Radiometric oil well assay for glucokinase in microscopic structures

The Central Role of Glucokinase in Glucose Homeostasis: A Perspective 50 Years After Demonstrating the Presence of the Enzyme in Islets of Langerhans

It is hypothesized that glucokinase (GCK) is the glucose sensor not only for regulation of insulin release by pancreatic β-cells, but also for the rest of the cells that contribute to glucose homeostasis in mammals. This includes other cells in endocrine pancreas (α- and δ-cells), adrenal gland, glucose sensitive neurons, entero-endocrine cells, and cells in the anterior pituitary. Glucose transport is by facilitated diffusion and is not rate limiting. Once inside, glucose is phosphorylated to glucose-6-phosphate by GCK in a reaction that is dependent on glucose throughout the physiological range of concentrations, is irreversible, and not product inhibited. High glycerol phosphate shuttle, pyruvate dehydrogenase, and pyruvate carboxylase activities, combined with low pentose-P shunt, lactate dehydrogenase, plasma membrane monocarboxylate transport, and glycogen synthase activities constrain glucose-6-phosphate to being metabolized through glycolysis. Under these conditions, glycolysis produces mostly pyruvate and little lactate. Pyruvate either enters the citric acid cycle through pyruvate dehydrogenase or is carboxylated by pyruvate carboxylase. Reducing equivalents from glycolysis enter oxidative phosphorylation through both the glycerol phosphate shuttle and citric acid cycle. Raising glucose concentration increases intramitochondrial [NADH]/[NAD⁺] and thereby the energy state ([ATP]/[ADP][Pi]), decreasing [Mg²⁺ADP] and [AMP]. [Mg²⁺ADP] acts through control of K_ATP channel conductance, whereas [AMP] acts through regulation of AMP-dependent protein kinase. Specific roles of different cell types are determined by the diverse molecular mechanisms used to couple energy state to cell specific responses. Having a common glucose sensor couples complementary regulatory mechanisms into a tightly regulated and stable glucose homeostatic network.

Differential metabolic adaptation to acute and long-term hypoxia in rat primary cortical astrocytes

Brain astrocytes provide structural and metabolic support to surrounding cells during ischemia. Glucose and oxygen are critical to brain function, and glucose uptake and metabolism by astrocytes are essential to their metabolic coupling to neurons. To examine astrocyte metabolic response to hypoxia, cell survival and metabolic parameters were assessed in rat primary cortical astrocytes cultured for 3 weeks in either normoxia or in either 1 day or 3 weeks sustained hypoxia (5% O2). Although cell survival and proliferation were not affected by the mildly hypoxic environment, substantial differences in glucose consumption and lactate release after either acute or prolonged hypoxia suggest that astrocyte metabolism may contribute to their adaptation. Hypoxia over a period of 1 day increased glucose uptake, lactate release, and glucose transporter 1 (GLUT1) and monocarboxylate transporter 1 (MCT1) expression, whereas hypoxia over a period of 3 weeks resulted in a decrease of all parameters. Furthermore, increased glucose uptake at 1 day of hypoxia was not inhibited by cytochalasin B suggesting the involvement of additional glucose transporters. We uncovered hypoxia-regulated expression of sodium-dependent glucose transporters (SGLT1) in astrocytes indicating a novel adaptive strategy involving both SGLT1 and GLUT1 to regulate glucose intake in response to hypoxia. Overall, these findings suggest that although increased metabolic response is required for the onset of astrocyte adaptation to hypoxia, prolonged hypoxia requires a shift to an energy conservation mode. These findings may contribute to the understanding of the relative tolerance of astrocytes to hypoxia compared with neurons and provide novel therapeutic strategies aimed at maintaining brain function in cerebral pathologies involving hypoxia.

