Conjugation and deamination of circulating dopamine: relationship between sulfated and free dopamine in man

In order to assess the importance of conjugation in dopamine metabolism, we infused dopamine (20 nmol/kg/min, 1 h) to seven male subjects, then measured the kinetics of free and conjugated dopamine and DOPAC (deaminated dopamine) in plasma both during and after the infusion. Dopamine increased as early as 2 min after the start of the infusion, then reached a plateau and remained elevated on a level of 25 times the basal value. Dopamine sulfate and DOPAC followed very similar patterns and increased continuously from the 30th and 20th min, respectively, until the end of the infusion (5.18- and 5.76-fold, respectively), but only dopamine sulfate remained elevated (1.65-fold) on the first day following infusion. DOPAC glucuronide increased moderately (2.58-fold) at the end of the experiment, but neither dopamine glucuronide nor DOPAC sulfate were increased despite high precursor circulating levels. Free DOPAC and dopamine sulfate levels during infusion were correlated with dopamine levels when all subjects were pooled. In addition, within a subject dopamine sulfate to dopamine correlations were found with slopes varying in a range of 1 to 15. Our results show that sulfation is significantly involved in the metabolism of circulating dopamine and is not easily saturated, suggesting that dopamine sulfate may be an index of endogenous dopamine release.