IRREVERSIBLE INHIBITION OF CATALASE BY 



THE 3- AMINO- 1 : 2 : 4-TRIAZOLE GROUP OF 

 INHIBITORS IN THE PRESENCE OF CATALASE 



DONORS 



By E. Margoliash and A. Schejter 



The Laboratory for the Study of Hereditary and Metabolic Disorders, and 

 the Departments of Biological Chemistry and Medicine, University of 

 Utah College of Medicine, Salt Lake City, Utah, and the Department of 

 Experimental Medicine and Cancer Research, The Hebrew University, 

 Hadassah Medical School, Jerusalem 



3-AMiNO-l :2:4-TRiAZOLE (AT) has come into widespread use as a plant 

 growth regulator. When injected into laboratory animals this drug causes a 

 rapid decrease of the catalatic activity of liver or kidney suspensions to low 

 levels but produces no change in the catalatic activity of erythrocyte haemo- 

 lysates (Heim, Appleman and Pyfrom, 1956). 3-Amino-l :2:4-triazole and a 

 number of related substances were shown to cause the irreversible inhibition 

 of purified recrystallized catalase preparations from liver and blood in vitro 

 but only in the presence of a continuous supply of hydrogen peroxide 

 (Margoliash and Novogrodsky, 1958). This reaction parallels the effect of 

 3-amino-l : 2 : 4-triazole on the liver and kidney catalase activities in laboratory 

 animals, but gives no explanation of the lack of effect of the drug on erythrocyte 

 catalase in vivo. 



A study of the kinetics of the irreversible inhibition demonstrated that this 

 reaction was second order between the inhibitors and catalase-hydrogen 

 peroxide complex I (Cat. HoOa I), during which the inhibitors become 

 irreversibly bound to the enzyme (Margohash, Novogrodsky and Schejter, 

 1960). If in addition to catalase, hydrogen peroxide and the inhibitor, the 

 reaction mixture also contains a sufficiently high concentration of a catalase 

 donor, i.e., a substance that can be oxidized by Cat. H2O2 1, the concentration 

 of Cat. H2O2 I available for reaction with the inhibitor will be decreased 

 (Chance, 1953) and the irreversible inhibition reaction will be slower. 



The purpose of this paper is to develop the kinetic equations that will 

 describe the irreversible inhibition of catalase by the 3-amino-l: 2 :4-triazole 

 group of substances in the presence of catalase donors, and to show that the 

 lack of effect of such inhibitors in blood haemolysates is due to the presence 

 of a naturally occurring catalase donor in erythrocytes. The existence of 



236 



