Enzymes of glucose metabolism in normal mouse pancreatic islets.
1. Glucose-phosphorylating and glucose 6-phosphatase activities, glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP(+)-linked isocitrate dehydrogenase, ;malic' enzyme and pyruvate carboxylase were assayed in homogenates of normal mouse islets. 2. Two glucose-phosphorylating activities were detected; the major activity had K(m) 0.075mm for glucose and was inhibited by glucose 6-phosphate (non-competitive with glucose) and mannoheptulose (competitive with glucose). The other (minor) activity had a high K(m) for glucose (mean value 16mm) and was apparently not inhibited by glucose 6-phosphate. 3. Glucose 6-phosphatase activity was present in amounts comparable with the total glucose-phosphorylating activity, with K(m) 1mm for glucose 6-phosphate. Glucose was an inhibitor and the inhibition showed mixed kinetics. No inhibition of glucose 6-phosphate hydrolysis was observed with mannose, citrate or tolbutamide. The inhibition by glucose was not reversed by mannoheptulose. 4. 6-Phosphogluconate dehydrogenase had K(m) values of 2.5 and 21mum for NADP(+) and 6-phosphogluconate respectively. 5. Glucose 6-phosphate dehydrogenase had K(m) values of 4 and 22mum for NADP(+) and glucose 6-phosphate. The K(m) for glucose 6-phosphate was considerably below the intra-islet concentration of glucose 6-phosphate at physiological extracellular glucose concentrations. The enzyme had no apparent requirement for cations. Of a number of possible modifiers of glucose 6-phosphate dehydrogenase, only NADPH was inhibitory. The inhibition by NADPH was competitive with NADP(+) and apparently mixed with respect to glucose 6-phosphate. 6. NADP(+)-isocitrate dehydrogenase was present but the islet homogenate contained little, if any, ;malic' enzyme. The presence of pyruvate carboxylase was also demonstrated. 7. The results obtained are discussed with reference to glucose phosphorylation and glucose 6-phosphate oxidation in the intact mouse islet, and the possible nature of the beta-cell glucoreceptor mechanism.
D000818:Animals; D002247:Carbon Isotopes; D002951:Citrates; D005941:Glucokinase / Q000032:analysis; D005942:Gluconates; D005947:Glucose / Q000378:metabolism*; D005952:Glucose-6-Phosphatase / Q000032:analysis / Q000037:antagonists & inhibitors; D005954:Glucosephosphate Dehydrogenase / Q000032:analysis; D006539:Heptoses; D006593:Hexokinase / Q000032:analysis; D006600:Hexosephosphates; D066298:In Vitro Techniques; D007515:Islets of Langerhans / Q000201:enzymology* / Q000378:metabolism; D007521:Isocitrate Dehydrogenase / Q000032:analysis; D007700:Kinetics; D008025:Ligases / Q000032:analysis; D008291:Malate Dehydrogenase / Q000032:analysis; D008358:Mannose; D051379:Mice; D009249:NADP / Q000378:metabolism; D010734:Phosphogluconate Dehydrogenase / Q000032:analysis; D011773:Pyruvates; D014044:Tolbutamide
D002247:Carbon Isotopes; D002951:Citrates; D005942:Gluconates; D006539:Heptoses; D006600:Hexosephosphates; D011773:Pyruvates; D009249:NADP; D014044:Tolbutamide; D008291:Malate Dehydrogenase; D007521:Isocitrate Dehydrogenase; D010734:Phosphogluconate Dehydrogenase; D005954:Glucosephosphate Dehydrogenase; D006593:Hexokinase; D005941:Glucokinase; D005952:Glucose-6-Phosphatase; D008025:Ligases; D005947:Glucose; D008358:Mannose