Estimation of the turnover of 3-methoxytyramine in the rat striatum by HPLC with electrochemical detection: implications for the sequence in the cerebral metabolism of dopamine.

A highly sensitive method for the determination of 3-methoxytyramine (3-MT) in nervous tissue is described. The method is based on a rapidly performed isolation of 3-MT on small columns of Sephadex G 10, followed by reverse-phase high-performance liquid chromatography in conjunction with a rotating disk electrochemical detector. The detection limit of the assay (0.5-1 pmol/tissue sample) is about 10% of control value for microwave-killed rats. 3-MT as well as dopamine could be quantified in the same chromatographic run. Inhibition of catechol-O-methyl transferase with tropolone resulted in an exponential decline of 3-MT. From this exponential decline a turnover rate for 3-MT of 1.9 nmol/gh/h was calculated. In the same group of rats the turnover rate of homovanillic acid was 9.1 nmol/g/h. From these data it is concluded that in the rat striatum about 80% of homovanillic acid is formed from 3,4-dihydroxyphenylacetic acid and 20% from 3-MT.