An allosteric model for the inhibition of glucokinase by long chain acyl coenzyme A

First evidence of changes in enzyme kinetics and stability of glucokinase affected by somatic cancer-associated variations

Recent investigation of somatic variations of allosterically regulated proteins in cancer genomes suggested that variations in glucokinase (GCK) might play a role in tumorigenesis. We hypothesized that somatic cancer-associated GCK variations include in part those with activating and/or stabilizing effects. We analyzed the enzyme kinetics and thermostability of recombinant proteins possessing the likely activating variations and the variations present in the connecting loop I and provided the first experimental evidence of the effects of somatic cancer-associated GCK variations. Activating and/or stabilizing variations were common among the analyzed cancer-associated variations, which was in strong contrast to their low frequency among germinal variations. The activating and stabilizing variations displayed focal distribution with respect to the tertiary structure, and were present in the surroundings of the heterotropic allosteric activator site, including but not limited to the connecting loop I and in the active site region subject to extensive rearrangements upon glucose binding. Activating somatic cancer-associated variations induced a reduction of GCK's cooperativity and an increase in the affinity to glucose (a decline in the S_0.5 values). The hotspot-associated variations, which decreased cooperativity, also increased the half-maximal inhibitory concentrations of the competitive GCK inhibitor, N-acetylglucosamine. Concluded, we have provided the first convincing biochemical evidence establishing GCK as a previously unrecognized enzyme that contributes to the reprogramming of energy metabolism in cancer cells. Activating GCK variations substantially increase affinity of GCK to glucose, disrupt the otherwise characteristic sigmoidal response to glucose and/or prolong the enzyme half-life. This, combined, facilitates glucose phosphorylation, thus supporting glycolysis and associated pathways.

Defining lipid mediators of insulin resistance - controversies and challenges

Essential elements of all cells, lipids play important roles in energy production, signalling and as structural components. Despite these critical functions, excessive availability and intracellular accumulation of lipid is now recognised as a major factor contributing to many human diseases, including obesity and diabetes. In the context of these metabolic disorders, ectopic deposition of lipid has been proposed to have deleterious effects of insulin action. While this relationship has been recognised for some time now, there is currently no unifying mechanism to explain how lipids precipitate the development of insulin resistance. This review summarises the evidence linking specific lipid molecules to the induction of insulin resistance, describing some of the current controversies and challenges for future studies in this field.

Long-chain acyl-CoA esters in metabolism and signaling: Role of acyl-CoA binding proteins

Long-chain fatty acyl-CoA esters are key intermediates in numerous lipid metabolic pathways, and recognized as important cellular signaling molecules. The intracellular flux and regulatory properties of acyl-CoA esters have been proposed to be coordinated by acyl-CoA-binding domain containing proteins (ACBDs). The ACBDs, which comprise a highly conserved multigene family of intracellular lipid-binding proteins, are found in all eukaryotes and ubiquitously expressed in all metazoan tissues, with distinct expression patterns for individual ACBDs. The ACBDs are involved in numerous intracellular processes including fatty acid-, glycerolipid- and glycerophospholipid biosynthesis, β-oxidation, cellular differentiation and proliferation as well as in the regulation of numerous enzyme activities. Little is known about the specific roles of the ACBDs in the regulation of these processes, however, recent studies have gained further insights into their in vivo functions and provided further evidence for ACBD-specific functions in cellular signaling and lipid metabolic pathways. This review summarizes the structural and functional properties of the various ACBDs, with special emphasis on the function of ACBD1, commonly known as ACBP.

