Parkinson's disease (PD) is a neurodegenerative disorder characterised by the loss of dopamine-producing neurons in the midbrain. Oxidative stress (OS) is thought to contribute to the initiation and progression of PD. Levodopa (L-DOPA), the primary treatment for PD, can increase OS and additionally be mistakenly incorporated into proteins. Conversion of L-tyrosine to L-DOPA can, therefore, occur from both hydroxyl radical attack on tyrosine residues and replacement of L-tyrosine with L-DOPA during protein synthesis. This study aimed to explore the presence of L-DOPA as a human protein constituent in addition to creating a synthetically modified proteome containing L-DOPA.
Tyrosinase was used to convert tyrosine residues in tryptic peptides of human neuroblastoma cell (SH-SY5Y) proteins to DOPA, creating a positive control for DOPA incorporation. Cells were also treated with L-DOPA to allow in vitro incorporation into proteins. The brain subset of the draft human proteome was reanalysed for sites of hydroxylated tyrosine as a negative control and compared to two publicly available Parkinson’s disease datasets to quantitatively analyse the undocumented presence of this modification. PEAKS Studio X® was utilised for all data analyses.
The tyrosinase reaction achieved a 10% conversion of detected tyrosine residues to L-DOPA. Analysis of L-DOPA treated cell cultures showed increased numbers of L-DOPA sites versus non-treated controls. Numerous sites of hydroxylated tyrosine and other residues in the draft human brain proteome were identified. It was noted that the substantia nigra contains a higher number of hydroxylated sites, even amongst controls.
This study has mapped tyrosine hydroxylation sites within the human brain proteome and generated a method for the specific conversion of tyrosine to L-DOPA. This work has additionally created a novel library of L-DOPA containing peptides for assaying clinical samples of Parkinson’s disease and exploring hydroxylation in other human diseases and tissues.