In the second step, the FADH2 is oxidized back to FAD, and molecular oxygen is reduced to H2O2. Notice that the hydrogen peroxide (H2O2) is of concern because of the possibility of its subsequent oxidation into free radicals. This model was proposed by P.Dostert et. al in Italy.
Figure 2 This figure shows a more detailed picture of the mechanism of MAO-B, and also shows how hydrogen peroxide is oxidized to make free radicals. Hydrogen peroxide is normally removed by glutathione peroxidase, but whenever iron is associated with neuromelanin, it will donate an electron to the hydrogen peroxide, which will then divide into two hydroxyl radicals (-OH). These are extremely toxic to brain tissue. The purpose of MAO-inhibitors is to block the formation of hydrogen peroxide so that these radicals cannot be made. The model above was proposed by M.B.H. Youdim and P. Riederer at the University of Wurzburg, Germany.
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