In vitro effects of myricetin, morin, apigenin, (+)-taxifolin, (+)-catechin, (-)-epicatechin, naringenin and naringin on cytochrome b5 reduction by purified NADH-cytochrome b5 reductase

The microsomal NADH-dependent electron transport system consisting of cytochrome b5 reductase and cytochrome b5 participates in a number of physiologically important processes including lipid metabolism as well as is involved in the metabolism of various drug and xenobiotics. In the present study, we assessed the inhibitory effects of eight dietary flavonoids representing five distinct chemical classes on cytochrome b5 reduction by purified cytochrome b5 reductase. From the flavonoids tested, myricetin was the most potent in inhibiting cytochrome b5 reduction with an IC50 value of 0.35μM. Myricetin inhibited b5 reductase noncompetitively with a Ki of 0.21μM with respect to cofactor NADH, and exhibited a non-linear relationship indicating non-Michaelis-Menten kinetic binding with respect to cytochrome b5. In contrast to the potent inhibitory activity of myricetin, (+)-taxifolin was found to be a weak inhibitor (IC50=9.8μM). The remaining flavonoids were inactive within the concentration range tested (1-50μM). Analysis of structure-activity data suggested that simultaneous presence of three OH groups in ring B is a primary structural determinant for a potent enzyme inhibition. Our results suggest that inhibition of the activity of this system by myricetin or myricetin containing diets may influence the metabolism of therapeutic drugs as well as detoxification of xenobiotics.

Homotropic allosteric regulation in monomeric mammalian glucokinase

Glucokinase catalyzes the ATP-dependent phosphorylation of glucose, a chemical transformation that represents the rate-limiting step of glycolytic metabolism in the liver and pancreas. Glucokinase is a central regulator of glucose homeostasis as evidenced by its association with two disease states, maturity onset diabetes of the young (MODY) and persistent hyperinsulinemia of infancy (PHHI). Mammalian glucokinase is subject to homotropic allosteric regulation by glucose-the steady-state velocity of glucose-6-phosphate production is not hyperbolic, but instead displays a sigmoidal response to increasing glucose concentrations. The positive cooperativity displayed by glucokinase is intriguing since the enzyme functions as a monomer under physiological conditions and contains only a single binding site for glucose. Despite the existence of several models of kinetic cooperativity in monomeric enzymes, a consensus has yet to be reached regarding the mechanism of allosteric regulation in glucokinase. Experimental evidence collected over the last 45 years by a number of investigators supports a link between cooperativity and slow conformational reorganizations of the glucokinase scaffold. In this review, we summarize advances in our understanding of glucokinase allosteric regulation resulting from recent X-ray crystallographic, pre-equilibrium kinetic and high-resolution nuclear magnetic resonance investigations. We conclude with a brief discussion of unanswered questions regarding the mechanistic basis of kinetic cooperativity in mammalian glucokinase.

Functional characterization of MODY2 mutations highlights the importance of the fine-tuning of glucokinase and its role in glucose sensing

Glucokinase (GK) acts as a glucose sensor in the pancreatic beta-cell and regulates insulin secretion. Heterozygous mutations in the human GK-encoding GCK gene that reduce the activity index increase the glucose-stimulated insulin secretion threshold and cause familial, mild fasting hyperglycaemia, also known as Maturity Onset Diabetes of the Young type 2 (MODY2). Here we describe the biochemical characterization of five missense GK mutations: p.Ile130Thr, p.Asp205His, p.Gly223Ser, p.His416Arg and p.Ala449Thr. The enzymatic analysis of the corresponding bacterially expressed GST-GK mutant proteins show that all of them impair the kinetic characteristics of the enzyme. In keeping with their position within the protein, mutations p.Ile130Thr, p.Asp205His, p.Gly223Ser, and p.His416Arg strongly decrease the activity index of GK, affecting to one or more kinetic parameters. In contrast, the p.Ala449Thr mutation, which is located in the allosteric activator site, does not affect significantly the activity index of GK, but dramatically modifies the main kinetic parameters responsible for the function of this enzyme as a glucose sensor. The reduced Kcat of the mutant (3.21±0.28 s(-1) vs 47.86±2.78 s(-1)) is balanced by an increased glucose affinity (S(0.5) = 1.33±0.08 mM vs 7.86±0.09 mM) and loss of cooperativity for this substrate. We further studied the mechanism by which this mutation impaired GK kinetics by measuring the differential effects of several competitive inhibitors and one allosteric activator on the mutant protein. Our results suggest that this mutation alters the equilibrium between the conformational states of glucokinase and highlights the importance of the fine-tuning of GK and its role in glucose sensing.

