862 THE CARCINOGENIC STIMULUS II 



factor in influencing the development of hepatic tumors ; administration of ribo- 

 flavin inhibits the tumor-inciting action of butter yellow and a riboflavin deficient 

 diet favors the induction of liver tumors in rats by this compound (Kensler et al., 

 1940, 1 941). Binding of the azo dye by liver proteins is inhibited by riboflavin and 

 dye-binding can be correlated with hepatic tumor formation (Miller and Miller, 



1947)- 



The studies of Miller and Miller (1947, 1952) indicate that the azo dyes, which 

 are hepatic carcinogens, are "bound" by nucleoprotein and soluble cytoplasmic 

 proteins of the susceptible liver, but not by nucleic acids. Administration of 

 methylcholanthrene (Richardson and Cunningham, 1951) or nitrogen mustard 

 (GriflEin et al., ig5ib) simultaneously with the hepatic carcinogen reduces the 

 number of induced tumors. Protein binding of the hepatic carcinogen is inhibited 

 by methylcholanthrene. 



According to the enzyme deletion hypothesis, certain enzymes are important in 

 limiting the growth of normal cells, and if these are lost due to the action of a 

 carcinogenic agent, then an irreversible change has occurred, the loss of growth 

 restriction resulting in what is termed neoplastic growth (Potter, 1944). Since 

 this metabolic deviation is heritable, passed on to the progeny of altered cells, the 

 alteration would presumably be genie with cytoplasmic enzymatic control lost 

 as a result. The pattern for deviation of this type has been demonstrated in Neuro- 

 spora by the classical investigations of Beadle and Tatum (1945). 



Hepatic nucleic acid is not bound by azo dyes (Miller and Miller, 1952). If the 

 nuclear genes which are rich in deoxyribonucleic acid do not react with the chem- 

 ical carcinogen, then the reproducible alteration (resulting in neoplasia) must 

 go via cytoplasmic genie material. Such "plasmagenes" have been postulated by 

 Spiegelman (1946). 



[c) Amino compounds 



Certain amitio compounds {e.g., acetylaminofluorene) are of particular interest 

 because of the diversity of their carcinogenic effects [e.g., induction of hepatic, 

 mammary, urinary bladder, thyroid and ear-duct tumors). As in the case of azo 

 dyes, the carcinogen is bound by protein of the liver for which it is carcinogenic 

 (Weisburger et al., 1953). Following the ingestion of acetylaminofluorene, the 

 compound is apparently metabolized so that a breakdown product (or products) 

 is tumor-provoking. Local application of acetylaminofluorene yields no neoplasm 

 (Bielschowsky, 1944a). In the case of 2-naphthylamine, which induces urinary 

 bladder tumors, a carcinogenic metabolite, 2-amino-i-naphthol has been 

 identified (Bonser et al., 1951) in the urine. 



Haddow (1953) has suggested that certain of the amino compounds {e.g., 

 alkylating agents such as nitrogen mustard) aflfect genie components of the cell. 

 These agents are both radiomimetic and mutagenic, and have also been demon- 

 strated to be carcinogenic (Heston, 1949; Griffin et al., 1950; Boyland and Horning, 

 1949). Haddow, proposes that the primary step in carcinogenesis induced by 

 these agents is the "inhibition of certain fundamental processes of genetic syn- 

 thesis", followed by the "generation of a new self-duplicating fiber or template 

 chemically, and hence genetically, modified". This first step may be the primary 



