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Kong LD, Cheng CH, Tan RX. 
“Inhibition of MAO A and B by some plant-derived alkaloids, phenols and anthraquinones”. 
J Ethnopharmacol. 2004 Apr 03;91(2-3):351-5.
A total of seventeen phytochemicals including seven alkaloids (piperine, strychnine, brucine, stachydrine, tetrandrine, frangchinoline and sinomenine), four phenols (paeonol, honokiol, magnolol and eugenol) and six anthraquinones (emodin, rhein, chrysorphanol, aloe-emodin, physcion and 1,8-dihydroxyanthraquinone) was examined for inhibitory activity of monoamine oxidase (MAO) A and B from rat brain mitochondrial. Among these compounds, piperine and paeonol were found to be inhibitory against MAO A in a dose-dependent manner with IC(50) values of 49.3 and 54.6 microM, respectively. Piperine, paeonol and emodin were shown to inhibit MAO B in a dose-dependent manner with the IC(50) data of 91.3, 42.5 and 35.4 microM, respectively. Lineweaver-Burk transformation of the inhibition data indicated that the inhibitory action of piperine on MAO A was of mixed type, and that of paeonol on the same type of the enzyme was of non-competitive type. For piperine, the K(i) and K(I) were determined to be 35.8 and 25.7microM, respectively. For paeonol, the K(i) was estimated to be 51.1 microM. The inhibition of piperine and paeonol on MAO B was of competitive type with K(i) values of 79.9 and 38.2 microM, respectively. The inhibition of emodin on MAO B was of mixed type with the K(i) and K(I) data of 15.1 and 22.9 microM, respectively. The present investigation showed that the phytochemicals piperine, paeonol and emodin are potent MAO inhibitors whereas other compounds were inactive against any type of MAO at 100 microM in the present assay.
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