Internal binding of halogenated phenols in dehaloperoxidase-hemoglobin inhibits peroxidase function

Dehaloperoxidase (DHP) from the annelid Amphitrite ornata is a catalytically active hemoglobin-peroxidase that possesses a unique internal binding cavity in the distal pocket above the heme. The previously published crystal structure of DHP shows 4-iodophenol bound internally. This led to the proposal that the internal binding site is the active site for phenol oxidation. However, the native substrate for DHP is 2,4,6-tribromophenol, and all attempts to bind 2,4,6-tribromophenol in the internal site under physiological conditions have failed. Herein, we show that the binding of 4-halophenols in the internal pocket inhibits enzymatic function. Furthermore, we demonstrate that DHP has a unique two-site competitive binding mechanism in which the internal and external binding sites communicate through two conformations of the distal histidine of the enzyme, resulting in nonclassical competitive inhibition. The same distal histidine conformations involved in DHP function regulate oxygen binding and release during transport and storage by hemoglobins and myoglobins. This work provides further support for the hypothesis that DHP possesses an external binding site for substrate oxidation, as is typical for the peroxidase family of enzymes.

Insulin resistance and metabolic syndrome in the pediatric population

The metabolic syndrome is a constellation of specific anthropometric, physiological, and biochemical abnormalities predisposing affected individuals to the development of diabetes and cardiovascular disease. The syndrome is well described in the adult literature. However, its description in the pediatric literature is more limited. Due in large part to the normal physiological changes that occur in children and adolescents with respect to growth and puberty, investigators have also struggled to establish a standard definition of the syndrome in the pediatric age group, hindering coordinated research efforts. However, whatever definition of the syndrome is used, the prevalence of the metabolic syndrome in the pediatric age group has increased worldwide. Insulin resistance is the principal metabolic abnormality that is common to the development of the metabolic syndrome in both children and adults. This review summarizes current research regarding the pathophysiology of insulin resistance and how this may contribute to specific abnormalities seen in children and adolescents with the metabolic syndrome. Specifically, insulin resistance in pediatric patients is correlated with cardiovascular risk factors such as elevated blood pressure, dyslipidemia, and type 2 diabetes mellitus, all of which are significant risk factors for adult disease. In addition, current treatment and prevention strategies, including lifestyle modifications, pharmacologic agents, and certain surgical therapies, are highlighted. The need for collaborative changes at the family, school, city, state, and national levels to address the growing prevalence of the metabolic syndrome in the pediatric age group is also reviewed.

Long-chain fatty acyl-coenzyme A-induced inhibition of glucokinase in pancreatic islets from rats depleted in long-chain polyunsaturated omega3 fatty acids

The metabolism of D-glucose was recently reported to be impaired in pancreatic islets from second generation rats depleted in long-chain polyunsaturated omega3 fatty acids. Considering the increased clearance of circulating non-esterified fatty acids prevailing in these rats, a possible inhibition of glucokinase in insulin-producing cells by endogenous long-chain fatty acyl-CoA was considered. The present study was mainly aimed at assessing the validity of the latter proposal. The activity of glucokinase in islet homogenates, as judged from the increase in D-glucose phosphorylation rate in response to a rise in the concentration of the hexose represented, in the omega3-depleted rats, was only 81.8 +/- 4.8% (n = 11; p < 0.005) of the paired value recorded in control animals. This coincided with the fact that the inclusion of D-glucose 6-phosphate (3.0 mM) and D-fructose 1-phosphate (1.0 mM) in the assay medium resulted in a lesser fractional decrease of D-glucose phosphorylation in omega3-depleted rats than in control animals. Moreover, whereas palmitoyl-CoA (50 microM) decreased the activity of glucokinase by 38.0 +/- 6.0% (n = 4; p < 0.01) in islet homogenates from normal rats, the CoA ester failed to affect significantly the activity of glucokinase in islet homogenates from omega3-depleted rats. These findings afford direct support for the view that glucokinase is indeed inhibited by endogenous long-chain fatty acyl-CoA in islets from omega3-depleted rats, such an inhibition probably participating to the alteration of D-glucose catabolism prevailing in these islets.

