Rabbit intestinal microsomal glucose 6-phosphatase has been shown to possess inorganic pyrophosphatase-and inorganic pyrophosphate- (and nucleoside 5′-di- and triphosphate-) glucose phosphotransferase activities. All these activities responded in a parallel fashion in studies of: (a) subcellular location, (b) distribution along the length of the small intestine, and (c) stability to partial thermal inactivation. Activation energies for all were quite similar. Inhibitions by molyb-date and by citrate, which previously has been shown to inhibit activities of the rat liver enzyme in a highly pH-dependent manner, were studied at pH 6.0. Ki values for each of these compounds, determined with the various activities, agreed closely. Km values for pyrophosphate in both phosphohydrolase and phosphotransferase reactions were nearly identical, as were Km values for glucose in reactions in which either pyrophosphate or nucleotide served as phosphoryl donor. Km values for the various substrates also were determined with microsomal preparations from rabbit liver and kidney and were found to correspond well with their counterparts evaluated for the intestinal enzyme. The common genetic identity of glucose 6-phosphatases from these various tissues is supported by these observations, as well as by a variety of other similarities noted in catalytic properties of the rabbit intestinal enzyme and those previously described for liver and kidney activities of rat. The presence in rat intestine of an inhibitor of glucose 6-phosphate phosphohydrolase has been confirmed, and inhibition of phosphotransferase activity by this same, heat-labile factor also has been observed. © 1968, American Chemical Society. All rights reserved.
