610 2. ANALOGS OF ENZYME REACTION COMPONENTS 



two cationic groups on the enzyme, since the 3-substituted gkitarates are 

 bound more tightly than the 2-substituted isomers. 



The administration of nicotinylalanine to rats leads to a 4-fold increase 

 in the urinary level of A"-methylnicotinamide (Decker et al., 1963). Nicotin- 

 ylalanine is possibly formed from tryptophan through 3-hydroxykynure- 

 nine but studies with tryptophan-C^^ indicate it not to be a metabolite but 

 actually a strong inhibitor of kynureninase and kynurenine hydroxylase. 

 This inhibition presumably occurs in vivo, resulting in a sequential block 

 in the major route of kynurenine degradation. The effect of this analog on 

 the transaminase is not known. 



Inhibition of Urease by Methylurea and Thiourea 



There is a good deal of disagreement on the analog inhibitions of urease 

 and the kinetics are certainly not simple. Takeuchi (1933) originally found 

 very little inhibition (perhaps 5%) by 83 mM methylurea (although mark- 

 ed inhibition by oxyurea — which may be HgN — CO — NHgOH — occurs, 

 this is probably not competitive). Sophianopoulos and Corley (1959) ob- 

 tained competitive inhibition by methylurea at lower concentrations (un- 

 specified), but higher concentrations (300-1000 mM) increase the substrate 

 inhibition produced by urea; these latter effects may be related to enzyme 

 denaturation. The inhibition was found to depend on the pH by Shaw and 

 Raval (1961), it being noncompetitive between pH 7 and 8.9, and competi- 

 tive below pH 7. Furthermore, the kinetics correspond to the reaction of 2 

 molecules of methylurea with the active center, which may relate this type 

 of inhibition to substrate inhibition. 



Thiourea was claimed to stimulate urease at 5 mM (Sizer and Tytell, 

 1941), to have no effect below 50 mM, and to inhibit 35% at 500 mM 

 (Kistiakowsky and Shaw, 1953). This inhibition is completely reversible 

 and occurs rapidly. At pH 6 the inhibition is competitive but as the pH is 

 raised, deviation occurs. The kinetics again point to 2 molecules of thiourea 

 reacting with the enzyme. The reaction 



E -f 2 I -> EI2 



is not affected by change of pH, whereas the reaction 



ES + 2 I -> ESl2 



is sensitive to pH, which serves to explain the change in inhibition type 

 with the pH. Lister (1956) reported that of the 17 urea analogs tested, 

 only thiourea is inhibitory — 35% at 200 mM, 70% at 400 mM, and 85% 

 at 1000 mM, when urea is 500 mM. The inhibition is prevented by cysteine, 

 which brings up the possibility of disulfide bond formation by thiourea at 

 high concentrations. 



