668 6. ARSENICALS 



ACCUMULATION OF KETO ACIDS AND OTHER 

 SUBSTANCES 



The inhibition of keto acid oxidation should under certain circumstances 

 lead to the accumulation of the keto acids, and this has been found to be 

 the case in a variety of organisms and tissues (Table 6-5). These results 

 can perhaps be taken as the best evidence that a relatively selective inhi- 

 bition can be exerted by the arsenicals. The keto acid levels often rise 

 very markedly; sometimes no pyruvate can be detected in the controls while 

 it progressively accumulates in the inhibited preparations to quite high 

 levels, e. g., in Microsporum cams (Chattaway et al., 1956) and Bacillus 

 megaterium (Duff and Webley, 1958). The pattern and degree of accumula- 

 tion will depend on several factors, particularly on the pathways by which 

 the keto acids are formed, the pathways by which they are metabolized, 

 and the relative effects of the arsenical on these pathways. 



Let us first consider a simple pathway such as 



glutamate -> a-ketoglntarate -> succinate 



(or a comparable system for alanine). The effect of an arsenical on a-keto- 

 glutarate concentration will depend on the relative inhibitions on the two 

 reactions. Since glutamate dehydrogenase and transaminases are moderately 

 sensitive to the arsenicals (Table 6-3), one would expect, as the arsenical 

 concentration is increased, to find first a rise in the level of a-ketoglutarate 

 and then a fall, so that if too high a concentration of arsenical is used, no 

 a-ketoglutarate would be formed. Thus one must exert care in choosing 

 the proper concentration of arsenical if marked accumulation is to be dem- 

 onstrated. This will, of course, apply to any pathway of keto acid meta- 

 bolism. If we now turn to the usual glycolytic formation of pyruvate and 

 its oxidation through the cycle, represented schematically by 



Glucose -> Xj ->• Xj -> ... -> pyruvate -> CO2 + HgO 



the arsenicals will induce accumulation of pyruvate only if they act more 

 strongly on the entry of pyruvate into the cycle than on the EM pathway. 

 Since there are numerous instances in which pyruvate levels are increased 

 with glucose as the substrate, it is evident that arsenicals can depress the 

 utilization of pyruvate more than its formation. Nevertheless, since some 

 of the glycolytic enzymes are inhibitable by the arsenicals, the concentration 

 should be chosen with care, and preferably several concentrations tested, 

 a procedure which has actually been used very seldom. The degree of pyru- 

 vate accumulation will also depend on the rate at which it is formed and 

 hence on the choice of the substrate. Chattaway et al. (1956) showed that 

 much pyruvate was formed from glucose (78), less from fructose, mannose, 



