Gastric (H super(+), K super(+))-ATPase

Gastric acid secretion results from the activity of a specific ATPase, the (H super(+), K super(+))-ATPase. This enzyme, discovered in 1973, exchanges H super(+) for K super(+). It has two ATP binding sites, both involved in enzyme activity, whose affinities vary as a function of the H super(+) and K super(+) concentrations. Hydrolysis of ATP at the highest affinity site leads to the synthesis of a covalent aspartyl phosphate which accumulates in the absence of K super(+). The presence of this cation accelerates dephosphorylation resulting in the stimulation of ATPase (and PNPPase) activity. The structure of membranous (H super(+), K super(+))-ATPase is poorly defined. n-Octylglucoside solubilizes an active enzyme of 390-420 kDa which can be partly depolymerized using cholate. The monomer, characterized in SDS has a 95 kDa molecular mass and is inactive.