title: Evidence for the participation of independent translocation for phosphate and glucose 6-phosphate in the microsomal glucose-6-phosphatase system. Interactions of the system with orthophosphate, inorganic pyrophosphate, and carbamyl phosphate.
abstract: The interactions of Pi, PPi, and carbamyl-P with the hepatic glucose-6-phosphatase system were studied in intact and detergent-disrupted microsomes. Penetration of PPi and carbamyl-P into intact microsomes was evidenced by their reactions with the enzyme located exclusively on the luminal surface. Lack of effects of carbonyl cyanide m-chlorophenylhydrazone and valinomycin + KCl indicated that pH gradients and/or membrane potentials that could influence the kinetics of the system are not generated during metabolism of PPi and glucose-6-P by intact microsomes. With disrupted microsomes, only competitive interactions were seen among glucose-6-P, Pi, PPi, and carbamyl-P. With intact microsomes, Pi, PPi, and carbamyl-P were relatively weak, noncompetitive inhibitors of glucose-6-phosphatase, and PPi hydrolysis was inhibited competitively by Pi and carbamyl-P but noncompetitively by glucose-6-P. Analysis of the kinetic data in combination with findings from other studies that a variety of inhibitors of the glucose-6-P translocase (T1) does not affect PPi hydrolysis provide compelling evidence that permeability of microsomes to Pi, PPi, and carbamyl-P is mediated by a second translocase (T2). Some properties of the microsomal anion transporters are described. If the characteristics of the glucose-6-phosphatase system as presently defined in intact microsomes apply in vivo, glucose-6-P hydrolysis appears to be the predominant, if not the exclusive, physiologic function of the system. Both the "noncompetitive character" and the relative ineffectiveness of Pi as an inhibitor of glucose-6-phosphatase of intact microsomes result from the rate limitation imposed by T1 that prevents equilibration of glucose-6-P across the membrane. In microsomes from fed rats, where T1 is less rate restricting, about one-half as much Pi was required to give 50% inhibition compared with microsomes from fasted or diabetic rats. Thus, any treatment or agent that alters the kinetic relationship between transport and hydrolysis of glucose-6-P (e.g. endocrine or nutritional status) is an essential consideration in analyses of kinetic data for the glucose-6-phosphatase system.
mesh_terms: D000818:Animals; D017920:Antiporters; D002221:Carbamyl Phosphate / Q000494:pharmacology; D011756:Diphosphates / Q000494:pharmacology; D005947:Glucose / Q000494:pharmacology; D005952:Glucose-6-Phosphatase / Q000378:metabolism*; D007425:Intracellular Membranes / Q000201:enzymology; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D009004:Monosaccharide Transport Proteins; D009097:Multienzyme Complexes / Q000378:metabolism*; D010710:Phosphates / Q000494:pharmacology; D010770:Phosphotransferases / Q000378:metabolism*; D051381:Rats
chemical_list: D017920:Antiporters; D011756:Diphosphates; D009004:Monosaccharide Transport Proteins; D009097:Multienzyme Complexes; D010710:Phosphates; C503533:Slc37a4 protein, rat; C111261:glucose 6-phosphate(transporter); D002221:Carbamyl Phosphate; D010770:Phosphotransferases; D005952:Glucose-6-Phosphatase; D005947:Glucose