l-DOPA is incorporated into brain proteins of patients treated for Parkinson's disease, inducing toxicity in human neuroblastoma cells in vitro

Levodopa (l-dopa), a close structural analogue of the protein amino acid l-tyrosine, can substitute for l-tyrosine in protein synthesis and be mistakenly incorporated into newly synthesised proteins in vitro. We show that l-dopa-containing proteins are present in the brain in l-DOPA-treated Parkinson's disease patients and accumulate in specific brain regions. In vitro studies demonstrate that substitution of l-tyrosine residues in proteins with l-DOPA causes protein misfolding and promotes protein aggregation in SH-SY5Y neuroblastoma cells resulting in the appearance of autofluorescent bodies. We show that the presence of l-DOPA-containing proteins causes profound changes in mitochondria and stimulates the formation of autophagic vacuoles in cells. Unlike l-DOPA, which is toxic to cells through its ability to generate radicals, proteins containing incorporated l-DOPA are toxic to SH-SY5Y cells by a mechanism independent of oxidative stress and resistant to antioxidants. These data suggest that the accumulation of l-DOPA-containing proteins in vulnerable cells might negatively impact on cell function.