Analysis of monoamine oxidase enzymatic activity by reversed-phase high performance liquid chromatography and inhibition by β-carboline alkaloids occurring in foods and plants

Monoamine oxidase (MAO) is a flavin adenine dinucleotide (FAD)-containing enzyme located at the outer membranes of mitochondria that catalyzes the oxidative deamination of biogenic and xenobiotic amines. We have used a chromatographic method to measure MAO-enzymatic activity by using kynuramine as a non-selective substrate with its MAO-oxidation product subsequently analyzed by RP-HPLC–DAD and HPLC–mass spectrometry (MS). This method was applied to study the kinetic parameters, inhibition and reaction products of MAO recombinant enzymes in presence of tetrahydro-β-carboline and β-carboline alkaloids occurring in foods, plants and mammals. Analysis by HPLC showed that tetrahydro-β-carbolines or β-carbolines were not modified by MAO. Several β-carbolines such as tryptoline (1,2,3,4-tetrahydro-β-carboline) and 1-methyltryptoline (1-methyl-1,2,3,4-tetrahydro-β-carboline) were inhibitors of MAO-A; instead their corresponding 6-hydroxy-derivatives (6-hydroxytryptoline and 6-hydroxy-1-methyltryptoline) lacked this activity. Tetrahydro-β-carboline-3-carboxylic acids were unable to inhibit MAO enzymes. In contrast, their oxidation products, i.e. the fully aromatic β-carbolines (norharman and harman), acted as good inhibitors of MAO. Two tetrahydro-β-carbolines (i.e. tryptoline and 1-methyltryptoline) occurring in foods were isolated by solid-phase extraction (SPE) and RP-HPLC from selected samples of sausages and the corresponding extracts exhibited good inhibition properties over MAO-A. These results suggest that β-carbolines from foods, plants, and mammals may exert inhibitory actions on MAO enzymes.