Immunological and kinetic properties of pyruvate kinase in rat pancreatic islets

Direct measurements of oscillatory glycolysis in pancreatic islet β-cells using novel fluorescence resonance energy transfer (FRET) biosensors for pyruvate kinase M2 activity

Pulses of insulin released from pancreatic β-cells maintain blood glucose in a narrow range, although the source of these pulses is unclear. We and others have proposed that positive feedback mediated by the glycolytic enzyme phosphofructokinase-1 (PFK1) enables β-cells to generate metabolic oscillations via autocatalytic activation by its product fructose 1,6-bisphosphate (FBP). Although much indirect evidence has accumulated in favor of this hypothesis, a direct measurement of oscillating glycolytic intermediates has been lacking. To probe glycolysis directly, we engineered a family of inter- and intramolecular FRET biosensors based on the glycolytic enzyme pyruvate kinase M2 (PKAR; pyruvate kinase activity reporter), which multimerizes and is activated upon binding FBP. When introduced into Min6 β-cells, PKAR FRET efficiency increased rapidly in response to glucose. Importantly, however, metabolites entering downstream of PFK1 (glyceraldehyde, pyruvate, and ketoisocaproate) failed to activate PKAR, consistent with sensor activation by FBP; the dependence of PKAR on FBP was further confirmed using purified sensor in vitro. Using a novel imaging modality for monitoring mitochondrial flavin fluorescence in mouse islets, we show that slow oscillations in mitochondrial redox potential stimulated by 10 mm glucose are in phase with glycolytic efflux through PKM2, measured simultaneously from neighboring islet β-cells expressing PKAR. These results indicate that PKM2 activity in β-cells is oscillatory and are consistent with pulsatile PFK1 being the mediator of slow glycolytic oscillations.

Mitochondrial malic enzyme (ME2) in pancreatic islets of the human, rat and mouse and clonal insulinoma cells

Despite interest in malic enzyme(ME)s in insulin cells, mitochondrial malic enzyme (ME2) has only been studied with estimates of mRNA or with mRNA knockdown. Because an mRNA's level does not necessarily reflect the level of its cognate enzyme, we designed a simple spectrophotometric enzyme assay to measure ME2 activity of insulin cells by utilizing the distinct kinetic properties of ME2. Mitochondrial ME2 uses either NAD or NADP as a cofactor, has a high Km for malate and is allosterically activated by fumarate and inhibited by ATP. Cytosolic ME (ME1) and the other mitochondrial ME (ME3) use only NADP as a cofactor and have lower Kms for malate. The assay easily showed for the first time that substantial ME2 activity is present in pancreatic islets of humans, rats and mice and INS-1 832/13 cells. ME2's presence was confirmed with immunoblotting. There was no evidence that ME3 is present in these tissues.

Expression of liver type pyruvate kinase in insulinoma cells: involvement of LF-B1 (HNF1)

The messages for LF-B1, which interacts with the cis-acting element of PKL-I to play an essential role in expression of L-type pyruvate kinase (PK) in the liver, and L-type PK were found to be present in RIN-m5F insulinoma cells as well as the liver, kidney and small intestine, although the levels of the two mRNAs in these tissues were not correlated. Gel retardation assay suggested that similar nuclear proteins bound to two other cis-acting elements, PKL-II and PKL-III, were expressed in both liver and insulinoma cells, and that additional PKL-III-binding proteins were present only in RIN-m5F cells. Thus, we suggest that the mechanism of L-type PK expression in pancreatic B cells is similar to that in the liver.

Evidence for phosphorylation of pancreatic islet pyruvate kinase

Rabbit pancreatic islet cytosol catalyzes the calcium-activated phosphorylation by [gamma 32P]ATP of a protein with a molecular weight of 57,000 that is precipitated with antipyruvate kinase antibodies. We were unable to demonstrate that phosphorylation in the presence of calcium or cAMP had any immediate effect on rat pancreatic islet pyruvate kinase activity. This finding is consistent with our inability to confirm the finding of others that pancreatic islets contain phosphoenolpyruvate carboxykinase activity (Diabetes, 34:246, 1985). Since the carboxykinase catalyzes phosphoenolpyruvate formation and pyruvate kinase catalyzes essentially the opposite reaction, if the carboxykinase were present in the beta cell, pyruvate kinase would need to be inhibited to prevent recycling of phosphoenolpyruvate.

The role of phosphoenolpyruvate in insulin secretion: the effect of L-phenylalanine

Incubation of rat islets with phenylalanine increased the tissue content of phosophoenolpyruvate, both in the presence and in the absence of glucose. At the same time, L-phenylalanine neither stimulated nor inhibited insulin release. It is unlikely that insulin secretion is tightly coupled to the availability of phosphoenolpyruvate in rat islets.

Phosphoenolpyruvate carboxykinase in mouse pancreatic islets. ATP-induced changes in sensitivity to Mn2+ activation

The presence of high phosphoenolpyruvate carboxykinase (EC 4.1.1.32) activity in mouse islet cytosol has been demonstrated. The enzyme was activated by Mn2+ with a Ka of 100 X 10(-6) mol/l. The mean total activity of the Mn2+-stimulated phosphoenolpyruvate carboxykinase in islet cytosol estimated at 22 degrees C with saturating concentrations of the substrates oxaloacetate and ITP was 146 pmol/min per micrograms DNA. Km was calculated to be 6 X 10(-6) mol/l for oxaloacetate and 140 X 10(-6) mol/l for ITP. The islet phosphoenolpyruvate carboxykinase activity was not increased after starvation of the animals for 48 h. Preincubation of the cytosol at 4 degrees C with Fe2+, quinolinate, ATP, Pi, glucose 6-phosphate, fructose 1,6-bisphosphate, NAD+, NADH, oxaloacetate, ITP, cyclic AMP and Ca2+ had no effect on the enzyme activity. However, preincubation of the cytosol at 37 degrees C with ATP-Mg inhibited the Mn2+-stimulated phosphoenolpyruvate carboxykinase activity progressively with time and in a concentration-dependent manner. A similar but weaker inhibitory effect was observed with p[NH]ppA, whereas p[CH2]ppA, ADP, AMP, adenosine and Pi had no effect. It is tentatively suggested that ATP and p[NH]ppA either by adenylation or otherwise affect the interaction between islet phosphoenolpyruvate carboxykinase and the recently discovered Mr = 29000 protein modulator of the enzyme in such a way - perhaps by causing a dissociation between them - that phosphoenolpyruvate carboxykinase loses its sensitivity to Mn2+ activation.

New perspectives on pancreatic islet glucokinase

Control of blood sugar involves the complex interaction of the pancreatic glucose-sensing beta-cells with the liver, which serves as the primary site of glucose disposal after a meal. Glucokinase occupies an important role in controlling glucose phosphorylation and metabolism both in the liver and in pancreatic islets. In the beta-cells, glucokinase functions as pacemaker of glycolysis at physiological glucose levels. It determines the unique characteristics of islet hexose usage, that is, the rate, affinity, cooperativity, and anomeric discrimination of glucose metabolism. Because glycolysis controls hexose-induced insulin release, glucokinase is considered the best-qualified candidate for the elusive glucose sensor of beta-cells. A deficiency of glucokinase would disturb glucose homeostasis. Decreased islet glucokinase would diminish islet glycolysis and would result in a higher set point of beta-cells for glucose-induced insulin release. Decreased liver glucokinase would cause less efficient hepatic glucose disposal. Human maturity-onset diabetes (type II diabetes) has these characteristics. It is thus conceivable that certain forms of type II diabetes are due to a glucokinase deficiency.