The Inhibition by Phlorizin of Kidney Microsomal Inorganic Pyrophosphate-Glucose Phosphotransferase and Glucose GPhosphatase*

Glucose 6-phosphatase from rat liver and kidney microsomes previously has been shown to catalyze an inorganic pyrophosphate-glucose phosphotransferase reaction. Nordlie and Soodsma recently have suggested that formation and then degradation of glucose 6-phosphate by the combined action of phosphotransferase and phosphohydrolase activities of this enzyme may under certain conditions constitute part of a mechanism for facilitated transport of glucose across the kidney tubular cell. To attempt to gain additional insight regarding the possible significance of such a physiological role for the enzyme, studies of the effect of phlorizin, a potent inhibitor of tubular sugar transport, on both phosphohydrolase and phosphotransferase activities of this enzyme were undertaken. Inhibition, in vitro, of both glucose 6-phosphate phosphohydrolase and pyrophosphate-glucose phosphotransferase activities of kidney homogenates and microsomal suspensions prepared in isotonic sucrose solution was noted over the entire range of phlorizin concentrations studied, 6.7 x 1O-5 M to 6.7 IIIM. Sensitivity of activities to inhibition by this compound was either partially or completely abolished by supplementation of the homogenates or microsomal suspensions, to 0.2% (w/v), with sodium deoxycholate, sodium cholate, digitonin, Triton X-100, or Tween 20. Phlorizin was without discernible effect on activity of partially purified enzyme preparations obtained by fractional ammonium sulfate precipitation of deoxycholate-dispersed microsomal suspensions. Inhibition of phosphotransferase activity in homogenates or microsomes was markedly potentiated by the cationic detergent trimethylhexadecylammonium bromide. Kinetic analyses indicated that inhibition of both phosphohydrolase and phosphotransferase activities by phlorizin was of the classical noncompetitive type. A value of ap-