650 



6. ARSENICALS 



begin at the same pH with all the substances tested, irrespective of their 

 p^g's? It certainly seems unlikely that the marked rise in the inhibition 

 of cholinesterase around pH 5.4 is related to ionization of arsenious acid. 

 Is the reaction of arsenicals with SH groups accelerated by a rise in the pH ? 

 Eagle (1945) reported that pH has little effect on the reactions between 



Fig. 6-1. Activation of fumarase by various anions. 

 (A) Absence of added anions; (B) selenate (30 mM) 

 or sulfate (25 mM); (C) arsenite (10 mM); (D) citrate 

 (10 mM); (E) borate (200 mM); (F) phosphate (60 

 mM); (G) arsenate (50 mM); (H) chloride (100 mi/). 

 (From Massey, 1953. a) 



organic arsenicals and simple thiols, and we know nothing about the effect 

 of pH on the reactivity of protein or enzyme SH groups. The inhibition 

 of /5-amylase seems to involve SH groups because of the potent inhibition 

 by the mercurials, and here there is also an elevation of the effect of jo-ami- 

 nophenylarsenoxide as the pH is changed from 5.5 to 7.2 (Ghosh, 1958), 

 so that this might indicate a greater SH group reactivity at the higher pH, 

 unless it is a matter of the ionization of the ^j-amino group; arsenite does 

 not inhibit this enzyme. Both Barron and Singer (1945) and Mounter and 

 Whittaker (1953) noted that phenylarsenoxide-enzyme complexes tend 

 to dissociate when the pH is raised, this leading to reactivation of the en- 

 zymes. Although this corresponds to the observation that the As — S bond 

 is more readily hydrolyzed under alkaline conditions it is, difficult to under- 

 stand in view of the fact that inhibitions generally rise with the pH. There 

 are some intriguing problems here which might repay further investigation. 



