Gastric K +-stimulated adenosine triphosphatase. Demonstration of an endogenous activator

Abstract

A K’stimulated, ouabain insensitive ATPase activity associated with the apical and tubulovesicular membranes of the acid secreting cells of fundic mucosa from a number of different species has recently been reported [ 1,2] and has been strongly implicated to be involved in H+ transport [3-S]. During isolation of purified microsomal membrane vesicles enriched in K’stimulated ATPase from the fundus of various species, we observed that the degree of stimulation of the enzyme by K’ in the presence of gramicidin varied considerably from one preparation to another. The ionophore, gramicidin, has been shown to be very effective for maximal activation of the microsomal K+-stimulated ATPase [2] . The variability in enzymatic activity observed in presence of gramicidin indicated that there could be some dissociable regulatory factor(s) bound to the enzyme under in vivo conditions. The degree of dissociation of the factor(s) during isolation of the purified membranes might account for the variable activity of the enzyme. In this report, we present evidence that there is a heat-labile, non-dialysable and protease-sensitive activator in the soluble supernatant fraction from rabbit fundic mucosal cells, which markedly activates the K+-stimulated ATPase and pNPPase activities of rabbit, frog, and hog gastric microsomes. Furthermore, Ca2+ at low concentrations has been shown to obliterate the activating effects of the activator(s).