Inhibition of the bioactivation of the neurotoxin MPTP by antioxidants, redox agents and monoamine oxidase inhibitors

Abstract

Monoamine oxidase (MAO) enzymes located in human mitochondria oxidize neurotransmitters and bioactivate the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) by oxidation to directly-acting neurotoxic pyridinium cations (MPDP⁺/MPP⁺) that produce Parkinsonism. Antioxidants and MAO inhibitors are useful as neuroprotectants. Naturally-occurring substances, antioxidants and redox agents were assessed as inhibitors of the oxidation (bioactivation) of MPTP by human mitochondria and MAO enzymes. Methylene blue, 5-nitroindazole, norharman (β-carboline), 9-methylnorharman (9-methyl-β-carboline) and menadione (vitamin-K analogue) highly inhibited the oxidation of MPTP to the neurotoxic species, MPDP⁺/MPP⁺, in human mitochondria (IC₅₀ of 0.18, 3.1, 9.9, 7.3, and 12.6 μM, respectively). Inhibition by methylene blue was similar to R-deprenyl (IC₅₀ of 0.15 μM), a known neuroprotectant. The naturally-occurring β-carbolines, harmine, harmaline and tetrahydro-β-carboline, and the antioxidants, melatonin, resveratrol, quercetin and catechin showed little or no inhibition. Oxidation of MPTP in mitochondria was performed by human MAO-B and the above active compounds were also inhibitors of this isozyme. Norharman and 5-nitroindazole were competitive inhibitors of MAO-B whereas methylene blue inhibited MPTP oxidation (IC₅₀ of 50 nM) under a mixed type and predominantly uncompetitive mechanism. Methylene blue, 5-nitroindazole, norharman, 9-methylnorharman and menadione inhibit MAO-B in mitochondria and afford protective effects, as suggested by a reduced conversion of MPTP to neurotoxic species.