The scavenging of superoxide radical by manganous complexes: In vitro

Abstract

Dialyzable manganese has been shown to be present in millimolar concentrations within cells of Lactobacillus plantarum and related lactic acid bacteria. This unusual accumulation of Mn appears to serve the same function as Superoxide dismutase (SOD), conferring hyperbaric oxygen and Superoxide tolerance on these SOD-free organisms. The form of the Mn in the lactic acid bacteria and the mechanisms whereby it protects the cell from oxygen damage are unknown. This report examines the mechanisms by which Mn catalytically scavenges O 2 −, both in the xanthine oxidase/cytochrome c SOD assay and in a number of in vitro systems relevant to the in vivo situation. In all the reaction mixtures examined, Mn(II) is first oxidized by O 2 − to Mn(III), and H 2O 2 is formed. In pyrophosphate buffer the Mn(III) thus formed is re-reduced to Mn(II) by a second O 2 −, making the reaction a true metal-catalyzed dismutation like that catalyzed by SOD. Alternatively, if the reaction takes place in orthophosphate or a number of other buffers, the Mn(III) is preferentially reduced largely by reductants other than O 2 −, such as thiols, urate, hydroquinone, or H 2O 2. H 2O 2, a common product of the lactic acid bacteria, reacted rapidly with Mn(III) to form O 2, apparently without intermediate O 2 release. Free hexaquo Mn(II) ions were shown by electron spin resonance spectroscopy and activity assays in noncomplexing buffers to be poorly reactive with O 2 −. In contrast, Mn(II) formed complexes having a high catalytic activity in scavenging O 2 − with a number of organic acids, including malate, pyruvate, propionate, succinate, and lactate, with the Mn-lactate complex showing the greatest activity.