Reactive oxygen species production by monoamine oxidases in intact cells

Monoamine oxidase (MAO) A and B are mitochondrial enzymes involved in the oxidative deamination of endogenous and exogenous amines. At present, the production of H₂O₂ by MAO in intact cells and its functional consequences in cell function have not been extensively investigated. The aim of this study was to define whether, in intact cells, the metabolism of small amounts of MAO substrates was able to induce a detectable H₂O₂ production. Hydrogen peroxide production was measured using a luminol-amplified chemiluminescence assay in three cell types, rat mesangial cells, rabbit proximal tubule cells and Hep-G2 cells, containing different MAO A/MAO B ratios. Our results showed that cell incubation with tyramine (50 μmol/l) led to a time-dependent H₂O₂ generation which was fully inhibited by MAO A (clorgyline and RO 41–1049) and MAO B (selegiline and RO 19–6327) inhibitors. The extent of inhibition of H₂O₂ production by selective inhibitors was in agreement with the amount of MAO isoforms expressed in each cell type, as determined by Western blot analysis and enzyme assay. Altogether, these findings show that, in a normal cell environment, MAO can be a source of reactive oxygen species which could have a functional impact on cell functions. In addition, we propose the luminol-amplified chemiluminescence assay as a rapid and sensitive procedure to characterize the monoamine oxidase isoforms and their regulation in intact cells. Received: 15 December 1998 / Accepted: 17 February 1999