GLUCOSE-6-PHOSPHATASE SPECIFICITY AFTER MEMBRANE SOLUBILIZATION BY DETERGENT TREATMENTS

Glucose-6-phosphatase (Glc6Pase) is a liver microsomal protein exhibit ing a high specificity for glucose-6-phosphate (Glc6P) when present in the intact membrane. On the contrary, Glc6Pase of detergent-treated l iver microsomes is able to hydrolyze both Glc6P and mannose-6-phosphat e (Man6P) with the same affinities and rates (K-m and V-max) at pH 6.5 [Ajzannay, A., Minassian, C., Riou, J.P., and Mithieux, G. (1993) fur . J. Biochem. 212, 335-338]. We have carried out competition experimen ts between constant micromolar concentrations of [U-C-14]Glc6P and [U- C-14]Man6P and increasing millimolar concentrations of unlabeled Glc6P and Man6P. The rate of hydrolysis of [U-C-14]Glc6P was progressively inhibited as the competitor concentration increased, but it was signif icantly higher in the presence of Man6P than in the presence of Glc6P (twofold higher at 20 mM). In the same manner, the rate of hydrolysis of [U-C-14]Man6P was inhibited by increasing concentrations of competi tor, but it was significantly higher when the competitor was Man6P tha n when the competitor was Glc6P. These data provide direct evidence th at, in the presence of both Glc6P and Man6P, Glc6Pase is able to exhib it a specific, albeit limited, kinetic behavior towards the former aft er detergent treatment of the membrane. They also emphasize the role o f the membrane in the acquisition of the final specificity of Glc6Pase under physiological conditions.