698 



5. OXIDANTS 



Furthermore, the reaction is initially rapid but in most instances slows 

 down suddenly, which is difficult to explain for the simple thiols. The oxi- 

 dation proceeds much faster at pH 7 than at pH 5 (see accompanying 

 tabulation). 



Fig. 5-1. Rates of reaction of 

 10 mil/ tetrathionate with the 

 SH groups of thiols and proteins 

 at pH 5. (From Goffart and 

 Fischer, 1948.) 



Inhibition of Enzymes and Metabolism 



Succinate dehydrogenase is inhibited around 90% by 0.1 mM tetrathi- 

 onate (Keilin and Hartree, 1940). This is an effect on the dehydrogenase SH 

 groups according to these authors and PhiHps et al. (1947), who confirmed 

 the inhibition on succinate dehydrogenases from several tissues. No inhi- 

 bition of ascorbate oxidation, and hence of the cytochrome system, is ob- 

 served even with 10 mM. Succinate protects the enzyme; when tetrathionate 

 is 0.5 mM, succinate reduces the inhibition from 96% to 39%, and when 

 it is 0.1 mM from 79% to 10% (pigeon breast enzyme). The inhibition is 

 only partially reversible with glutathione or cysteine. In work with suc- 

 cinate dehydrogenase it may be well to consider the possibility that some 

 of the inhibition results from a competitive action of the tetrathionate, since 

 it has negative charges appropriately separated. Choline dehydrogenase 

 from rat liver is also quite sensitive to tetrathionate, 38% inhibition re- 



