Chromaffin Granule Proton Pump

This chapter focuses on the chromaffin granule proton pump. A continuous process of catecholamine uptake takes place in chromaffin granules of adrenal medullae cells. The driving force for this uptake is a proton-motive force generated by ATP hydrolysis, which is catalyzed by an ATPase located in the granule membrane. Though ATPase activity of chromaffin granule membranes was discovered over two decades ago, until recently, the identity of the enzyme responsible for the formation of proton-motive force was not clear. The revelation of the true identity of the chromaffin granule proton-ATPase was hampered by contamination of mitochondrial membranes in the granule membrane preparation and the instability of the genuine enzyme. The proton-ATPase of chromaffin granules may be defined as the minimal structure, isolated from chromaffin granule membranes that, upon reconstitution into phospholipid vesicles, will catalyze the reaction of ATP-dependent proton uptake. It is quite likely that only part of this structure will be sufficient to catalyze the ATPase activity of this protein complex. Chromaffin granules contain catecholamines and ATP at high concentrations; therefore, it is convenient to isolate them on the basis of their heavy density. Depending on the required purity of the membranes the preferred isolation method can be more or less rigorous.