870 7. MERCURIALS 



ELECTRON TRANSPORT 

 AND OXIDATIVE PHOSPHORYLATION 



The majority of dehydrogenases are quite sensitive to the mercurials, 

 and inspection of Table 7-13 shows that 50% inhibition is commonly pro- 

 duced by concentrations of 0.001-0.05 mM. If we define NADH dehydro- 

 genase as the enzyme component responsible for the transfer of electrons 

 to a variety of acceptors, it must be placed in the same category with 

 respect to sensitivity, and, indeed, NADH-cytochrome c reductase is usual- 

 ly even more susceptible, often being completely inhibited by concentra- 

 tions of 0.001-0.01 mM. One is thus tempted to attribute the inhibition 

 of various oxidations by the mercurials to an action early in the electron 

 transport chain, at least pre-cytochrome. Furthermore, Barron and Singer 

 (1945) had reported that the oxidation of reduced cytochrome c by a cyto- 

 chrome oxidase preparation is not affected by p-MB, and this has more 

 recently been observed with Arum (Simon, 1957) and Penicillium (Sih et 

 al., 1958) cytochrome oxidases. Finally, some have found that certain oxi- 

 dases and the corresponding dehydrogenases are inhibited equally by mer- 

 curials, although the different conditions of testing in such cases make ac- 

 curate comparison difficult. 



This simple picture of inhibition in the electron transport sequence has, 

 however, been questioned by workers at the Institutum Divi Thomae, who 

 from 1946 to 1957 obtained increasing evidence that the cytochrome system 

 may not be as immune to mercurials as generally imagined. Their results 

 may be summarized in the four following categories. (1 ) Cytochrome oxidase 

 activity, as determined by the oxidation of ascorbate, is inhibited rather 

 potently, 50% depression being observed with 0.006-0.012 mM PM and 

 0.032 mM p-MB (Cook et al, 1946; Kreke et al, 1950). (2) Succinate oxi- 

 dase is much more sensitive to mercurials than is succinate dehydrogenase. 

 It requires around 10 times the concentration to inhibit rat heart succinate 

 dehydrogenase compared to the oxidase (Cook et al, 1946; Kreke et al, 

 1949; Smalt et al, 1957). (3) The inhibitions of cytochrome oxidase and 

 succinate oxidase are not reversed by thiols (Cook and Perisutti, 1947; 

 Kreke et al, 1949, 1950). This led them to suppose that the inhibition might 

 not involve SH groups, but this conclusion, as we have seen, is not valid. 

 (4) No evidence for reaction of the mercurials with ascorbate or cytochrome 

 c could be obtained by spectroscopic or preincubation techniques (Cook et 

 al, 1946; Kreke et al, 1950). It may also be mentioned that Boeri and Tosi 

 (1954) found no reaction of p-MB with cytochrome c, and that Strittmatter 

 and Velick (1956) likewise found no change in microsomal cytochrome spec- 

 tral absorption after incubation with 1 mM p-MB. All of these data have 

 been interpreted as indicating that the mercurials may exert a major part 

 of their effect on cytochrome oxidase. 



