Dopamine β-hydroxylase (DBH) is an enzyme that converts dopamine to norepinephrine. DBH is a 290 kDa copper-containing oxygenase consisting of four identical subunits, and its activity requires ascorbate as a cofactor. It is the only enzyme involved in the synthesis of small-molecule neurotransmitters that is membrane-bound, making norepinephrine and epinephrine the only transmitters synthesized inside vesicles. It is expressed in noradrenergic nerve terminals of the central and peripheral nervous systems, as well as in chromaffin cells of the adrenal medulla. DBH is inhibited by disulfiram, tropolone, and, most selectively, by nepicastat.