872 7. MERCURIALS 



in the dehydrogenase test the pH was 7.2 and the succinate concentration 

 0.33 rciM, whereas in the oxidase test the pH was 7.4 and the succinate 

 88 mM; the former was done in strong phosphate buffer, whereas the latter 

 medium contained 0.7 mM Ca++ and A1+++; in addition, the times for 

 equilibration and incubation were different. When the conditions are so 

 diverse, it is impossible to compare these two systems quantitatively. The 

 structural interference theory of Slater must also be considered and has as 

 much evidence as the other explanations (i.e., none). It would be important 

 to know just how much effect mercurials can exert on the cytochrome sys- 

 tem, inasmuch as it has obvious bearing in considerations of the actions 

 on various oxidations, mitochondrial systems, and respiration. 



A comparable situation with NADH dehydrogenase, NADH: cytochrome 

 c reductase, and NADH oxidase has been noted by Minakami et al. (1963). 

 The total oxidase and the cytochrome c reductase are very sensitive to p-MB 

 whereas the dehydrogenase, as determined by ferricyanide reduction, is not 

 as sensitive. It was postulated that two types of SH group are involved in 

 NADH oxidation, one readily accessible to mercurials and functioning be- 

 tween the dehydrogenase active site and the distal respiratory chain (this 

 SH group is not required for ferricyanide reduction), and a second con- 

 cealed in the dehydrogenase complex as isolated, and exposed on degrada- 

 tion to the cytochrome c reductase. Such an explanation could apply to the 

 succinate oxidase as well, as was suggested above relative to methylene 

 blue as an acceptor for the determination of dehydrogenase activity. 



Oxidative Phosphorylation 



The results summarized in Table 7-15 show that mercurials are not par- 

 ticularly specific or effective uncouplers of oxidative phosphorylation in 

 mitochondria, but that a fair degree of uncoupling can occur under certain 

 circumstances. It is especially interesting that high toxic doses of the mer- 

 curial diuretics and HgClg can often reduce the P : ratio in the mitochon- 

 dria of excised kidneys several hours after the administration, without sim- 

 ultaneously affecting oxidative phosphorylation in the liver, but this is 

 undoubtedly due to the higher concentration of mercurial in the kidney. 

 The P : ratio is, however, not altered significantly by the ordinary diuretic 

 doses, so that it is questionable if this action is related to diuresis. I know 

 of no instance in which the mercurials augment O2 uptake while simul- 

 taneously reducing the P, incorporation, so that they are not true uncou- 

 plers in the same sense as the nitrophenols. 



The Pj^^-ATP exchange is quite potently inhibited by mercurials in the 

 mitochondria obtained from mosquitoes (Avi-Dor and Gonda, 1959), pig 

 liver (Chiga and Plant, 1959), and rat liver (Plaut, 1957; Cooper and Leh- 

 ninger, 1957; Lehninger et al., 1958; Low et al., 1958). For rat liver mito- 

 chondria the exchange is sometimes inhibited 50% by concentrations 



