse cells in culture by 6-mercaptopurine is po- 



^ See Chapter i. 



