Electrogenic Partial Reactions of the Gastric H,K-ATPase

Abstract

The fluorescent styryl dye RH421 was used to identify and investigate electrogenic reaction steps of the H,K-ATPase pump cycle. Equilibrium titration experiments were performed with membrane vesicles isolated from hog gastric mucosa, and cytoplasmic and luminal binding of K+ and H+ ions was studied. It was found that the binding and release steps of both ion species in both principal conformations of the ion pump, E1 and P-E2, are electrogenic, whereas the conformation transitions do not contribute significantly to a charge movement within the membrane dielectric. This behavior is in agreement with the transport mechanism found for the Na,K-ATPase and the sarcoplasmic reticulum Ca-ATPase. The data were analyzed on the basis of the Post-Albers reaction cycle. The equilibrium dissociation constants for K1 binding on the cytoplasmic side were 11 and 16 mM. The respective equilibrium dissociation constants on the luminal side were obtained via K+ concentration dependence of the enzyme activity and determined to be 0.11 mM for both luminal binding sites.