272 2. ANALOGS OF ENZYME REACTION COMPONENTS 



glucoside inhibits while /5-methylglucoside does not, indicating the impor- 

 tance of the carbon 1 configuration (Campbell et al., 1952). In general, the 

 /5-anomers cannot act as either substrates or inhibitors of phosphorylase. 

 The /^-glucuronidase of mouse liver is also stereospecific, since menthyl- 

 /?-glucoronide is a substrate but menthyl-a-glucuronide only a very weak 

 inhibitor (Levvy and Marsh, 1952). 



Positional Analogs 



Isomers in which a ring group is moved from one position on the ring 

 to another are generally not inhibitory due to the fairly marked structural 

 changes involved. There are exceptions, however, and one of the most 

 striking is the inhibition of the oxidation of p-hydroxyphenylpyruvate to 

 homogentisate by m-hydroxyphenylpyruvate in preparations from dog li- 

 ver (La Du and Zannoni, 1955). A depression of 50% is seen with 0.2 mM 

 and over 90% with 0.5 mM m-hydroxyphenylpyruvate when the substrate 

 concentration is presumably 1.2 raM, indicating a tighter binding to the 

 enzyme of the m-isomer. Other examples of positional isomers will be 

 encountered in later sections. 



Geometric Isomeric Analogs 



One would not expect that potent inhibitors would be found in cis 

 and trans pairs because of the different molecular configurations. We have 

 already seen that fumarate and maleate differ markedly in their reactions 

 with succinate dehydrogenase (page 34). The outstanding exception to 

 this rule is the well-known inhibition of aconitase by fraws-aconitate. This 

 enzyme catalyzes the interconversion of the tricarboxylates: 



Citrate ^ cts-aconitate ;fi isocitrate 



although perhaps cis-aconitate is not an obligatory intermediate between 

 citrate and isocitrate. Bernheim (1928), impressed by the results obtained 

 by Quastel with malonate, tested the effect of frans-aconitate on liver 

 "citric dehydrogenase" (the reduction of the methylene blue used in this 

 system was actually due to the oxidation of isocitrate formed from 

 citrate via aconitase) and found definite inhibition. He believed the inhi- 

 bition to be related to the structural similarity between citrate and trans- 

 aconitate, stating, "The curve obtained seems to indicate that the aconitic 

 acid is adsorbed on the enzyme so that part of the surface is unavailable 

 for citric acid." Twenty years later a thorough study of this inhibition was 

 made by Saffran and Prado (1949), using aconitase from pigeon breast 

 muscle. Both the conversion of m-aconitate to citrate and the disappearance 

 of citrate are inhibited by irans-aconitate. However, trans-acomta.te is 



