Biphasic character of fungal catalases inhibition with hydroxylamine in presence of hydrogen peroxide

The kinetics of hydrogen peroxide decomposition with fungal catalases, i.e. Aspergillus niger (ANC), Penicillium vitale (PVC), Scytalidium thermophilum (STC), exhibits biphasic character in the presence of hydroxylamine (HA). HA is inactive in the initial phase of the hydrogen peroxide decomposition. In the second phase, at pH 7.2, 30 °C and 1 mM H2O2 the concentration of the inhibitor that results in 50% inhibition was estimated as 25, 40 and 261 nM for STC, PVC and ANC, respectively. In acidic solutions, the inhibition decreases with an apparent pKa 5.3–5.8 that is related to HA protonation. The inhibition of ANC at pH 7.2 is reversible since dialysis of the samples restores activity and spectral properties of ferricatalase. Hydrazine (HZ) does not inhibit ANC even at the pH values larger than pKa of HZ protonation. During the reaction of ANC with HA and H2O2, catalase intermediate is generated showing maximum absorbance at 428 nm with a little bit smaller extinction compared to the maximal absorbance of ferricatalase. heme of catalase inhibition includes an intermediate, possibly nitric oxide-ferrocatalase. Good agreement with the experimental data is achieved if the kinetic constants estimated from the non-stationary hydrogen peroxide decomposition model are used for the calculations of the inhibition of catalases at the second phase, and for the kinetics of intermediate formation.