?-Carbolines as selective monoamine oxidase inhibitors:In vivo implications

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Abstract
The inhibitory action of a range ofβ-carbolines on human and rat monoamine oxidase (MAO) A and B has been studied. Concentrations of 5-hydroxytryptamine and phenylethylamine, approximately at their Km values, were used as substrates for MAO A and B respectively. A wide variation in selectivity was found, with harmaline being 10,000 times more potent an inhibitor of A than B whereas, using tetrahydro-β-carboline and harmane, the difference was nearer to ten-fold. Of the carbolines which have been found endogenously, tetrahydro-β-carboline, 6-methoxytetrahydro-β-carboline and harmane are all sufficiently potent inhibitors of human MAO A, with I50 values of 5×10−6, 10−6, 5×10−7M respectively, for this property to be of possible physiological significance. Harmane, with an I50 of 5×10−6M, might also play a role as an inhibitor of MAO B.