Glucose‐6‐phosphate phosphohydrolase of detergent‐treated liver microsomal membranes exhibits a specific kinetic behaviour towards glucose 6‐phosphate

With the aim of questioning the apparent loss of specificity of the microsomal glucose‐6‐phosphate phosphohydrolase after detergent‐treatment, we performed competitive inhibition experiments among the four best substrates of the enzyme, i. e. the 6‐phosphates of glucose (Glc6 P ), mannose‐6 (Man6 P ), glucosamine (GlcN6 P ) and 2‐deoxyglucose (dGlc6 P ). The K m and V max of glucose‐6‐phosphatase (Glc6 P ase) and mannose‐6‐phosphatase (Man6 P ase), assayed either by complex formation determination of P i produced or by radiometric determination of [U‐ 14 C]Glc or [U‐ 14 C]Man, were very close to 1 mM and 0.64 μmol · min −1 · mg −1 microsomal protein, respectively. The K m of the enzyme for GlcN6 P and for dGlc6 P , determined by colorimetric assay of P i , were equal to 1.53 ± 0.07 mM and 2.35 ± 0.15 mM, respectively, whilst the V max was not different from that of Glc6 P ase and Man6 P ase. Unexpectedly, the K i of Man6 P (1.61 ± 0.22 mM), GlcN6 P (2.24 ± 0.17 mM) and dGlc6 P (3.40 ± 0.07 mM) for Glc6 P ase, assayed by liberation of [U‐ 14 C]Glc, were significantly (50%) higher than their K m previously determined. The K i of Glc6 P (0.66 ± 0.05 mM) for Man6 P ase, assayed by liberation of [U‐ 14 C]Man, was significantly lower than its K m previously determined. In contrast, the K i of GlcN6 P (1.55 ± 0.05 mM) for Man6 P ase, assayed by the radiometric assay, was not different from its K m previously determined. It can be inferred from these data that Glc6 P phosphohydrolase exhibits specific behaviour towards Glc6 P after the detergent‐treatment of the microsomal membrane.