Covalent protein binding of a metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine to mouse and monkey brain in vitro and in vivo

We have recently reported that a reactive metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is formed in rat brain in vitro by the type B monoamine oxidase (MAO). In the present study, we further characterize the irreversible binding in vitro using tissues from mice and monkeys, two species more sensitive than rats to MPTP neurotoxicity. We also report the occurrence of irreversible binding of radioactivity after administration of tritiated MPTP in the same species in vivo. Tissue homogenates were incubated at 37° with 1-[methyl-3H]MPTP in in vitro experiments. Animals were injected with labeled MPTP and sacrificed at different times in in vivo experiment. The perchloric acid precipates of tissue homogenates from either procedure were washed exhaustively with organic solvents and counted for radioactivity. The amount of recovered radioactivity in in vitro experiments was similar brain homogenates from mice and monkeys, whereas a considerably lower amount was found in mouse liver. MAO-B inhibitors decreased the covalent binding. However, the combined MAO-B/MAO-A inhibitor pargyline had no effect if added after 2 hr of incubation. Sulfhydryl-containing compounds decreased the covalent binding in a concentration-related manner. GSH reduced the rate of the reaction throughout the incubation. The covalent binding slowly increased in time in vivo in mouse brain, not in liver. There was a two-fold variation of covalently bound radioactivity in different brain areas of 3H2-MPTP-treated monkey. This reactive metabolite may play a role in MPTP neurotoxicity.