Inactivation of Coprinus cinereus peroxidase by 4-chloroaniline during turnover: comparison with horseradish peroxidase and bovine lactoperoxidase

Abstract

The peroxidase from Coprinus cinereus (CPX) catalyzed oxidative oligomerization of 4-chloroaniline (4-CA) forming several products: N-(4-chlorophenyl)-benzoquinone monoamine (dimer D), 4,4′-dichloroazobenzene (dimer E); 2-(4-chloroanilino)- N-(4-chlorophenyl)-benzoquinone (trimer F); 2-amino-5-chlorobenzoquinone-di-4-chloroanil (trimer G); 2-(4-chloroanilino)-5-hydroxybenzoquinone-di-4-chloroanil (tetramer H) and 2-amino-5-(-4-chlroanilino)-benzoquinone-di-4-chloroanil (tetramer I). In the presence of 4-CA and H 2O 2, CPX was irreversibly inactivated within 10 min. Inactivation of CPX in the presence of H 2O 2 was a time-dependent, first-order process when the concentration of 4-CA was varied between 0 and 2.5 mM. The apparent dissociation constant ( K i) for CPX and 4-CA was 0.71 mM. The pseudo-first order rate constant for inactivation ( k inact), was 1.15×10 −2 s −1. Covalent incorporation of 20 mole 14C-4-CA per mole of inactivated CPX was observed. The partition ratio was about 2200 when either 4-CA or H 2O 2 was used as the limiting substrate. These results show that 4-CA is a metabolically activated inactivator (i.e. a suicide substrate). Unmodified heme and hydroxymethyl heme were isolated from native, 4-CA-inactivated and H 2O 2-incubated CPX. Inactivation resulted in significant losses in both heme contents. Analysis of tryptic peptides from 4-CA-inactivated CPX by MALDI-TOF/MS and UV-VIS spectrophotometry suggested that trimer G and tetramer H were the major 4-CA derivatives that were covalently bound, including to a peptide (MGDAGFSPDEVVDLLAAHSLASQEGLNSAIFR) containing the heme binding site. These studies show that heme destruction and covalent modification of the polypeptide chain are both important for the inactivation of CPX. These results were compared with similar studies on 4-CA-inactivated horseradish peroxidase (HRP) and bovine lactoperoxidase (LPO) during the oxidation of 4-CA.