INHIBITION OF ENZYMES 647 



E — SH form, which is susceptible to inhibition by arsenite, A^-ethylma- 

 leimide, and p-merciiribenzoate. These results have bearing on the effects 

 of arsenicals in vivo, since the susceptibilities of the metabolic systems may 

 well depend on the redox states of certain enzymes and hence on the supply 

 of substrates and oxygen. Other instances of potentiation of arsenical 

 inhibition are not so clearly understood. Arsenite depresses succinate 

 dehydrogenase activity more strongly when 0.01 mM Ca++ is added 

 (Kushnarev and Blagoveshchenskii, 1961), and the oxidation of glutamate 

 by Vigna seedling mitochondria is insensitive to arsenite unless AMP is 

 added (Das and Roy, 1962). Both of these effects may be related to alter- 

 ations in the equilibrium between S — S and SH groups. For example, AMP 

 stimulates glutamate oxidation and could thus shift this equilibrium. 



Types of Inhibition Observed with the Arsenicals 



Arsenical inhibitions may be recorded as either competitive or noncom- 

 petitive, depending on how the tests are made and how tightly the arsenical 

 is bound to the enzyme. Most inhibitions are presumably due to the arsen- 

 ical reacting with SH groups at or near the active center, and it is very 

 commonly observed that the enzyme is protected by its substrate. If, 

 however, the arsenical is bound tightly to the enzyme, the final equilibrium 

 inhibition will not depend to any extend on the substrate. Thus one can 

 obtain competitive, noncompetitive, and mixed kinetics from the same 

 system if the determination of the inhibition is done at different times. 

 Thus Peters and Sanadi (1961) showed that xanthine oxidase is protected 

 against arsenite by the substrate, but that the final inhibition is formally 

 noncompetitive with respect to xanthine. Competitive inhibition has been 

 reported. The inhibition of D-amino acid oxidase by butarsen has been 

 claimed to be competitive with respect to alanine (Frisell and Hellerman, 

 1957) but, since apparently only 15 min was allowed for the reaction, it 

 is questionable if equilibrium conditions had been reached, so that only 

 a competitive substrate protection was measured. On the other hand, the 

 point was made that butarsen may be inhibiting by virtue of its benzoate- 

 like structure, rather than as an SH reagent, especially when FAD is 

 present to protect certain SH groups. Flamm and Crandall (1963) report 

 that the inhibition of homogentisate oxidase by lewisite is noncompetitive 

 with respect to substrate but competitive with respect to Fe++, and the 

 double-reciprocal plots obtained certainly seem to be beyond objection. 

 Since the Fe++ and the arsenical may react with the same SH group, true 

 competitive inhibition is not so surprising. Liver aldehyde oxidase is in- 

 hibited competitively by arsenite and, since the iiC, of 3 mM indicates a 

 rather low affinity of the enzyme for the arsenical, a true competitive 

 situation could exist here (Palmer, 1962). On the other hand, rabbit liver 

 aldehyde oxidase is inhibited competitively by arsenite with K, = 0.0063 



