g28 ANTIMETABOLITES AS MITOTIC POISONS 13 



land, 1950); the yield of high-energy phosphate bonds may be used in the syn- 

 thesis of nucleic acids and proteins. Adenosine triphosphate enters repeatedly into 

 the series of reactions leading from thymidine and the deoxynucleosides of adenine, 

 guanine, and cytosine through their triphosphates to DNA-like polynucleotides 

 in the enzymatic synthesis studied by Kornberg, Lehmann, Bessman, and Simms 

 (1956). Similarly, Grunberg-Manago, Ortiz and Ochoa (1955) have fovmd that 

 RNA-like polynvicleotides are enzymatically fashioned from nvicleoside diphos- 

 phates. Phosphorylation is also involved in several hypotheses of protein synthesis 

 on RNA templates (Bounce, 1952; Bounce, Work and Campbell, 1953; Zamec- 

 nik and Keller, 1954; Borsook, 1956). 



In the soluble protein fraction of rat liver homogenates, Zamecnik, Keller, 

 Littlefield, Hoagland and Loftfield (1956) have found that amino acids in the 

 presence of ATP and sokible enzymes are activated by conversion to aminoacyl- 

 adenosine monophosphate compounds, which are built into proteins in microsomal 

 ribonucleoprotein particles with the assistance of guanosine polyphosphate. 



Protein synthesis may depend on ribonucleic acid, because ribonuclease 

 abolishes protein synthesis in disrupted staphylococcal cells (Gale and Folkes, 

 1953), in onion root cells (Brachet, 1954), and in rat liver microsomes (Zamecnik 

 and Keller, 1954). Addition of purines and pyrimidines to staphylococcal cells 

 in a medium containing glucose and amino acids enhances protein synthesis and 

 permits a concomitant synthesis of nucleic acids; if amino acids are absent, no 

 nucleic acid is synthesized ; and if nucleic acid content falls too low, protein syn- 

 thesis ceases (Gale and Folkes, 1953). Specific nucleotide patterns on the RNA 

 template govern the synthesis of proteins with specific amino acid sequences 

 (Bounce, 1952; Gamow, 1954; Bounce, Gamow, Spiegelman, Newmark, Harker 

 and Soodak, 1956; Gale and Folkes, 1955). 8-Azaguanine, 2,6-diaminopurine, 

 and to some extent benzimidazole, inhibit formation of certain bacterial enzymes, 

 probably because the incorporation of these purine analogues into RNA inter- 

 feres in its action as a template in protein synthesis (Greaser, 1955a, 1955b). The 

 experiments of Pardee (1954) suggest the necessity of continued synthesis of RNA 

 for maintaining the synthesis of [^-galactosidase in uracil-requiring mutants of 

 E. coli, because the enzyme is produced only when uracil is present in the medium. 

 Formation of this enzyme is inhibited in an adenine-requiring mutant of £'. coli by 

 5-hydroxyuridine, which presumably has the initial effect of inhibiting RNA 

 synthesis (Spiegelman, Halvorson and Ben-Ishai, 1955). The evidence indicated 

 to Borsook (1956) that new RNA must continually be synthesized in order to 

 maintain protein synthesis because of a continuing denaturation of RNA. 



All this is pertinent to the analysis of mitotic inhibition by purine and pyrimidine 

 analogues. Their incorporation into BNA may interfere in the reduplication of 

 the BNA and in its direction of the syntheses implied in the gene-enzyme concept. 

 Incorporation of the analogues into RNA may interfere in the functions of this 

 substance (or substances) in mitosis, including synthesis of special proteins. The 

 presence of the analogues may prevent the autosynthesis of nucleic acids. Mitotic 

 inhibition by amino acid analogues may also be considered here, for protein syn- 

 thesis on the nucleic acid template may be regarded as impossible unless all the 

 necessary amino acids are present at the same time (Borsook, 1956). Inhibitory 