Peroxisome proliferator-activated receptor (PPAR) activation induces tissue-specific effects on fatty acid uptake and metabolism in vivo--a study using the novel PPARalpha/gamma agonist tesaglitazar

Agonists of peroxisome proliferator-activated receptors (PPARs) have emerged as important pharmacological agents for improving insulin action. A major mechanism of action of PPAR agonists is thought to involve the alteration of the tissue distribution of nonesterified fatty acid (NEFA) uptake and utilization. To test this hypothesis directly, we examined the effect of the novel PPARalpha/gamma agonist tesaglitazar on whole-body insulin sensitivity and NEFA clearance into epididymal white adipose tissue (WAT), red gastrocnemius muscle, and liver in rats with dietary-induced insulin resistance. Wistar rats were fed a high-fat diet (59% of calories as fat) for 3 wk with or without treatment with tesaglitazar (1 micromol.kg(-1).d(-1), 7 d). NEFA clearance was measured using the partially metabolizable NEFA tracer, (3)H-R-bromopalmitate, administered under conditions of basal or elevated NEFA availability. Tesaglitazar improved the insulin sensitivity of high-fat-fed rats, indicated by an increase in the glucose infusion rate during hyperinsulinemic-euglycemic clamp (P < 0.01). This improvement in insulin action was associated with decreased diglyceride (P < 0.05) and long chain acyl coenzyme A (P < 0.05) in skeletal muscle. NEFA clearance into WAT of high-fat-fed rats was increased 52% by tesaglitazar under basal conditions (P < 0.001). In addition the PPARalpha/gamma agonist moderately increased hepatic and muscle NEFA utilization and reduced hepatic triglyceride accumulation (P < 0.05). This study shows that tesaglitazar is an effective insulin-sensitizing agent in a mild dietary model of insulin resistance. Furthermore, we provide the first direct in vivo evidence that an agonist of both PPARalpha and PPARgamma increases the ability of WAT, liver, and skeletal muscle to use fatty acids in association with its beneficial effects on insulin action in this model.

Non-esterified fatty acids impair insulin-mediated glucose uptake and disposition in the liver

AIMS/HYPOTHESIS

We investigated the effect of elevated circulating NEFA on insulin-mediated hepatic glucose uptake (HGU) and whole-body glucose disposal (M) in eight healthy male subjects.

METHODS

Studies were performed using positron emission tomography (PET) and [(18)F]-2-fluoro-2-deoxyglucose ([(18)F]FDG) during euglycaemic hyperinsulinaemia (0-120 min) and an Intralipid/heparin infusion (IL/Hep; -90-120 min). On a different day, similar measurements were taken during euglycaemic hyperinsulinaemia and saline infusion (SAL). Graphical and compartmental analyses were used to model liver data.

RESULTS

Circulating NEFA increased approximately three-fold during IL/Hep, and declined by 81+/-7% in the SAL study ( p</=0.01). Both M (-28+/-7%) and HGU (-25+/-9%) were significantly lowered by NEFA elevation ( p=0.004 and p=0.035 respectively). In the whole data set, the decreases in M and HGU were positively correlated ( r=0.78, p=0.038). No evidence of [(18)F]FDG outflow was detected during the scanning time. HGU was correlated with the phosphorylation rate parameter ( r=0.71, p=0.003) as derived by compartmental modelling.

