652 6. ARSENICALS 



the results obtained with arsenicals. Many of the intermediates in the se- 

 quence are hypothetical. One scheme involves the following reactions: 



Pyruvate + TPP -> a-hydroxyethyl-TPP + CO2 

 a-Hydroxyethyl-TPP + lipS^ -> acetyl-TPP + lip(SH)2 

 Acetyl-TPP + CoASH -> acetyl-S-CoA + TPP 

 Lip(SH)2 + NAD+ -► lipSa + NADH + H+ 



Pyruvate + CoA-SH + NAD+ -► acetyl-S-CoA + NADH + H+ + 00^ 



The acetyl-CoA enters the cycle or is metabolized in other ways, while 

 the NADH is oxidized through the usual metallo-flavo-cytochrome system. 

 Another scheme also involves the acetylation of lipoate: 



"Aldehyde-TPP" + lipS^ ->• acetyl-S-lipSH + TPP 

 Acetyl-S-lipSH + CoASH -> liplSH)^ + acetyl-S-CoA 



where the dihydrolipoate is oxidized via NADH by a dihydrolipoate dehy- 

 drogenase. Various electron acceptors other than NAD can function in the 

 system. 



The inhibition of these systems will depend on the method by which 

 the activity is measured. If lipoate is inactivated by an arsenical, one would 

 expect inhibition of the O2 uptake and of the anaerobic dismutation of 

 pyruvate, but not much inhibition of CO2 formation (decarboxylase activ- 

 ity), since lipoate does not function in this early step. These results have, 

 in general, been obtained; the inhibition of pyruvate decarboxylation occurs 

 only at higher concentrations of arsenical than are required to block Og 

 uptake (Table 6-3 and Fig. 6-3). The difficulties ^rise when artificial elec- 

 tron acceptors are used. Although the reduction of NAD is potently inhib- 

 ited by arsenite (Sanadi et al., 1959; Goldman, 1959 a) in both the pyru- 

 vate and a-ketoglutarate oxidase systems, there is usually much less inhi- 

 bition on the reduction of other acceptors. Methylene blue reduction is 

 inhibited moderately (Peters et al, 1946), but the reduction of ferricyanide 

 is not inhibited up to 10 rriM arsenite (Sanadi et al., 1952, 1959). When 

 2,6-dichlorophenolindophenol is used as the acceptor, only 4% inhibition 

 from 30 mM arsenite was reported by Moyed and O'Kane (1956) with the 

 Proteus pyruvate oxidase, while no inhibition by 0.5 mM arsenite was 

 observed on the a-ketoglutarate oxidase from rat heart, at which concen- 

 tration O2 uptake is abolished (Montgomery et al., 1956). The failure to 

 inhibit with this dye as acceptor is surprising because is was assumed from 

 various evidence that lipoate is involved in its reduction. How can these 

 results be reconciled with the reaction sequences such as discussed above? 

 Both schemes postulate that the primary reduction involves lipoate. One 

 must first admit that lipoate has not been directly demonstrated to be 



