Catalase-like oxygen production by horseradish peroxidase must predominantly be an enzyme-catalyzed reaction

When hydrogen peroxide (H2O2) was provided as the only substrate for horser adish peroxidase C (HRP-C) the catalase-like emission of oxygen gas was obs erved. The reaction was favored at neutral compared to acidic pH. Addition of the superoxide radical scavengers tetranitromethane (TNM) or superoxide dismutase (SOD) increased activity. TNM's effect was concentration dependen t but SOD's was not, indicating that only some of the superoxide generated was released into solution. Manganous ions (Mn2+) react with superoxide rad icals to regenerate H2O2 but not oxygen; when added to the reaction medium oxygen production was reduced but not abolished. The effect was essentially concentration independent, suggesting that most oxygen was produced enzyma tically and not by chemical disproportionation of superoxide. The catalase- like activities of some site-directed mutants of HRP-C suggest that active site residues histidine 42 and arginine 38 are influential in determining t his activity. A clear correlation also existed between catalase activity an d the enzymes' resistance to inactivation by H2O Computer simulation of a r eaction scheme that included catalase-like activity agreed well with experi mental data. (C) 2001 Academic Press.