Dopamine agonist treatment ameliorates hyperglycemia, hyperlipidemia, and the elevated basal insulin release from islets of ob/ob mice

One of the characteristics of obesity-associated diabetes is an elevated fasting plasma insulin concentration with a weak insulin secretory response to subsequent glucose stimulation. Evidence suggests that hyperglycemia and hyperlipidemia may contribute to the initiation and progression of this disordered islet glucose sensing. It has been proposed that reducing hyperglycemia and hyperlipidemia per se may improve islet glucose sensing. Here we studied glucose-dependent insulin release in islets isolated from ob/ob mice treated with dopamine agonists (bromocriptine and SKF38393, BC/SKF) which significantly reduced circulating glucose and lipid levels of ob/ob mice. Islets from BC/SKF-treated mice showed a marked decrease of the elevated basal insulin release to levels similar to lean mice. Such treatment also induced a higher secretory response to glucose stimulation compared with that in ob/ob mice with sustained hyperglycemia and hyperlipidemia. Similarly, when islets from untreated ob/ob mice were cultured for 7 days in 11 mM glucose in the absence of free fatty acid, the basal insulin release was significantly decreased and high glucose stimulated insulin release increased compared with that from islets cultured in medium containing 30 mM glucose and 2 mM oleate. The BC/SKF-induced reduction of elevated basal insulin release was associated with decreased hexokinase activity and basal cyclic AMP content in islet tissue. Our results demonstrate that dopamine agonist treatment improves basal insulin release in ob/ob mice and this effect may be mediated, in part, by a reduction of hyperglycemia and hyperlipidemia.

Structural instability of mutant beta-cell glucokinase: implications for the molecular pathogenesis of maturity-onset diabetes of the young (type-2)

The catalytic function and thermal stability of wild-type and mutant recombinant human pancreatic beta-cell glucokinase was investigated. The mutants E70K and E300K, which are thought to be the cause of impaired insulin production by the pancreatic beta-cell and decreased glucose uptake by the liver of patients with maturity-onset diabetes of the young, were found to be functionally indistinguishable from the wild-type, i.e. their kcat.S0.5, inflection point and h were normal. However, these two mutants showed markedly reduced stability under a variety of test conditions. Glucokinase instability, not low enzyme catalytic activity, may be the cause of diabetes mellitus with E70K and E300K mutants.

Unequal potency of transgenic yeast hexokinase on pancreatic beta cell metabolism and secretion

Recent reports have proposed that sequence specific interactions between glucokinase and other beta cell proteins are important to glucokinase regulation of beta cell activity. We have previously reported enhancement of beta cell function by a transgenic hexokinase derived from yeast which has only 30% amino acid sequence homology to glucokinase. To test the functional significance of the amino acid sequence of islet glucokinase we have made a quantitative study of the effect of yeast hexokinase on beta cell glucose metabolism and insulin secretion. Transgenic and normal islets were assayed for hexokinase activity, glucose usage and insulin secretion. Most parameters were measured at six glucose concentrations between 0.5 and 20 mM glucose. Transgenic islet hexokinase activity measured in islet extracts exceeded normal islet hexokinase activity by 31 to 77 percent at different glucose concentrations. At all glucose concentrations tested the percentage increase in transgenic glucose metabolism greatly exceeded the percentage increase in transgenic hexokinase activity. The increase in transgenic glucose metabolism produced a proportional reduction in the threshold for glucose stimulated insulin secretion. However, yeast hexokinase had little if any effect on the first phase of insulin secretion. The finding that metabolism was very sensitive to yeast hexokinase but first phase secretion was not, supports recent proposals that hexokinase and glucokinase may be physically and functionally separated in the beta cell.

Glucokinase and cytosolic phosphoenolpyruvate carboxykinase (GTP) in the human liver. Regulation of gene expression in cultured hepatocytes

Glucokinase and phosphoenolpyruvate carboxykinase are key enzymes of glucose metabolism in the rat liver. The former is considered to be instrumental in regulating glucose hepatic release/uptake according to the glycaemia level, and cytosolic phosphoenolpyruvate carboxykinase is a major flux-generating enzyme for gluconeogenesis. The level of expression of both enzymes and the regulation of their mRNAs in the human liver cell were investigated. Surgical biopsies of liver from patients undergoing partial hepatectomies and parenchymal hepatocytes derived from the biopsies were used to assay glucokinase, hexokinase and phosphoenolpyruvate carboxykinase activities. Hepatocytes were placed in culture and the actions of insulin, glucagon and cAMP on glucokinase and phosphoenolpyruvate carboxykinase mRNAs were studied. The main results are: (a) glucokinase accounts for 95% of the glucose phosphorylation activity of human hepatocytes, although this fact is masked in assays of total liver tissue; (b) glucokinase activity is set at a lower level in human hepatocytes than in rat hepatocytes, and vice-versa for the gluconeogenic enzyme phosphoenolpyruvate carboxykinase; and (c) as previously shown in rat liver, glucokinase and phosphoenolpyruvate carboxykinase mRNAs are regulated in a reciprocal fashion in human hepatocytes, insulin inducing the first enzyme and repressing the latter, whereas glucagon has opposite effects. These data have interesting implications with respect to metabolic regulation and intracellular hormone signaling in the human liver.

