The regulation of stable catalase from Aspergillus niger was investigated. The preincubation of catalase with peroxynitrite (PN) resulted in a significant decrease in the production of O2, while the subsequent incubation with reduced glutathione (GSH, 1mM) led to restoration of the enzymatic activity. Western blot analysis revealed not only the increased immunoreactivities of 3-nitrotyrosine and S-nitrosocysteine in a PN-dose-dependent manner, but also conversely decreased immunoreactivity of 3-nitrotyrosine by the subsequent preincubation of catalase with GSH (1mM)/glutathione S-transferase (GST). The inhibition of the catalase after PN-treatment may be due to conformational changes of the enzyme via tyrosine-nitration/cysteine-nitrosation and the binding of active nitrogen/oxygen species to the Fe3+-protoporphyrin groups of the enzyme. The reverse of these processes to restore enzymatic activity by GSH/GST may be a vital antioxidative mechanism.