DISCUSSION ON TUMOR RESPIRATION 247 



rate of fermentation in the yeast system were measured. In Table 1 

 are presented the results of tests of only those substances of which 

 the carcinogenic potency of the primary azo compound for rat 

 livers has been tested. However, in further tests of the inhibitory 

 effect on the yeast system of seventeen of the methyl derivatives of 

 p-phenylenediamine, as well as of those hsted, the inhibition was 

 found to correlate closely with the stabihty of the semiquinone 

 intermediary oxidation product as reported by Michaelis, Schubert, 

 and Granick (5). 



In the experiments with the yeast system it was noted, further- 

 more, that those compounds which were toxic and which formed 

 stable free radicals (semiquinones) were oxidized readily by the 

 yeast apozymase used. The substances which were shown by 

 Michaelis and his associates not to form stable free radicals were 

 not so oxidized. This suggested, therefore, that the stable free radi- 

 cal or some further oxidation product of it was responsible for the 

 inhibition observed. 



Among the possible end products of the oxidation that might be 

 responsible for the inhibition are quinone, methylamine, dimethyl- 

 amine, and formaldehyde. Of these compounds only quinone was 

 found to have significant toxicity. This compound, however, was less 

 than half as toxic as the diamine in equivalent concentrations. Hydro- 

 gen peroxide, which can be formed under certain conditions of 

 oxidation, was also proved to be non-toxic. The acetylated p-phenyl- 

 enediamine, which is quite stable and is a metaboHte of dimethyl- 

 aminoazobenzene, was found to be non-toxic. 



This inhibition of the diphosphopyridine nucleotide system by 

 the p-aromatic diamines made it desirable to test another yeast 

 enzyme system. Similar results were obtained when a carboxylase- 

 cocarboxylase system was employed, except for one difference. The 

 addition of reducing agents such as cysteine, glutathione, and as- 

 corbic acid reduced markedly the toxicity of the p-aromatic diamines 

 for the diphosphopyridine nucleotide system. In the carboxylase- 

 cocarboxylase system, on the other hand, to which the same sub- 

 stances are inhibitory and where there is no complicating catalytic 

 oxidation of the inhibiting agent, the addition of the reducing sub- 

 stances prevents any inhibition at all. 



In experiments with both the diphosphopyridine nucleotide sys- 

 tem and the cocarboxylase system, where the inhibition by the toxic 

 compounds was large in the presence of low coenzyme concentra- 

 tions, it was found that the addition of large amounts of coenzyme 



