Oxidative decarboxylation of tris-(p-carboxyltetrathiaaryl)methyl radical EPR probes by peroxidases and related hemeproteins: Intermediate formation and characterization of the corresponding cations

Tris(p-carboxyltetrathiaaryl)methyl radicals (TAM) are good EPR probes for measurement of dioxygen concentration in biological systems and for EPR imaging. It has been previously reported that these radicals are efficiently oxidized by superoxide, O2−, or alkylperoxyl radicals, ROO, and by liver microsomes via an oxidative decarboxylation mechanism leading to the corresponding quinone-methides (QM). This article shows that peroxidases, such as horseradish peroxidase (HRP), lactoperoxidase (LPO) and prostaglandin synthase (PGHS), and other hemeproteins, such as methemoglobin (metHb), metmyoglobin (metMb) and catalase, also efficiently catalyze the oxidation of TAM radicals to QM by H2O2 or alkylhydroperoxides. These reactions involve the intermediate formation of the corresponding cations TAM+ that have also been cleanly generated by K2Ir(IV)Cl6 and characterized by UV–Visible spectroscopy and mass spectrometry, and through their reactions with ascorbate or H2O2. Labelling experiments on HRP-catalyzed oxidation of TAM to QM using H218O or 18O2 in the presence of glucose and glucose oxidase (GOX) showed that the oxygen atom incorporated into QM came both from O2 and from H2O. Mechanisms for these reactions in agreement with those data were proposed. Oxidative decarboxylation of TAM to QM is a new reaction catalyzed by peroxidases. Such reactions should be considered when using TAM as EPR oximetry probes in vivo or in vitro in complex biological media.