MPTP and MPTP analogs induced cell death in cultured rat hepatocytes involving the formation of pyridinium metabolites.

MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) which has been shown to produce a Parkinson-like syndrome in humans and monkeys also causes cell death in cultures of rat hepatocytes. Treatment of cells with MPTP or its metabolite MPP+ (1-methyl-4-phenyl pyridinium ion), resulted in leakage of lactic acid dehydrogenase and 14C-labeled adenine nucleotides, as well as marked depletion of ATP and glutathione. Deprenyl, a specific inhibitor of monoamine oxidase-B, the enzyme catalyzing the oxidation of MPTP into MPP+, blocked the lethal effect of MPTP, but gave no protection from MPP+-induced cell death. The 4'-fluoro and 4'-chloro analogs of MPTP evoked toxicities similar to that of the parent compound, while N-butyl-PTP, 4'-amino-MPTP, and 2'-methyl-MPTP were relatively less toxic. N-Acetylamino-MPTP was found virtually nontoxic. The cell death produced by these analogs was also associated with leakage of [14C]adenine nucleotides, which is an indicator of loss of ATP from cells. All these compounds except the N-acetylamino analog were converted to corresponding pyridinium metabolites by liver cells when analyzed by high-pressure liquid chromatography and plasma desorption mass spectrometry. MPTP and its analogs also served as substrates for rat liver mitochondrial monoamine oxidase to varying degrees. Toxicity of various analogs, with the noticeable exception of 2'-methyl-MPTP, was inhibited by deprenyl. These findings indicate that the conversion of MPTP and its analogs to corresponding pyridinium metabolites is essential for the expression of toxicity.