Contribution of Sulfate Conjugation, Deamination, and O‐Methylation to Metabolism of Dopamine and Norepinephrine in Human Brain

Abstract: The kinetic constants were determined for dopamine (DA) and norepinephrine (NE) metabolism by phenolsulfotransferase (PST), type A and B monoamine oxidase (MAO), and membrane‐bound and soluble catechol‐O‐ methyltransferase (COMT) in frontal lobe preparations of human brain. PST and membrane‐bound COMT were found to have the lowest K_m, values for both catecholamines. By means of the appropriate rate equations and the calculated kinetic constants for each enzyme, the activity of each enzymatic pathway was determined at varying concentrations of DA and NE. Results indicate that deamination by MAO is the principal pathway for the enzymatic inactivation of DA whereas NE is largely metabolized by MAO type A and membrane‐bound COMT under the in vitro assay conditions used. At concentrations less than 100 μM, soluble COMT’contributes less than 5% to the total catabolism of either catecholamine. PST can contribute up to 15% of the total DA metabolism and 7% of NE metabolism.