648 6. ARSENICALS 



mM (Rajagopalan and Handler, 1964). Pseudomonas acetamidase has also 

 been claimed to be competitively inhibited, but the iC , of 2 mM indicates 

 that SH groups are not involved (Jakoby and Fredericks, 1964). Markwardt 

 (1953 b) claimed that arsenite competitively inhibits human erythrocyte 

 cholinesterase, but his double-reciprocal plots actually seem to meet fairly 

 well on the 1/(S) axis, indicating noncompetitive inhibition. Where arsenite 

 is inhibiting as an anion, rather than by reacting with SH groups, as in 

 nitrite oxidation (Butt and Lees, 1960), competitive inhibition would be 

 expected. The relationship between inhibition and substrate may be com- 

 plicated by the shift in the S — S and SH group ratio, as we have seen for 

 certain oxidizing enzymes. When the presence of the substrate favors the 

 inhibition, one cannot immediately conclude that uncompetitive (coupling) 

 inhibition occurs, i.e., the arsenical combines with the ES complex. Thus 

 the inhibition of S-acetoacetyldithioloctanoate thioesterase by arsenite re- 

 quires the presence of the substrate; since arsenite reacts with neither the 

 enzyme nor the substrate alone, it was suggested that the reaction is with 

 the ES complex (Drummond and Stern, 1961). This may well be so, but it 

 may also imply that SH groups become available during the catalysis. 

 Mutual depletion kinetics must also be expected in many arsenical inhibi- 

 tions, and evidence for this is to be found in the results of Rona et al. (1922) 

 on the inhibition of maltase by methylarsenoxide, and of Stoppani and 

 Brignone (1957) on succinate dehydrogenase inhibited by methylarsenoxide 

 and oxophenarsine. 



Protection of an enzyme by a substrate, coenzyme, or cofactor implies 

 that the inhibitor reacts with the substrate site or with some group within 

 the radius of steric or electrostatic influence of the bound substrate, pro- 

 viding the protector does not directly react with the inhibitor, or merely 

 stabilize the enzyme structure against secondary inactivation induced 

 by the inhibitor. Some further examples of substrate protection will be 

 mentioned: the inhibition of muscle succinate dehydrogenase by arsenite 

 (Bergstermann and Mangier, 1948), the inhibitions of urease, succinate 

 dehydrogenase, and choline dehydrogenase by oxophenarsine (Gordon 

 and Quastel, 1948), the inhibition of pyruvate decarboxylase by oxophera- 

 sine (Stoppani et al, 1952), and the inhibition of yeast fumarase by Melarsen 

 (Favelukes and Stoppani, 1958). The mechanisms of these protections are 

 not known but it is generally assumed that they point to a binding of the 

 arsenical at the substrate site. Protection by a coenzyme similarly suggests 

 a common binding region, as the protection of aldehyde dehydrogenases 

 by NAD and NADP observed by Stoppani and Milstein (1957 a, b). Other 

 types of protection present greater difficulties in interpretation; for example, 

 the protection of the malate oxidative decarboxylase by Mn++ when arsenite 

 is the inhibitor (Rutter and Lardy, 1958). Finally, we have protection 

 against arsenical inhibition by more readily reversible inhibitors, such as 



