ELECTKON TRANSPORT 871 



Slater (1949) had also observed that succinate oxidase is inhibited more 

 strongly than the dehydrogenase by p-MB (and also by o-iodosobenzoate 

 and oxidized glutathione), although the difference was not as great as re- 

 ported by Cook, Kreke, and their co-workers, and attributed this in the 

 particulate preparations used to an effect on some link between the dehy- 

 drogenase and the oxidase, presumably occurring before cytochrome c in 

 the chain. This effect might be a structural disorganization of the complex 

 to interrupt the flow of electrons. Nevertheless, Slater observed some inhibi- 

 tion of cytochrome oxidase. Seibert et al. (1950) made a solubilized deoxy- 

 cholate preparation of cytochrome oxidase and found by both manometric 

 and spectrophotometric tests that it is inhibited to the same degree as the 

 crude preparation: they also demonstrated shifts in the spectral bands of the 

 oxidase following treatment with the mercurials. The final conclusion of the 

 Institutum Divi Thomae group is that the actions of the mercurials on heme 

 enzymes may be nonspecific, may involve denaturation (which could ac- 

 count for the spectral shifts), and do not involve SH groups, but I doubt 

 if there is sufficient evidence for any of these statements. However, their 

 data, which are definite and consistent, must be explained on some basis. 

 It is important to realize that the inhibitions reported for "cytochrome 

 oxidase" were all obtained with ascorbate (and occasionally hydroquinone) 

 as the substrate. Now neither ascorbate nor hydroquinone is oxidized di- 

 rectly by cytochrome oxidase and the electron transfer occurs through a 

 series of components. It has usually been assumed that ascorbate reduces 

 cytochrome c^ or c, in which case the action of the mercurials could be on 

 some component or link in the cytochrome sequence, rather than on cyto- 

 chrome oxidase itself. It will be remembered that the work quoted at the 

 beginning of this section showed that, when cytochrome c is used as sub- 

 strate, the mercurials do not inhibit. Is it possible that there is a com- 

 ponent which might be designated as ascorbate dehydrogenase, which is 

 sensitive to the mercurials? Seibert et al. (1950) actually observed relative- 

 ly little inhibition of the purified system when determined spectrophoto- 

 metrically with cytochrome c as the substrate. 



There are several ways of explaining the differential inhibitions of suc- 

 cinate dehydrogenase and oxidase. Since the activities of these two sys- 

 tems are determined very differently — the dehydrogenase usually by 

 methylene blue reduction and the oxidase manometrically — one must 

 question if this could be responsible for the different sensitivities observed. 

 The dehydrogenase activity associated with methylene blue reduction might 

 not be exactly the same as in the normal transfer of electrons to the cyto- 

 chromes; i.e., a region of the enzyme surface, or another component in the 

 chain, might be involved in the normal transfer but not in the dye reduc- 

 tion, and this part of the system could be sensitive to the mercurials. If 

 we look into the details of the procedures (Kreke et al., 1949), we find that 



