Ultra-performance liquid chromatography-mass spectrometry analysis of post-mortem brain tissue reveals specific amino acid profile dysregulation in Parkinson's disease and Alzheimer's disease patients.

Previous metabolomic studies have reported significant alterations in circulating amino acids in Parkinson's disease (PD). However, it remains unclear whether these changes reflect central nervous system pathology or are restricted to peripheral metabolism. To address this issue, here we measured the levels of a panel of amino acids in post-mortem brain samples from MPTP-intoxicated monkeys, with and without L-DOPA treatment, and from PD patients at different Braak Lewy body (LB) stages through targeted UPLC-MS. In untreated MPTP monkeys, the putamen showed significant increases in glutamate, aspartate, GABA, phenylalanine, branched-chain amino acids, and serine. L-DOPA treatment further altered this profile, increasing glycine, threonine, and citrulline levels. In contrast, no amino acid changes were detected in the superior frontal gyrus (SFG) of MPTP monkeys, regardless of treatment. In PD patients, caudate-putamen analysis revealed consistent serine upregulation at Braak LB stages 3-4 and 6, with stage 6 additionally showing increased proline and reduced phosphoethanolamine. No amino acid changes were observed in the PD SFG, whereas Alzheimer's disease SFG samples showed marked amino acid increases. Together, these findings demonstrate region-specific amino acid dysregulation in PD, preferentially affecting nigrostriatal targets and supporting disease-specific metabolic signatures across neurodegenerative disorders.