Polyphenol-Enriched Extracts from Leaves of Mediterranean Plants as Natural Inhibitors of Monoamine Oxidase (MAO)-A and MAO-B Enzymes

Background: Alzheimer’s disease and Parkinson’s disease are multifactorial disorders causing severe disability, rising with the increase in life expectancy. Currently, the identification of natural compounds useful against these disorders is becoming an urgent necessity. In this study, we used polyphenol-enriched extracts obtained from leaves of Mediterranean plants, which are important in animal feeding (Lotus ornithopodioides, Hedysarum coronarium, Medicago sativa) and in the human Mediterranean diet (Cichorium intybus). Objectives: The aims of this study were as follows: (i) tentative identification of the organic compounds present in the extracts; (ii) determination of their effect on the activity of monoamine oxidase (MAO)-A and MAO-B, key enzymes involved in the metabolism of aminergic neurotransmitters, as well as on protein expression level of these enzymes in cell lines expressing basal MAO-A and MAO-B. Methods: The ability of plant polyphenol extracts to inhibit MAO-A and MAO-B activity was assessed by in vitro enzyme assays. The protein expression level was analyzed by Western blotting. Results: Our data demonstrate that all the extracts behaved as MAO-A and MAO-B inhibitors, although to a different extent and enzyme inhibition mechanism; among them, the extract from L. ornithopodioides induced a decrease in MAO-A protein level in human AGS gastric adenocarcinoma and SH-SY5Y neuroblastoma cell lines. Conclusions: These data reinforce the hypothesis that a plant-based diet and/or integrative supplementation of pharmacological treatments can be considered for preventing and relieving symptoms of neurodegenerative diseases.