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Non-oxygen-forming pathways of hydrogen peroxide degradation by bovine liver catalase at low hydrogen peroxide fluxes

Heme catalases are considered to degrade two molecules of H2O2 to two molecules of H2O and one molecule of O2 employing the catalatic cycle. We here studied the catalytic behaviour of bovine liver catalase at low fluxes of H2O2 (relative to catalase concentration), adjusted by H2O2-generating systems. At a ratio of a H2O2 flux (given in μM/min− 1) to catalase concentration (given in μM) of 10 min− 1 and above, H2O2 degradation occurred via the catalatic cycle. At lower ratios, however, H2O2 degradation proceeded with increasingly diminished production of O2. At a ratio of 1 min− 1, O2 formation could no longer be observed, although the enzyme still degraded H2O2. These results strongly suggest that at low physiological H2O2 fluxes H2O2 is preferentially metabolised reductively to H2O, without release of O2. The pathways involved in the reductive metabolism of H2O2 are presumably those previously reported as inactivation and reactivation pathways. They start from compound I and are operative at low and high H2O2 fluxes but kinetically outcompete the reaction of compound I with H2O2 at low H2O2 production rates. In the absence of NADPH, the reducing equivalents for the reductive metabolism of H2O2 are most likely provided by the protein moiety of the enzyme. In the presence of NADPH, they are at least in part provided by the coenzyme.