Key role of the medium in methemalbumin-catalyzed peroxidation of aromatic free radical scavengers and antioxidants

Participation of the complexes of hemin and albumins (or delipidated albumins) in peroxidation of aromatic free radical scavengers and antioxidants was studied at varying hemin/albumin ratios. The radicalscavenging amines includedo-phenylenediamine (OPD) and tetramethylbenzidine (TMB); the antioxidants were gallic acid (GA) and GA polydisulfide (GAPD). Peroxidation reactions were carried out in buffered physiological saline (BPS) supplemented with 2% dimethylsulfoxide (DMSO), pH 7.4 (medium A), or in 40% aqueous dimethylformamide (DMF), pH 7.4 (medium B). In all systems involving methemalbumins, kinetic constants (k_cat), Michaelis constants(K _M), and the ratios thereof(k _cat/K_M) were determined for OPD oxidation in medium A and TMB oxidation in medium B. Oxidation of OPD, GA, and GAPD in medium A was characterized by a decrease in the catalytic activity of hemin after the formation of hemin-albumin complexes. Conversely, oxidation of TMB and OPD in medium B was distinguished by pronounced activation of hemin present within methemalbumins.