Effects of cysteine on the inactivation of bovine liver catalase

Bovine liver catalase was exposed to cysteine, as a natural inactivator met abolite, causing autoxidation-generating H2O2 continuously. The catalase sp ecies concentrations and activity measurement were done by spectrophotometr y in phosphate buffer 10 mM, pH 6.5, and 27 degrees C. The activity of cata lase decreased continuously due to the conversion of active ferricatalase s pecies, E-Fe (III), to an inactive enzyme species, E-Fe (IV). This conversi on is related to the slow production of H2O2 generated by autoxidation of c ysteine. The free SH-group of cysteine has an essential role in production of H2O2 and hence inactivation of catalase. NADPH can protect catalase agai nst inactivation due to the conversion of inactive form of E-Fe (IV) to fer ricatalase species, E-Fe (III).