Inhibition of monoamine oxidase (MAO) by β-carbolines and their interactions in live neuronal (PC12) and liver (HuH-7 and MH1C1) cells

Interactions among monoamine oxidase (MAO) inhibitors in drugs, botanicals, and dietary supplements may lead to unpredictable neurochemical dysfunction due to excessive inhibition or therapeutic invalidation. Often recombinant MAO or brain tissue homogenates have been used to evaluate MAO inhibitors such as the β-carboline alkaloids (harmane, harmine, harmaline, and harmalol). However, there is a lack of cellular systems for evaluation of MAO activity, which represents a more advanced in vitro model compared to recombinant enzymes or tissue lysates. Using kynuramine assays, intracellular MAO inhibition by β-carbolines was measured in PC12 (rat pheochromocytoma), MH1C1 (rat liver), and HuH-7 (human liver) cell lines, which were compared with human recombinant MAO and cell lysates. β-Carbolines (1 μM, 90 min incubations) inhibited MAO by 40-99% in live PC12 cells where MAO A was the active isoform, and <12% in HuH-7 and MH1C1 cells where MAO B was primarily active. The combination index (CI), which serves as a quantitative indicator of pharmacological interactions, was determined for harmaline/harmine (CI, 1.01-1.25) and methylene blue/harmine (CI, 0.74-1.07) in PC12 cells. Overall, this study illustrates applications of cell-based in vitro assay platforms to gain a comprehensive understanding of intracellular MAO inhibitors and their interactions.