Abstract
Bovine liver catalase was exposed to cysteine, as a natural inactivator metabolize, causing autoxidation-generating $H_2O_2$ continuously. The catalase species concentrations and activity measurement were done by spectrophotometry in phosphate buffer 10mM, pH 6.5, and 27 $^{\circ}C$. The activity of catalase decreased continuously due to the conversion of active ferricatalase species, E-Fe (III), to an inactive enzyme species, E-Fe (IV). This conversion is related to the slow production of $H_2O_2generated$ by autoxidation of cysteine. The free SH-group of cysteine has an essential role in production of $H_2O_2$ and hence inactivation of catalase. NADPH can protect catalase against inactivation due to the conversion of inactive form of E-Fe (IV) to ferricatalase species, E-Fe (III).