646 6. ARSENICALS 



butyrate apodehydrogenase is incubated with lecithin and a thiol it be- 

 comes active; the complex with cysteine is unaffected by 1 mM arsenite 

 while the dimercaprol complex is inhibited completely (Jurtshuk et al., 

 1963). This phospholipid complex is probably the form present in mito- 

 chondria and it is interesting to speculate whether the lecithin interferes 

 with the access of the arsenite to the active site unless dimercaprol is 

 present. We shall see that certain electron transport systems behave in 

 the same way (page 659) and lipid there may also play a role in restricting 

 certain polar inhibitors from getting to the site at which they would nor- 

 mally react when the enzymes are purified. 



The mechanisms involved in the thiol potentiation of arsenical inhibi- 

 tions are unknown. Most workers have assumed that the arsenical-thiol 

 complex can better penetrate to the functional SH groups perhaps because 

 it is more lipid-soluble. Another possibility is the formation of a ternary 

 complex of arsenical-thiol-enzyme, as discussed previously. A further expla- 

 nation, which seems to have been generally ignored, is the simple reduction 

 of S — S to SH groups in a region near the active site; the SH groups need 

 not be involved in the catalysis but, when combined with arsenical, would 

 interfere with the approach of the substrate to the active site. On the other 

 hand, one must bear in mind the fact that only certain thiols are poten- 

 tiating for a particular enzyme. It is quite certain that some enzymes al- 

 ternate between the S — S and SH forms during activity — i. e., these groups 

 participate in the electron transfer — as in various metalloflavoenzymes 

 concerned with NAD(P)H oxidation or in systems such as sulfite oxidase 

 (Fridovich and Handler, 1956). First, oxidation of SH groups to the S — S 

 form implies that the SH groups are quite close and hence might react 

 readily with the monosubstituted arsenicals. Second, it would cause the 

 susceptibility of the enzyme to vary with the state of the SH groups. 

 This is perhaps why it has been found that addition of substrate occasion- 

 ally potentiates the inhibition by an arsenical. Arsenite has no effect on 

 dihydrolipoate dehydrogenase but in the presence of NADH the inhi- 

 bition is marked (Searls and Sanadi, 1960 a). This was explained as a re- 

 duction of enzyme S — S groups to SH groups by the NADH through 

 FAD. Similar results were obtained with a-ketoglutarate dehydrogenase 

 with NAD as the acceptor, although no potentiation of arsenite inhibition 

 occurs when an indophenol dye is used as the acceptor (Searls and Sanadi, 

 1960 b). It was further shown that the enzyme exhibits paired SH groups 

 after incubation with NADH (Searls et al., 1961). Glutathione reductase 

 behaves in the same fashion in that the activity is unaffected by 1-10 mM 

 arsenite in the absence of NADPH, whereas strong inhibition is seen when 

 NADPH is added with the arsenite (Mapson and Isherwood, 1963). The 

 enzyme was pictured as existing in three states in equilibrium — E — SH, 

 E=S, and E — S— S — E — and NADPH was considered as generating the 



