title: Action of orthophosphate on rabbit red blood cell hexokinase.
abstract: 1. Orthophosphate (Pi) at low concentrations, is able to activate the glucose 6-phosphate, glucose-1,6-diphosphate or 2,3-diphosphoglycerate-inhibited hexokinase (E.C. 2.7.1.1). In contrast, at high concentrations it is competitive inhibitor of MgATP2- with a Ki of 20 mM. 2. The Pi effect on inhibited hexokinase is pH dependent and becomes lower as the pH increase. The similar pH-dependence of the Ki of glucose-1,6-diphosphate with the pH-independence of the Ki of glucose 6-phosphate here reported, suggest the coincidence of the C-1 phosphate, of glucose-1,6-diphosphate and Pi binding sites. 3. Activation of uninhibited hexokinase by Pi at pH lower than the physiological ones has been demonstrated to be an artefact.
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000818:Animals; D004789:Enzyme Activation; D004912:Erythrocytes / Q000201:enzymology*; D005947:Glucose / Q000494:pharmacology; D019298:Glucose-6-Phosphate / Q000031:analogs & derivatives; D005958:Glucosephosphates / Q000494:pharmacology; D006593:Hexokinase / Q000037:antagonists & inhibitors / Q000097:blood*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D010710:Phosphates / Q000494:pharmacology*; D011817:Rabbits
chemical_list: D005958:Glucosephosphates; D010710:Phosphates; D019298:Glucose-6-Phosphate; D000255:Adenosine Triphosphate; C031590:glucose-1-phosphate; C016473:glucose-1,6-bisphosphate; D006593:Hexokinase; D005947:Glucose