Differential inhibition of mitochondrial respiratory control by N-1-substituted, 3-,4-substituted pyridinium halides

Studies on the neurotoxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine: inhibition of NAD-linked substrate oxidation by its metabolite, 1-methyl-4-phenylpyridinium

The effects of the parkinsonism-inducing neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and its 4-electron oxidation product 1-methyl-4-phenylpyridinium (MPP+) were studied in isolated mitochondria and in mouse brain striatal slices. ADP-stimulated oxidation of NAD-linked substrates was inhibited in a time-dependent manner by MPP+ (0.1-0.5 mM), but not MPTP, in mitochondria prepared from rat brain, mouse brain, or rat liver. Under identical conditions, succinate oxidation was relatively unaffected. In neostriatal slices prepared from the mouse, a species susceptible to the dopaminergic neurotoxicity of MPTP, incubation with either MPP+ or MPTP caused metabolic changes consistent with inhibition of mitochondrial oxidation, i.e., an increase in the formation of lactate and accumulation of the amino acids glutamate and alanine with concomitant decreases in glutamine and aspartate levels. The changes resulting from incubation with MPTP were prevented by the monoamine oxidase inhibitor pargyline, which blocks formation of MPP+ from MPTP. The results suggest that compromise of mitochondrial function and its metabolic sequelae within dopaminergic neurons could be an important factor in the neurotoxicity observed after MPTP administration.

Inhibition of NADH-linked oxidation in brain mitochondria by 1-methyl-4-phenyl-pyridine, a metabolite of the neurotoxin, 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine

1-methyl-4-phenylpyridine (MPP+), a major metabolite of the neurotoxin, 1-methyl-4-phenyl-1,2,5,6-tetrahydropyridine (MPTP) inhibited the ADP-stimulated and uncoupled oxidation of NADH-linked substrates by brain mitochondrial preparations. MPTP itself was ineffective. The apparent Ki's for MPP+ inhibition of pyruvate or glutamate oxidation by purified rat brain mitochondria were approximately 300 and 400 microM, respectively; with mouse brain mitochondria the values were lower, 60 and 150 microM, respectively. Succinate oxidation was unaffected by either compound. Compromise of mitochondrial oxidative capacity by MPP+ could be an important factor in mechanisms underlying the toxicity of MPTP.