CONCLUSIONS/INTERPRETATION

In healthy men, NEFA impair insulin-mediated HGU and whole-body glucose uptake to a similar extent. Our data suggest that multiple intracellular NEFA targets may concur to down-regulate glucose uptake by the liver.

Muscle long-chain acyl CoA esters and insulin resistance

A common observation in animal models and in humans is that accumulation of muscle triglyceride is associated with the development of insulin resistance. In animals, this is true of genetic models of obesity and nutritional models of insulin resistance generated by high-fat feeding, infusion of lipid, or infusion of glucose. Although there is a strong link between the accumulation of triglycerides (TG) in muscle and insulin resistance, it is unlikely that TG are directly involved in the generation of muscle insulin resistance. There are now other plausible mechanistic links between muscle lipid metabolites and insulin resistance, in addition to the classic substrate competition proposed by Randle's glucose-fatty acid cycle. The first step in fatty acid metabolism (oxidation or storage) is activation to the long-chain fatty acyl CoA (LCACoA). This review covers the evidence suggesting that cytosolic accumulation of this active form of lipid in muscle can lead to impaired insulin signaling, impaired enzyme activity, and insulin resistance, either directly or by conversion to other lipid intermediates that alter the activity of key kinases and phosphatases. Actions of fatty acids to bind specific nuclear transcription factors provide another mechanism whereby different lipids could influence metabolism.

Mechanisms linking diet and colorectal cancer: the possible role of insulin resistance

Diet is clearly implicated in the origin of colorectal cancer, with risk factors for the disease including reduced consumption of vegetables, fiber, and starch and increased consumption of red meat and animal fat. Several hypotheses have been developed to explain these associations. Most recently, McKeown-Eyssen and Giovannucci noted the similarity of the risk factors for colorectal cancer and those for insulin resistance and suggested that insulin resistance leads to colorectal cancer through the growth-promoting effect of elevated levels of insulin, glucose, or triglycerides. We briefly review the evidence from observational, epidemiological, and experimental animal studies linking diet with insulin resistance and colorectal cancer. The evidence suggests that diets high in energy and saturated fat and with high glycemic index carbohydrate and low levels of fiber and n-3 fatty acids lead to insulin resistance with hyperinsulinemia, hyperglycemia, and hypertriglyceridemia. We then consider how insulin, the related insulin-like growth factors, triglycerides, and nonesterified fatty acids could lead to increased growth of colon cancer precursor lesions and the development of colorectal cancer. Finally, we consider the implications of this scheme on possible future research directions, including studies of satiety and clinical tests of the importance of insulin resistance in the colon carcinogenesis process.

Secreção da insulina: efeito autócrino da insulina e modulação por ácidos graxos

A insulina exerce um papel central na regulação da homeostase da glicose e atua de maneira coordenada em eventos celulares que regulam os efeitos metabólicos e de crescimento. A sub-unidade beta do receptor de insulina possui atividade tirosina quinase intrínseca. A autofosforilação do receptor, induzida pela insulina, resulta na fosforilação de substratos protéicos intracelulares, como o substrato-1 do receptor de insulina (IRS-1). O IRS-1 fosforilado associa-se a domínios SH2 e SH3 da enzima PI 3-quinase, transmitindo, desta maneira, o sinal insulínico. A insulina parece exercer feedback positivo na sua secreção, pela interação com seu receptor em células B pancreáticas. Alterações nos mecanismos moleculares da via de sinalização insulínica sugerem uma associação entre resistência à insulina e diminuição da secreção deste hormônio, semelhante ao observado em diabetes mellitus tipo 2. Uma das anormalidades associadas à resistência à insulina é a hiperlipidemia. O aumento do pool de ácidos graxos livres circulantes pode modular a atividade de enzimas e de proteínas que participam na exocitose da insulina. Essa revisão descreve também os possíveis mecanismos de modulação da secreção de insulina pelos ácidos graxos em ilhotas pancreáticas.

