II PREPROPHASIC INHIBITION 929 



analoguesof leucine, methionine, and phenylalanine prevent the net incorpcjration 

 into protein not only of their homologues but also of all other amino acids. Finally, 

 stimulation or inhibition of the synthesis of nucleic acid or of protein tends to have 

 a similar effect on synthesis of the other (Borsook, 1956). 



The effect of ribonuclease on mitosis illuminates some of these considerations. 

 In onion roots, ribonuclease produces mitotic inhibition and abnormalities 

 (Kaufman and Das, 1954). The effect on living chick fibroblasts and myoblasts 

 has been studied by Firket, Chevremont-Comhaire, and Chevremont (1955). 

 Measurement of Feulgen stain in intermitotic cell nuclei showed that a greater 

 proportion of nuclei in treated cultures have doubled amounts of DNA or amounts 

 approaching that needed for division. Mitoses become less common in treated 

 cultures, and they are held at prophase. Evidently the action of ribonuclease as a 

 prophasic mitotic inhibitor in these cells is not exerted through an interference 

 in DNA synthesis. Ribonucleic acid, however, is in some way essential for mitosis. 



Brachet (1955) suggested that the soluble pentose nucleic acid of the cell sap 

 is concerned with cell division. Of the several classes of cytoplasmic ribonucleo- 

 protein macromolecules (Petermann, Hamilton and Mizen, 1954), one class 

 becomes especially prominent when liver regenerates after partial hepatectomy 

 (Petermann, Mizen and Hamilton, 1953). Perhaps a special class of cytoplasmic 

 ribonucleoprotein is required to synthesize proteins of the mitotic apparatus, for 

 example. 



In summary, it is seen that inhibitors of various phases of intermediary metabol- 

 ism or of the metabolism of nucleic acids or proteins may reduce the incidence of 

 mitosis in some cell types and in that sense act as preprophasic mitotic poisons. 



There are numerous examples of such inhibitors. Among them are many 

 antimetabolites that interfere in the metabolism of nucleic acids, nucleotides, or 

 proteins. Certain groups of these are considered below. 



(a) Antifolic acids 



Folic acid is important as a precursor of coenzymes of one-carbon transfers, 

 including those involved in synthesis of purines and thymine leading to nucleic 

 acids (Schlenk, 1955). Analogues of folic acid are consequently inhibitors of 

 growth, and their effect may be seen in a diminution of mitotic activity. The 

 effects of such antifolics as aminopterin (4-aminopteroylglutamic acid) in arrest- 

 ing cells in the metaphase of mitosis is discussed in a later section (see p. 942). 

 Arrest in metaphase may contribute to the overall suppression of mitotic activity, 

 and it is of interest that Hughes (1950) noted no preprophasic inhibition of chick 

 fibroblasts in vitro early in the exposure to concentrations of aminopterin and 

 a-methopterin that cause metaphase arrest. Longer exposure to the antifolic, 

 however, may lead to general mitotic inhibition. Mouse leukemia cells sensitive 

 to a-methopterin showed no specific phase inhibition after 24-hour exposure to the 

 drug in vitro, and it was suggested that the general suppression of mitotic activity by 

 doses as small as 0.0 1 mg/ml (about 0.02 mM) indicated an interference in meta- 

 boUc reactions concerned with the initiation of mitosis (Kieler and Kieler, 1954). 



Mitotic incidence is also decreased in mouse tissue cultures by exposure for 24 h. 

 to 5 different antifolic acids (Biesele, ig54b). Mitosis is more readily suppressed 



Literature p. g4y 



