Reduction of aromatic L-amino acid decarboxylase activity in clonal pheochromocytoma PC12h cells by culture in the presence of N-methyl-4-phenylpyridinium ion (MPP+).

Rat clonal pheochromocytoma PC12h cells were cultured in the presence of N-methyl-4-phenylpyridinium ion (MPP+) and the activity of aromatic L-amino acid decarboxylase (AADC) was reduced after 3 and 6 days of culture. AADC activity in control cells was increased markedly by addition of pyridoxal 5-phosphate (PLP) to the reaction mixture, but that in the cells cultured in the presence of MPP+ was not increased by addition of PLP. After 6 days culture, AADC activity was almost negligible in the cells cultured in the presence of 1 mM MPP+, but PLP concentration in the cells was not reduced. AADC in the cells cultured in the presence of MPP+ reduced the affinity to PLP, but the affinity to a substrate, L-DOPA, did not change. Intracellular concentration of AADC protein was not reduced, as shown by an immunobinding assay with anti-AADC antibody. These data indicate that MPP+ may induce conformational changes in AADC protein and reduces its affinity to the cofactor, PLP.