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Metabolic studies on the nigrostriatal toxin MPTP and its MAO B generated dihydropyridinium metabolite MPDP+
E Wu1, T Shinka, P Caldera-Munoz
1Division of Toxicology, University of California School of Pharmacy, San Francisco 94143-0446.
Abstract:
The metabolic fates of the nigrostriatal toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its two-electron oxidation product, the 1-methyl-4-phenyl-2,3-dihydropyridinium species (MPDP+), have been examined in mouse brain and liver tissue fractions. Incubations of MPTP (50 microM and 1 mM) with mouse brain preparations result in the expected MAO B catalyzed formation of MPDP+. The four-electron oxidation product, the 1-methyl-4-phenyl-pyridinium species (MPP+), was the only other metabolite detected. The oxidation of 50 microM MPDP+ to MPP+ in the same preparations appears to be mediated by unidentified components present in membrane containing structures. The behavior of MPTP in mouse liver subcellular fractions is considerably more complex. NADPH-supplemented liver microsomes convert MPTP to the desmethyl and N-oxide metabolites. At high (1 mM) initial concentrations of MPTP there is evidence that NADPH-dependent, pargyline-insensitive liver microsomal enzymes also catalyze the oxidation of MPTP to MPDP+. Incubations of MPDP+ with mouse liver preparations containing the cytosolic fraction led to the rapid disappearance of the substrate and the quantitative formation of a metabolic product with mass spectral and diode array UV characteristics expected for a lactam structure. Menadione, an inhibitor of the cytosolic enzyme aldehyde oxidase, inhibits the formation of this product. Unambiguous characterization of this metabolite as 1-methyl-4-phenyl-5,6-dihydro-2-pyridone was achieved by comparison of its high-resolution 1H NMR and high-resolution EI mass spectra with the corresponding spectra of a synthetic standard.(ABSTRACT TRUNCATED AT 250 WORDS)
Insights
The study investigated the metabolism of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and MPDP+ in mouse brain and liver. MPTP is converted to MPP+ in the brain, while liver metabolism yields different products, including a novel lactam metabolite.
Area of Science:
- Neuroscience
- Biochemistry
- Toxicology
Background:
- 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is a neurotoxin that selectively damages dopaminergic neurons in the nigrostriatal pathway.
- The metabolic activation of MPTP to its toxic species is crucial for its neurotoxicity, involving oxidation to MPDP+ and MPP+.
- Understanding the differential metabolism of MPTP and its metabolites in various tissues is essential for comprehending its toxicological profile.
Purpose of the Study:
- To elucidate the metabolic pathways of MPTP and MPDP+ in mouse brain and liver subcellular fractions.
- To identify and characterize the metabolites formed from MPTP and MPDP+ in these tissues.
- To investigate the enzymes and cellular components involved in MPTP metabolism.
Main Methods:
- Incubation of MPTP and MPDP+ with mouse brain and liver tissue fractions.
- Analysis of metabolites using mass spectrometry (MS) and diode array UV spectroscopy.
- Enzyme inhibition studies using menadione.
- Structural characterization of metabolites using high-resolution 1H NMR and EI mass spectrometry.
Main Results:
- In mouse brain, MPTP is metabolized to MPDP+ by MAO B, and further oxidized to MPP+ by membrane-associated components.
- Mouse liver microsomes convert MPTP to desmethyl and N-oxide metabolites, and at high concentrations, to MPDP+ via NADPH-dependent enzymes.
- MPDP+ is rapidly converted to a lactam metabolite, identified as 1-methyl-4-phenyl-5,6-dihydro-2-pyridone, in liver cytosols, a reaction inhibited by menadione, suggesting aldehyde oxidase involvement.
Conclusions:
- MPTP metabolism differs significantly between mouse brain and liver.
- The brain primarily converts MPTP to the toxic MPP+ species.
- Liver metabolism of MPTP and MPDP+ leads to detoxification via novel pathways, including the formation of a lactam metabolite.