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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 molybdate and by citrate, which previously has been shown to inhibit activities of the rat liver enzyme in a highly pHdependent manner, were studied at pH 6.0. Ki values for each of these compounds, determined with the various activities, agreed closely. K , values for pyrophosphate in both phosphohydrolase and phosphotransferase reactions were nearly identical, as were K, values C lassical liver and kidney microsomal glucose 6-phosphatases (D-glucose 6-phosphate phosphohydrolase, EC 3.1.3.9; reaction 1) have been shown to catalyze inorganic pyrophosphate hydrolysis (reaction 2 ) and the synthesis of glucose 6-phosphate by transfer of a phosphoryl group from pyrophosphate to glucose (reaction 3) Stetten and Taft, 1964; Nordlie and Soodsma, 1966). A variety of nucleoside di-and triphosphates, from which CDPl was selected for the present studies, also serve as phosphoryl donors for glucose 6-phosphate formation catalyzed by by this enzyme (reaction 4) (Nordlie and Arion, 1965; Nordlie and Soodsrna, 1966). ppi + glucose + glucose-6-P + PI (3)

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