DISCUSSION ON TUMOR RESPIRATION 251 



evidence available suggests, in addition, that the inhibition may be 

 produced by oxidation of, or combination with, sulfhydryl groups on 

 the protein enzyme. For example, in the diphosphopyridine nucleo- 

 tide system iodoacetate and alloxan, which are known to react with 

 sulfhydryl groups, also inhibit fermentation. The alloxan inhibition 

 is, like that caused by the p-aromatic diamines, a competitive one. 

 Vassel (6) has found that dimethyl-p-phenylenediamine will con- 

 dense with cysteine in strongly acid solutions in the presence of 

 Fe+++ and ZnCL to yield a colored (blue) product. In several 

 experiments done in our laboratory we found that in the presence 

 of a liver suspension (brei) an orange product is formed when both 

 dimethyl-p-phenylenediamine and cysteine are added. This product 

 is not formed on the addition of either alone. 



The work of White (7) has shown that organic sulfur (cystine, 

 methionine) can counteract the growth-inhibiting properties of both 

 the azo and hydrocarbon carcinogens and supports the view that 

 the carcinogen may combine with the sulfhydryl groups of proteins. 

 Further evidence is provided by the studies of L. F. Fieser and 

 his collaborators (8). These concern experiments with the hydro- 

 carbon carcinogens in which an entirely different approach was fol- 

 lowed. The results suggest that the action of the hydrocarbon carcin- 

 ogens may be on an S— S link of a protein and serve to emphasize 

 the need for further information on the mode of action of the 

 p-aromatic diamines in catalytically active systems. 



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5. Michaelis, L., Schubert, M. P., and Granick, S., J. Amer. Chem. Soc, 61, 

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6. Vassel, R., Jour. Biol. Chem., 140, 323 (1941). 



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11. Yosida, T., Virchow's Archiv., 283, 29 (1932). 



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