Dopamine, B-hydroxylase (D, BH) catalyzes the conversion of dopamine to the neurotransmitter and hormone norepinephrine within the catecholamineн secreting vesicles (chromaffin granules) of the adrenal medulla and the large dense-cored synaptic vesicles of the sympathetic nervous system. In adrenal chromaffin granules, D, BH is known to exist as both a soluble and membraneн bound protein, with an approximately equal distribution of enzyme activity between the two forms (for recent reviews of chromaffin granule organization and function, see 1-4). Upon stimulation of the chromaffin cell, granule membranes fuse with the plasma membrane and release their soluble contents, including the soluble form of dopamine, B-hydroxylase. The membrane-bound form of the enzyme is endocytosed from the cell surface and cycled to newly formed chromaffin granules via the Golgi region, where the granule memн …