In situ glucose uptake and glucokinase activity of pancreatic islets in diabetic and obese rodents

The present study evaluated the involvement of glucose transport and phosphorylation in glucose-stimulated insulin release from pancreatic islets. Using quantitative histochemical techniques, we investigated basal islet glucose content, islet glucose uptake in situ during acute extreme experimental hyperglycemia, and islet glucokinase activity in several animal models of diabetes and obesity. The basal islet glucose content in anaesthetized diabetic or obese rodents was either the same or higher than that in their relevant controls. The rate of glucose uptake of islet tissue in these animals after an i.v. glucose injection was different. The db+/db+ mouse and the obese Zucker rat exhibited significantly reduced islet glucose uptake rates. RIP-cHras transgenic mice, BHE/cdb rats and partially pancreatectomized rats showed normal islet glucose uptake rates. The activity of islet glucokinase was increased to a different degree related to the blood glucose level. All five animal models of diabetes or obesity exhibited either a delay or a reduction of insulin release in response to supra maximal glucose stimulation. Our results indicate that the impairment of glucose-induced insulin release in diabetes is not consistently associated with a reduction of islet glucose uptake nor a change of glucokinase activity.

Mammalian glucokinase and its gene

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Expression of yeast hexokinase in pancreatic beta cells of transgenic mice reduces blood glucose, enhances insulin secretion, and decreases diabetes

It has been proposed that endogenous hexokinases of the pancreatic beta cell control the rate of glucose-stimulated insulin secretion and that genetic defects that reduce beta-cell hexokinase activity may lead to diabetes. To test these hypotheses, we have produced transgenic mice that have a 2-fold increase in hexokinase activity specific to the pancreatic beta cell. This increase was sufficient to significantly augment glucose-stimulated insulin secretion of isolated pancreatic islets, increase serum insulin levels in vivo, and lower the blood glucose levels of transgenic mice by 20-50% below control levels. Elevation of hexokinase activity also significantly reduced blood glucose levels of diabetic mice. These results confirm the role of beta-cell hexokinase activity in the regulation of insulin secretion and glucose homeostasis. They also provide strong support for the proposal that reductions in beta-cell hexokinase activity can produce diabetes.

Characteristics of glucokinase of the Kirkman insulinoma

It was investigated whether the well-known transplantable insulinoma of the hamster (the Kirkman tumor) contains glucokinase and if so, what its kinetic characteristics are, and whether its cellular levels might be regulated in a manner typical for islet tissue. The supernatant of tumor homogenates contained a low-affinity component (Km 9.7 mmol/L) of glucose phosphorylating activity, apparently glucokinase. Partially purified insulinoma glucokinase exhibited similar kinetic characteristics to liver glucokinase (Km for glucose 5.0 and 5.3 mmol/L, half-maximal saturation 6.9 and 6.3 mmol/L, Hill coefficient 1.63 and 1.62, Ki for mannoheptulose 0.9 and 0.6 mmol/L in hamster insulinoma glucokinase and hamster liver glucokinase, respectively). Insulinoma glucokinase activity was not affected by the age of the tumor. Tumor-bearing hamsters without further treatment stayed normoglycemic (172 +/- 9.5 mg/dL) for the duration of the experiment. Fasting caused hypoglycemia (49 +/- 5.0 mg/dL), and pretreatment with streptozotocin prior to tumor transplantation caused hyperglycemia (393 +/- 20.6 mg/dL) in the tumor-bearing hamsters. Blood glucose levels of the host hamsters did not affect the content of the insulinoma glucokinase (83 +/- 3.5 mU/g in hypoglycemic group, 88 +/- 9.0 mU/g in hyperglycemic group, and 86 +/- 3.5 mU/g in normoglycemic group). Thus, biosynthesis and degradation of insulinoma glucokinase does not seem to be regulated by glucose as found to be true for islet glucokinase. Since glucokinase is constitutively present, the stable transplantable Kirkman tumor could serve as a useful model for studying the pancreatic B-cell glycolysis system which is characterized by the presence of both hexokinase and glucokinase.