Free radical generation and coupled thiol oxidation by lactoperoxidase/SCN −/H 2O 2

Abstract

The lactoperoxidase-catalyzed oxidation of glutathione (GSH) and thiocyanate (SCN −) was studied. Oxidation of SCN − was recorded by ultraviolet spectroscopy and by electron spin resonance (ESR). Consumption of GSH was measured by amperometric titration. One or two moles of GSH was oxidized per mole of H 2O 2 added, depending on the reaction conditions. Omission of SCN − prevented the oxidation of GSH. The oxidation of GSH required only catalytic amounts of SCN −, which was therefore recycled. Iodide (I −) could replace SCN −, while chloride or bromide were ineffective. The apparent Michaelis constant for SCN − was 17 μM. Oxidation of SCN − gave rise to two reactive intemediates, one stable and one unstable. The stable intermediate ( −OSC ·  N −(?)) decayed by a second-order reaction with a rate constant of 1.1 M −1 s −1. The decay of the unstable radical was very fast. The data (a) explain the short- and long-term antibacterial effects of lactoperoxidase-halide-H 2O 2 system. (b) point to possible deleterious effects due to glutathione depletion, (c) are of relevance for free radical disease involving sulphur-centered free radicals, and (d) support previous ovservations on lipid peroxidation/halogenation in biological membranes, liposomes, and unsaturated fatty acis.