Plasma levels of R- and S-salsolinol are not increased in "de-novo" Parkinsonian patients

Summary.

An augmented synthesis of tetrahydroisoquinolines, such as salsolinol (SAL) or an increased N-methylation of these compounds has been addressed by various investigators as putative pathophysiologic mechanisms in Parkinson's disease (PD). Aim of this study was (1) to investigate putative relations between plasma levels of dopamine and R- and S-enantiomers of SAL and (2) whether these metabolic precursors of the neurotoxic N-methylated-SAL (NMSAL) are elevated in untreated "de-novo" Parkinsonian patients compared to age- and sex-matched healthy controls. Plasma levels of R- and S-SAL and dopamine did not significantly (R-SAL: p = 0.61, S-SAL: p = 0.51, dopamine: p = 0.84) differ in both groups. Parkinsonian patients' R-SAL plasma levels were inversely related to intensity (p = 0.03, r = −0.42) and duration of PD (p = 0.03, r = −0.43) in contrast to S-SAL and dopamine. Dopamine levels were not associated to R-SAL (p = 0.88, r² = 0.0008) and S-SAL (p = 0.088, r² = 0.12) neither in Parkinsonian patients nor in controls. We conclude, that an upregulation of N-methylation of tetrahydroisoquinolines takes place in PD by enzymes such as neutral N-methyltransferase specific for R-SAL. The activity of this enzyme has been found elevated in parkinsonian lymphocytes. This increased N-methylation by the N-methyltransferase specific for R-SAL leads to the known augmented levels of neurotoxic R-NMSAL in Parkinsonian patients compared to controls in the cenral nervous system especially in the beginning of PD.