title: Glucose 6-phosphate causes translocation of phosphorylase in hepatocytes and inactivates the enzyme synergistically with glucose.
abstract: The role of glucose 6-P (glucose 6-phosphate) in regulating the activation state of glycogen synthase and its translocation is well documented. In the present study, we investigated the effects of glucose 6-P on the activation state and compartmentation of phosphorylase in hepatocytes. Glucose 6-P levels were modulated in hepatocytes by glucokinase overexpression or inhibition with 5-thioglucose and the effects of AMP were tested using AICAR (5-aminoimidazole-4-carboxamide 1-beta-D-ribofuranoside), which is metabolized to an AMP analogue. Inhibition of glucokinase partially counteracted the effect of glucose both on the inactivation of phosphorylase and on the translocation of phosphorylase a from a soluble to a particulate fraction. The increase in glucose 6-P caused by glucokinase overexpression caused translocation of phosphorylase a to the pellet and had additive effects with glucose on inactivation of phosphorylase. It decreased the glucose concentration that caused half-maximal inactivation from 20 to 11 mM, indicating that it acts synergistically with glucose. AICAR activated phosphorylase and counteracted the effect of glucose 6-P on phosphorylase inactivation. However, it did not counteract translocation of phosphorylase by glucose 6-P. Glucose 6-P and AICAR had opposite effects on the activation state of glycogen synthase, but they had additive effects on translocation of the enzyme to the pellet. There was a direct correlation between the translocation of phosphorylase a and of glycogen synthase to the pellet, suggesting that these enzymes translocate in tandem. In conclusion, glucose 6-P causes both translocation of phosphorylase and inactivation, indicating a more complex role in the regulation of glycogen metabolism than can be explained from regulation of glycogen synthase alone.
mesh_terms: D000620:Aminoimidazole Carboxamide / Q000031:analogs & derivatives* / Q000494:pharmacology; D000818:Animals; D002451:Cell Compartmentation; D002478:Cells, Cultured; D005941:Glucokinase / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology*; D019298:Glucose-6-Phosphate / Q000502:physiology*; D006006:Glycogen Synthase / Q000378:metabolism; D022781:Hepatocytes / Q000187:drug effects / Q000201:enzymology* / Q000378:metabolism; D008297:Male; D006005:Phosphorylases / Q000378:metabolism*; D021381:Protein Transport; D051381:Rats; D017208:Rats, Wistar; D012265:Ribonucleotides / Q000494:pharmacology
chemical_list: D012265:Ribonucleotides; D000620:Aminoimidazole Carboxamide; D019298:Glucose-6-Phosphate; D006005:Phosphorylases; D006006:Glycogen Synthase; D005941:Glucokinase; C031143:AICA ribonucleotide; D005947:Glucose