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Metabolism of [14C]MPTP in mouse and monkey implicates MPP+, and not bound metabolites, as the operative neurotoxin
S C Yang1, J N Johannessen, S P Markey
1Laboratory of Clinical Science, National Institute of Mental Health, Bethesda, Maryland 20892.
Abstract:
The distribution, identification, and binding of [phenyl-14C]- or [methyl-14C]MPTP metabolites have been determined in brains of mouse and monkey exposed to toxic doses of MPTP. The distribution of radiolabeled metabolites was heterogeneous, with levels of MPP+ 1-100 mumol/L in dissected and homogenized monkey brain tissues. MPP+ constituted greater than 98% of all tissue radioactivity remaining at 1-3 days in the monkey and was identified in both cortical and striatal tissue. The relevance of the 2% of unextractable ("bound") radiolabeled metabolite was assessed in mouse brain by using pargyline or mazindol pretreatments which block dopamine depletion. The amount of binding increased rather than decreased when [phenyl-14C]MPTP was used along with pargyline or mazindol but was unchanged when [methyl-14C]MPTP was employed. This demonstrates that bound metabolites are inversely correlated to neurotoxicity as well as being N-demethylated. Two extractable metabolites, demethylated MPTP (PTP) and 1-methyl-4-phenyl-2-pyridone, were found at 30-min survival times in mouse brain and probably derive from peripheral metabolism of MPTP. At 4 h, mouse brain profiles of extractable metabolites resembled those from monkey brain, containing MPP+ as the predominant (greater than 90%) constituent. The similarity of MPP+ concentrations in mouse and monkey brain homogenates with those concentrations of MPP+ known to produce biological effects in vitro, along with the inverse relationship between bound metabolites and neurotoxicity, supports the intermediacy of MPP+ as the operative neurotoxin.
Insights
The neurotoxin MPTP forms MPP+, which is the primary metabolite found in monkey and mouse brains. Bound metabolites, inversely related to neurotoxicity, are not MPP+ and indicate N-demethylation.
Area of Science:
- Neuroscience
- Toxicology
- Pharmacology
Background:
- MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) is a neurotoxin that causes Parkinsonism.
- Understanding MPTP metabolite distribution and binding is crucial for elucidating its neurotoxic mechanism.
Purpose of the Study:
- To determine the distribution, identification, and binding of MPTP metabolites in mouse and monkey brains.
- To assess the role of bound metabolites in MPTP-induced neurotoxicity.
Main Methods:
- Administered radiolabeled MPTP ([phenyl-14C]- or [methyl-14C]) to mice and monkeys.
- Analyzed tissue radioactivity, metabolite identification (MPP+, PTP, 1-methyl-4-phenyl-2-pyridone), and metabolite binding.
- Utilized pargyline or mazindol pretreatments in mice to block dopamine depletion.
Main Results:
- MPP+ was the predominant metabolite (>98%) in monkey brains 1-3 days post-MPTP exposure, found in cortical and striatal tissues.
- Bound metabolites constituted ~2% of radioactivity and were inversely correlated with neurotoxicity.
- Binding increased with [phenyl-14C]MPTP and blocking agents, suggesting N-demethylation, but was unchanged with [methyl-14C]MPTP.
- Extractable metabolites PTP and 1-methyl-4-phenyl-2-pyridone were found in mouse brains at 30 min, likely from peripheral metabolism.
- Mouse brain metabolite profiles at 4 h resembled monkey brains, with MPP+ >90%.
Conclusions:
- MPP+ is the primary neurotoxin responsible for MPTP-induced Parkinsonism.
- Bound metabolites are not MPP+ and their inverse correlation with neurotoxicity suggests they are N-demethylated products.
- MPP+ concentrations in brain homogenates align with in vitro toxic levels, supporting its role as the operative neurotoxin.