FERRICYANIDE 673 



inhibitions are summarized in Table 5-3. Certain enzymes which possess SH 

 groups reactive with other reagents, e.g. urease, are not inhibited by even 

 high concentrations of ferricyanide. One might imagine ferricyanide to be 

 unable to gain access to the SH groups. Ferricyaiade is not only a fairly 

 large ion but has a strong negative charge. If the enzyme SH group occu- 

 pied a region of high negative charge, this might repel the ferricyanide and 

 reduce the reaction. Indeed, one must always consider the possibility that 

 ferricyanide inhibits certain enzymes by mechanisms other than oxidation, 

 and related more to its charge and structure. For example, it would not be 

 so surprising if ferricyanide inhibits succinate dehydrogenase to some ex- 

 tent because it can interact with the cationic groups normally binding suc- 

 cinate. One notes also that ferrocyanide generally inhibits aconitase more 

 strongly than does ferricyanide, and here redox reactions may be of no sig- 

 nificance (Rahatekar and Rao, 1963). On the other hand, some enzymes 

 are inhibited just as rapidly and completely by ferricyanide as by the more 

 commonly used SH reagents; myosin ATPase is one example (Singer and 

 Barron, 1944). The inhibitions of papain and aldolase are quite reversible 

 with cysteine, indicating that reversible oxidation is the mechanism of the 

 inhibitions. Oxidation of coenzymes or cof"ctors can also occur. Ferricyanide 

 can directly oxidize NADH but the rate is slow (Schellenberg and Heller- 

 man, 1958). In the case of homogentisicase it may well be the Fe++ that is 

 oxidized but, on the other hand, there appears to l)e a tyrosine phenolic 

 group at the active site (Tokuyama, 1959). 



It is interesting that Weill and Caldwell (1945 b) report /5-amylase to be 

 not readily inhibited by either ferricyanide or Cu+^ alone but strongly in- 

 hibited when both are present, even when the ferricyanide is at a concen- 

 tration noninhibitory by itself (see accompanying tabulation). Could this 



Ferricyanide Cu++ 



(mM) (mM) 



% Inhibition 



relate to the observation of Katyal and Gorin (1959) that Cu++ accelerates 

 the action of ferricyanide ? Or does the Cu"^+ in some manner alter the en- 

 zyme structure so that ferricyanide can attack the SH groups more easily? 



Effects on Cellular Metabolism and Function 



Mendel (1937) reported that Balogh mouse tumor glycolysis is markedly 

 depressed by 10 mM ferricyanide and that the inhibition is maintained 



