244 E. Margoliash and A. Schejter 



neither the kinetic expression for the irreversible inhibition of catalase by 

 the AT group of inhibitors in the presence of hydrogen peroxide but in the 

 absence of catalase donors (Margoliash and Schejter, 1960), nor that developed 

 above for the case in which a catalase donor is present, predict any effect 

 of increasing the enzyme concentration on the relative rate of inhibition, 

 under the usual conditions where hydrogen peroxide, inhibitor and donor 

 are at molar concentrations far above those of the enzyme. Thus the 

 decrease in the rate of inhibition observed by Margoliash and Novogrodsky 

 (1958) on increasing the catalase concentration was probably due to the 

 presence of a catalase donor in the relatively crude enzyme preparation used. 



SUMMARY 



(1) A kinetic equation for the rate of the irreversible inhibition of catalase 

 by the 3-amino-l : 2 : 4-triazole group of substances in the presence of hydrogen 

 peroxide and a catalase donor, has been developed. 



(2) The lack of inhibition of catalatic activity by 3-amino-l : 2 : 4-triazole in 

 rat erythrocyte haemolysates has been shown to be due to a normally occur- 

 ring, dialysable, sulfhydryl catalase donor. The lack of inhibition of blood 

 catalase following the administration of 3-amino-l :2:4-triazole to laboratory 

 animals is considered to be due to this donor. 



A cknowledgement 



This investigation was aided in part by research grants from the National 

 Institutes of Health, United States Public Health Service, Bethesda, Maryland. 



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