Hydrogen peroxide enhances the activity of monoamine oxidase type-B but not of type-A: a pilot study.

The effect of hydrogen peroxide on monoamine oxidase (MAO) activity has been determined in homogenates of human brain areas taken postmortem. It could be shown that hydrogen peroxide enhances significantly the activity of MAO-B after short-term incubation (2 min), while no changes have been noted after long-term preincubation (60 min) indicating reversibility of this effect. MAO-A activity was not changed or decreased after preincubation with hydrogen peroxide. Freezing and thawing procedures did not change hydrogen peroxide stimulation of MAO in the caudate nucleus, while MAO-A activity dose dependently decreased. Inhibition of hydrogen peroxide stimulated MAO-B activity in human cortex by (-)deprenyl was found to be of similar potency compared to hydrogen peroxide free estimations. Glutathione, ascorbic acid and mannitol did not block MAO stimulation by hydrogen peroxide, while sodium azide led to a complete inhibition of hydrogen peroxide derived MAO activitation. Interorgan comparison showed increase of MAO-B activity in crude mitochondrial fractions of rat liver after preincubation with hydrogen peroxide, while with rat heart a reduction of MAO activity was detectable. As a conclusion these data indicate a possible role of hydrogen peroxide in the age-dependent increase of MAO-B in platelets and brain. Furthermore, increase of cytotoxic hydrogen peroxide via combined L-dopa therapy cannot be excluded to be of importance in the appearance of adverse reactions after long-term treatment with high doses. To reduce hydrogen peroxide production due to MAO activity, a combined treatment of L-dopa plus a selective MAO inhibitor and eventually additional administration of radical scavengers (ascorbic acid, vitamin E etc.) seems to be indicated.