46 



1. lODOACETATE AND lODOACETAMIDE 



than at 7 (McKinley-McKee, 1963). The results with carboxydismutase 

 suggested an enzyme group with a pK^ of 8.82, which fits well with an SH 

 group, and the authors also isolated *S-carbamylmethylcysteine from the 

 iodoacetamide-C^*-treated enzyme. 



An interesting situation was postulated by Hagen (1957) as a result of 

 his finding that liver cathepsin is inhibited more at lower pH's (see Table 

 1-8). Cysteine was added as an activator and it may be that iodoacetate 

 reacts with it, this occurring more rapidly at higher pH's and thus deplet- 

 ing the iodoacetate so that less is available for inhibition of the enzyme. 



Protection of Enzymes against Iodoacetate 



Protection of 3-PGDH by substrate and various phosphorylated sub- 

 stances, of alcohol dehydrogenase by substrate and NAD, and of creatine 

 kinase by either substrate with Mg++ has been discussed. There are many 

 other examples, some of which are given in the accompanying tabulation. 



Enzyme 



Protector 



Reference 



Aldehyde dehydrogenase 



Homogentisicase 

 ^-Hydroxyisobutyrate 



dehydrogenase 

 Hydroxypyruvate 



reductase 

 ProHdase 



PjTuvate decarboxylase 

 Pyruvate oxidase 

 Ribonuclease 

 Succinate dehydrogenase 



Acetaldehyde 

 NADP 



Homogentisate 

 NAD 



HydroxypjTuvate 



Mn++ 



Pyruvate 



Diphosphothiamine 



Cytidylate 



Fluoride + phosphate 



Suramin 



Mahler et al. (1954) 

 Stoppani and Milstein (1957 b) 

 Tokuyama (1959) 

 Robinson and Coon (1957) 



Behal and Hamilton (1962) 



Davis and Smith (1957) 

 Stoppani et al. (1953) 

 Baer (1948) 



Barnard and Stein (1959) 

 Stoppani and Brignone (1956) 

 Stoppani and Brignone (1957) 



The ability of a substance to protect apparently depends primarily on its 

 location of binding on the enzyme surface relative to the attacked SH group 

 and the energy of binding. Thus NADP protects the NADP-dependent alde- 

 hyde dehydrogenase of yeast, but NAD does not because the latter is not 

 bound significantly. Homogentisicase is protected by substrate but not by 

 Fe++, whereas /3-hydroxyisobutyrate dehydrogenase is protected by NAD 

 but not by substrate. Leucine decarboxylase is protected by pyridoxal-P 

 but leucine actually potentiates the inhibition by iodoacetate (Sutton and 

 King, 1959, 1962). In this case it was postulated that pyridoxal-P covers 

 the SH group whereas binding of the substrate exposes it. Malate dehydro- 



