| Abstract: | The ATP-dependent H+ pump from adrenal chromaffin granules is, like the platelet-dense granule H+ pump, essentially insensitive to the mitochondrial ATPase inhibitors sodium azide, efrapeptin, and oligomycin and also insensitive to vanadate and ouabain, agents that inhibit the Na+,K+-ATPase. The chromaffin granule H+ pump is, however, sensitive to low concentrations of NEM (N-ethylmaleimide) and Nbd-Cl (7-chloro-4-nitro-2,1,3-benzoxadiazole). These transport ATPases may thus belong to a new class of ATP-dependent ion pumps distinct from F1F0-and phosphoenzyme-type ATPases. Comparisons of ATP hydrolysis with ATP-dependent serotonin transport suggest that approximately 80% of the ATPase activity in purified chromaffin granule membranes is coupled to H+ pumping. Most of the remaining ATPase activity is due to contaminating mitochondrial ATPase and Na+,K+-ATPase. When extracted with cholate and octyl glucoside, the H+ pump is solubilized in a monodisperse form that retains NEM-sensitive ATPase activity. When reconstituted into proteoliposomes with crude brain phospholipid, the extracted enzyme recovers ATP-dependent H+ pumping, which shows the same inhibitor sensitivity and nucleotide dependence as the native pump. These data demonstrate that the predominant ATP hydrolase of chromaffin granule membrane is also responsible for ATP-driven amine transport and granule acidification in both native and reconstituted membranes. |
