Determination of m- and p-O-methylated products of -3,4-dihydroxyphenylalanine using high-performance liquid chromatography and electrochemical detection

In a study of in vitro and in vivo metabolism of -3,4-dihydroxyphenylalanine (_Dopa), two methods of high-performance liquid chromatography (HPLC) were used to separate the m- and p-O-methylated products. A reversed-phase column and an aqueous mobile phase by gradient elution were used; the elute was analyzed electrochemically with a single amperometric and dual coulometric electrode. The -Dopa and its O-methylated products could be detected individually in the enzymatic methylation of rat liver homogenate and in patients with Parkinson's disease. Meta/para ratios of O-methylation are easily obtained by this method.