294 2. MALEATE 



azosulfathiazole at pH 5.82-6.67 but is ineffective at pH 8.64. This com- 

 petition is presumably not due to reaction of the maleate with protein SH 

 groups but to relatively nonspecific electrostatic interactions, and yet 

 the pH data are not entirely consistent with this explanation. It is interest- 

 ing that succinate is a poor displacer, which was attributed to a lesser degree 

 of ionization, but at the pH's used this is not the case. In any event, these 

 results demonstrate that maleate can interact significantly with seralbumin. 



INHIBITION OF ENZYMES 



The effects of maleate on enzymes are summarized in Table 2-2. These 

 data are often of little quantitative significance since the complete experi- 

 mental conditions, especially the incubation interval, are not given. Maleate 

 frequently inhibits slowly so that the time of contact with the enzyme is 

 very important. Morgan and Friedmann (1938 b) found, for example, 

 that papain is inhibited 32% after 2 hr and 69% after 16 hr by 80 mM 

 maleate. The results on succinate dehydrogenase and oxidase in the table 

 are quite variable and part of this is due to quite different conditions and 

 incvibation durations. Thus, Hopkins et al. (1938) obtained their inhibition 

 of succinate dehydrogenase by allowing the enzyme to react with maleate 

 overnight, whereas others who report little or no inhibition have usually 

 used more conventional intervals. However, perusal of the table provides 

 certain conclusions, particularly that maleate does not inhibit some enzymes 

 which appear to be typical SH enzymes, e. g., homogentisate oxidase 

 (which is inhibited 89% by 1 vaM p-MB), malate dehydrogenase (decar- 

 boxylating) (which is inhibited 87% by 1 mM PM), /5-amylase (which 

 is inhibited 73% by 0.001 mM PM), isocitrate dehydrogenase (which is 

 inhibited 100% by 0.04 milf j)-WE), and protein phosphokinase (inhibited 

 45% by 0.1 mM 2)-MB). Succinate dehydrogenase is inhibited quite weakly 

 by maleate, which is surprising in view of the probability that an SH 

 group occurs in the active site (Potter and DuBois, 1943) and that the 

 cationic sites might be expected to bind maleate in a position in which 

 reaction of the SH with the double bond would occur, but it is likely that 

 the configuration of maleate is such that the double bond may be hindered 

 from approaching the SH group, as discussed in Chapter II-l (page 34). 

 It is interesting that bromelain and solanain are scarcely affected by mal- 

 eate, whereas asclepain is strongly inhibited; yet all are SH enzymes and 

 well inhibited by Hg++ and iodine. Apparently only certain SH groups 

 on enzymes are subject to attack by maleate, so that maleate is certainly 

 not as general an inhibitor of SH enzymes as most SH reagents. The steric 

 and electrostatic requirements for maleate are perhaps more rigid. When 

 maleate approaches an enzyme surface it may be either repelled by neg- 

 atively charged groups or attracted by positively charged groups, but in 



