618 6. ARSENICALS 



to (p — As=0 than to cp^^X^ — CI* — e. g.. the a-ketoglutarate oxidase 

 of rat liver is some 11 times more sensitive, and the pyruvate oxidase some 

 40 times more sensitive (Aldridge and Cremer, 1955) whereas isocitrate 

 dehydrogenase is more sensitive to 9^2= As — CI than to (p — As=0. It was 

 conchided that isocitrate dehydrogenase can form a stable monothioarsinite 

 but not a cyclic thioarsinite, and hence possesses a single SH group at or 

 near the active site. Aconitase was found to behave similarly (Peters, 1955), 

 since it is readily inhibited by 9^2 =As — CI but is unaffected by lewisite. 

 The creatine kinase of rabbit muscle is unaltered by 0.25 mikf lewisite 

 but is inhibited 82% by the same concentration of 9^2= As — CI (Rosen- 

 berg and Ennor, 1955) and the arginine kinase from crayfish muscle is 

 insensitive to arsenite but is strongly inhibited by (f^=k8 — CI (Morrison 

 et al., 1957), so that on this basis these enzymes fall into the same mono- 

 thiol class. 



A number of enzymes have recently been classified as monothiol or di- 

 thiol''' from their responses to arsenicals. In some cases it has been con- 

 cluded that an enzyme is in the dithiol class simply because it is strongly 

 inhibited by arsenite or the arsenoxides: this is the case with dihydrolip- 

 oate dehydrogenase (Massey and Veeger, 1960), succinic semialdehyde 

 dehydrogenase (Albers and Koval, 1961), glycine reductase (Stadtman, 

 1962), and UDP-translucuronylase (Storey, 1964). Sometimes evidence 

 is adduced from the ability or inability of simple monothiols and dithiols 

 to protect or reverse the inhibition. Thus Storey (1964) felt that the inabil- 

 ity of glutathione or cysteine to completely reactivate arsenical-inhibited 

 UDP-translucuronylase supports the dithiol nature of this enzyme, while 

 Lakshmanan et al., (1964) doubted if dithiol groups occur on monkey 

 liver aldehyde oxidase because the inhibition by arsenite can be prevented 

 only by high concentrations of dimercaprol. Some have deduced a monothiol 

 group because the enzyme is inhibited by a mercurial but not by arsenite 

 or an arsenoxide; this is the situation with aspartase, which Ellfolk (1953) 

 designated as a monothiol since it is much more strongly inhibited by 

 j)-mercuribenzoate than by 9? — As==0. Cadmium ion is also used as a di- 

 thiol detector and Peters and Sanadi (1961) felt that, although xanthine 

 oxidase is well inhibited by arsenite, the failure of cadmium to inhibit is 

 evidence against the dithiol nature of the enzyme. There is thus a good 

 deal of confusion because of the lack of accepted criteria for distinguishing 

 between monothiol and dithiol enzymes. 



Since this is a very important aspect of the use of arsenicals as enzyme 

 inhibitors, it will be well to consider the problem in greater detail. Let us 



* The nucleus of the phenylarsenoxides will be designated as tp — As = and of 

 the diphenylchloroarsines as (p2~As — CI for convenience in the following discussion. 



t An enzyme will be designated as a dithiol when it is thought to have two vicinal 

 SH groups, i.e., two SH groups sufficiently close to form a cyclic thioarsinite complex. 



