Dopamine ß-monooxygenase (DßM) 2 and peptidylglycine α-hydroxylating monooxygenase (PHM) belong to a small class of copper proteins found exclusively in higher eukaryotes. These physiologically important enzymes catalyze the transformation of dopamine to norepinephrine (DßM)(Equation 1) and C-terminal glycine-extended peptides to their α-hydroxylated products (PHM)(Equation 2). Although their substrate specificities are grossly different, these enzymes greatly resemble each other in many other respects. Both enzymes are localized in subcellular compartments: the chromaffin vesicles of the adrenal gland or synaptic vesicles of the sympathetic nervous system (DßM)(1) and the secretory vesicles of the pituitary gland (PHM)(2). Although DßM and PHM exist in soluble and membrane-bound forms within their vesicular compartments (3, 4), the majority of mechanistic studies have been completed with …