Abstract
The proton-translocating adenosine triphosphatase (ATPase) of bovine chromaffin granules contains up to five different polypeptides. Its activity is inhibited by N-ethylmaleimide, and ATP protects the enzyme from inhibition. After treatment of membranes with N-[2-3H]ethylmaleimide, only one polypeptide is strongly radiolabelled: this is the largest (70 kDa) subunit of the proton-translocating ATPase. This subunit therefore contains the ATP-hydrolysing site. Two-dimensional electrophoresis reveals heterogeneity in this polypeptide.