Potassium ion-activated hydrolysis of p-nitrophenyl phosphate in pancreatic islet-cell membranes
Summary, in English
Hydrolysis of p nitrophenyl phosphate was measured in a fraction enriched in plasma membranes from pancreatic islets of non inbred ob/ob mice. Hydrolysis was stimulated by K+ (10mM) in the pH range 5-10; a small peak of K+ induced activation was observed between pH 7.5 and 8. Both the K+ induced activation and the hydrolysis in the absence of K+ were Mg2+ dependent; maximum activation was obtained with 10mM K+ plus 5mM Mg2+. Rb+ was as effective an activator as K+. Ouabain was inhibitory, the effect being inversely related to the K+ concentration; 0.1-0.2 mM ouabain caused about 50% inhibition in the presence of 1mM K+, but had no demonstrable effect in the presence of 4-5 mM K+. The K+ stimulated activity was markedly inhibited by 0.1 mM ATP, 35-140 mM Na+, or 0.01 mM p chloromercuribenzenesulphonic acid. Similarities to Rb+ accumulation suggest that catalysis of univalent cation flow in pancreatic β cells may be coupled to a phosphoryl transfer reaction with ATP as natural substrate or regulator.