TheinVivoandinVitro Inhibition ofCatalase fromLeaves ofNicotiana sylvestris by3-Amino-1,2,4-Tr iazole

Seedlings oftobacco (Nicotiana sylvestris) weretreated invivo with0.03to20millimolar 3-amino-1,2,4-triazole (aminotriazole). Therewasarapid lossofcatalase (EC1.11.1.6) activity overthe first 5hoursfollowed byaslower decrease forthenext4hours toalevel that was15to20%oftheinitial activity, withlittle orno changeforperiods upto3days.Fifty percent lossofcatalase activity occurred at0.10to0.15millimolar inhibitor (18-hour incubation). Theisozymes oftobacco catalase differed insensitivity totheinhibitor. Enhanced-peroxidatic catalase (EP-CAT) (Havir EA,McHaleNA,[1989] PlantPhysiol 91:812-815) decreased 35%underconditions inwhichthemajor isozyme decreased 85%.Theresistance toaminotriazole inhibition demonstrated invivobyEP-CATwasalsoobserved invitro. Thetimes for50% inhibition at0.67, 3.33, 5.0,10.0, and15millimolar aminotriazole were15,5,2.6, 2.5, and1.5minutes, respectively, forthemajorisozyme ofcatalase and60,18.5, 5.1, 4,and3.0 minutes, respectively, forEP-CAT. Increasing H202concentration didnotchangethesensitivity ofEP-CATtoaminotriazole. The majorformofcatalase contained 4.0± 0.4molesofhemeper moleenzymeandEP-CAT3.4±0.3. Thus, theresistance ofEPCATtoaminotriazole isprobably notduetolowered affinity for H202 oralteration inhemecontent buttostructural changes that impair inhibitor binding.