title: Inhibition of aromatic L-aminoacid decarboxylase in clonal pheochromocytoma PC12h cells by N-methyl-4-phenylpyridinium ion (MPP+).
abstract: N-Methyl-4-phenylpyridinium ion (MPP+), a reaction product of a neurotoxin, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), was found to inhibit aromatic L-aminoacid decarboxylase activity in rat clonal pheochromocytoma PC12h cells. The enzyme activity was enhanced to several folds by addition of a cofactor, pyridoxal phosphate, and MPP+ inhibited the enhancement of the activity by exogenously added pyridoxal phosphate. The inhibition was competitive to pyridoxal phosphate, and the Ki value of MPP+ was 26.7 +/- 0.4 microM, while the Km value of pyridoxal phosphate was 0.645 +/- 0.053 microM. The inhibition was partly irreversible. The enzyme sample was incubated with MPP+ and then dialyzed against phosphate buffer. After dialysis, the inhibited enzyme activity was only partly recovered by addition of pyridoxal phosphate, even though MPP+ was completely removed. Activity of other enzymes, tyrosine hydroxylase and monoamine oxidase could be recovered by dialysis. On the other hand, MPP+ did not affect the binding of the enzyme with the substrate, L-DOPA or 5-hydroxytryptophan.
authors.affiliation: Department of Biochemistry, Nagoya University School of Medicine, Japan.
authors.forename: M
authors.initials: M
authors.lastname: Naoi
authors.forename: T
authors.initials: T
authors.lastname: Takahashi
authors.forename: H
authors.initials: H
authors.lastname: Ichinose
authors.forename: T
authors.initials: T
authors.lastname: Nagatsu
mesh_terms: D015655:1-Methyl-4-phenylpyridinium; D000310:Adrenal Gland Neoplasms; D000818:Animals; D065105:Aromatic Amino Acid Decarboxylase Inhibitors; D001667:Binding, Competitive; D002460:Cell Line; D007700:Kinetics; D009498:Neurotoxins / Q000494:pharmacology*; D010673:Pheochromocytoma; D011726:Pyridinium Compounds / Q000494:pharmacology*; D011732:Pyridoxal Phosphate / Q000494:pharmacology
chemical_list: D065105:Aromatic Amino Acid Decarboxylase Inhibitors; D009498:Neurotoxins; D011726:Pyridinium Compounds; D011732:Pyridoxal Phosphate; D015655:1-Methyl-4-phenylpyridinium