INHIBITION OF ENZYMES 649 



the protection of succinate dehydrogenase by malonate (Barron and Singer, 

 1945) and the protection of pyruvate decarboxylase against arsenoxides 

 by p-mercuribenzoate (Stoppani et al., 1953). 



Kinetics of Arsenical Inhibition 



The rates at which arsenicals inhibit enzymes may be fast or slow. 

 Gordon and Quastel (1948) noted that the inhibition of urease by various 

 arsenoxides does not increase with time and concluded that equilibrium 

 is achieved very rapidly, and similar results have been obtained by Mont- 

 gomery et al. (1956) on rat heart a-ketoglutarate oxidase inhibited by arse- 

 nite, and by Drummond and Stern (1961 ) on their liver thioesterase inhibited 

 by arsenite. On the other hand, succinate oxidase is inhibited by p-amino- 

 phenylarsenoxide much more slowly, 30-60 min being required for the 

 maximal effect (Fig. 1-12-12) (Slater, 1949). /^-Amylase is also inhibited 

 slowly by the same inhibitor, the inhibition still increasing at 30 min 

 (Ghosh, 1958). The dihydrolipoate dehydrogenase from spinach is little 

 inhibited by 1 mM arsenite at 5 min but is almost completely inhibited 

 by 15 min (Basu and Burma, 1960). Unfortunately the kinetics of arsenical 

 inhibition have never been studied in sufficient detail to be interpreted, 

 nor have the rates of development of the inhibitions been correlated with 

 SH titrations or reversibility experiments. However, it is important to 

 realize that enzymes differ quite markedly with respect to their inhibition 

 kinetics, and it is evident that even simple rate studies would often enable 

 arsenical inhibitions to be understood better and applied with greater 

 confidence to in vivo effects. 



Effects of pH on Arsenical Inhibition 



Much of the work on the variation of enzyme inhibition with pH has 

 been done with enzymes for which an SH group mechanism is not certain. 

 The high sensitivity of certain lipases to atoxyl is not explained, but Rona 

 and Haas (1923) reported that minimal inhibition is seen around pH 6.9, 

 more at pH 6.57, and most at pH 7.63, using human kidney lipase, whereas 

 Rothschild (1929) found the inhibition of liver lipase by arsenite to be 

 generally increased moderately as the pH is raised from 6.59 to 7.59. 

 The activation of fumarase by arsenite is probably unrelated to SH groups 

 and here the activation is absent at pH 6.5 and increases rapidly up to 

 pH 7.8 (Fig. 6-1) (Massey, 1953 a). The inhibitions of cholinesterases also 

 increase with the pH (Fig. 6-2) (Mounter and Whittaker, 1953). It is not 

 easy to interpret these results. Are the changes related to ionization of the 

 arsenious acid, or of enzyme groups? The anion-type activation of fumarase 

 might be explained on the basis of the arsenious acid ionization, but how 

 does this fit in with the reported pK^ of 9.22, and why does the activation 



