Simultaneous determination of serum concentrations of levodopa, dopamine, 3-O-methyldopa and alpha-methyldopa by HPLC.

Levodopa is the medication of choice for Parkinson's disease. The biological complexity of levodopa and of its main derivatives makes their determination important in the clinical field. The aim of this study was to develop an HPLC method for the simultaneous determination of serum concentrations of levodopa, dopamine, 3-O-methyldopa and alpha-methyldopa. We compared UV and fluorimetric detection of native and derivatised compounds. Though less sensitive than other methods, UV detection is important to exclude naturally fluorescent, interfering substances. Fluorimetric detection of derivatised compounds is more sensitive than UV detection. Since 3-O-methyldopa does not react with the derivatising agent 1,2-diphenylethylenediamine, it cannot be detected. For simultaneous determination of the four compounds after pharmacological treatment of patients we therefore advise fluorimetric detection of the native compound.