Functional organization of adrenal chromaffin vesicles

The functions of the chromaffin vesicles are reflected in their molecular organization. They contain large amounts of water-soluble proteins and ATP, which act in concert with catecholamines to form a stable storage pool. The vesicles contain an ATP-driven catecholamine transport system that enables the vesicles to take up dopamine, the precursor to NE, and to recover amines that leak out of the granules. The transport system appears to be coupled to the ATPase activity of the vesicle membranes and to include an amine carrier. The enzyme DBH is present as a soluble protein within the vesicle and as a component of the membrane. However, substrates can be acted upon only after they have been transported into the vesicle. During secretion the vesicle membrane fuses with the plasma membrane, but little is known about how this occurs.