A novel glucokinase regulator in pancreatic beta cells: precursor of propionyl-CoA carboxylase beta subunit interacts with glucokinase and augments its activity

A glucokinase regulatory protein has been reported to exist in the liver, which suppresses enzyme activity in a complex with fructose 6-phosphate, whereas no corresponding protein has been found in pancreatic beta cells. To search for such a protein in pancreatic beta cells, we screened for a cDNA library of the HIT-T15 cell line with the cDNA of glucokinase from rat islet by the yeast two hybrid system. We detected a cDNA encoding the precursor of propionyl-CoA carboxylase beta subunit (pbetaPCCase), and glutathione S-transferase pull-down assay illustrated that pbetaPCCase interacted with recombinant rat islet glucokinase and with glucokinase in rat liver and islet extracts. Functional analysis indicated that pbetaPCCase decreased the K(m) value of recombinant islet glucokinase for glucose by 18% and increased V(max) value by 23%. We concluded that pbetaPCCase might be a novel activator of glucokinase in pancreatic beta cells.

Long-chain acyl-CoA esters as indicators of lipid metabolism and insulin sensitivity in rat and human muscle

Long-chain acyl-CoAs (LCACoA) are an activated lipid species that are key metabolites in lipid metabolism; they also have a role in the regulation of other cellular processes. However, few studies have linked LCACoA content in rat and human muscle to changes in nutritional status and insulin action. Fasting rats for 18 h significantly elevated the three major LCACoA species in muscle (P < 0.001), whereas high-fat feeding of rats with a safflower oil (18:2) diet produced insulin resistance and increased total LCACoA content (P < 0.0001) by specifically increasing 18:2-CoA. The LCACoA content of red muscle from rats (4-8 nmol/g) was 4- to 10-fold higher than adipose tissue (0.4-0.9 nmol/g, P < 0.001), suggesting that any contamination of muscle samples with adipocytes would contribute little to the LCACoA content of muscle. In humans, the LCACoA content of muscle correlated significantly with a measure of whole body insulin action in 17 male subjects (r(2) = 0.34, P = 0.01), supporting a link between muscle lipid metabolism and insulin action. These results demonstrate that the LCACoA pool reflects lipid metabolism and nutritional state in muscle. We conclude that the LCACoA content of muscle provides a direct index of intracellular lipid metabolism and its links to insulin action, which, unlike triglyceride content, is not subject to contamination by closely associated adipose tissue.

Long chain fatty acids inhibit and medium chain fatty acids activate mammalian cardiac hexokinase

We investigated the effect of non-esterified fatty acids (FAs) on bovine heart hexokinase (type I: ATP: D-hexose 6-phosphotransferase, EC 2.7.1.1). Long chain FAs (C14 to C20) inhibited the enzyme in a way that correlated positively with both the chain length and the degree of unsaturation. Medium chain FA with 12 or less carbons activated hexokinase in a chain length dependent manner with the greater activation shown by laurate. The activation constant of laurate was 91.5 microM with a maximal activation of 60.3%. Oleate caused a maximal decrease in specific activity of 25% with an inhibition constant of 79 microM. Using the fluorescent probe cis-parinarate, we found a saturable binding site with K(d) of 3.5 microM. Oleate competed the fluorescent probe from the protein with a K(d) of 1.4 microM. Medium chain FAs did not compete the probe from HK. The binding of fatty acid to the protein appears to be entropically driven as indicated by an Arrhenius analysis (DeltaS=+231.6 J mol(-1) deg(-1)). The presence of oleate significantly increased the K(ATP)(m) from 0.47 mM to 0.89 mM while the K(glucose)(m) in the presence of the FA (0.026+/-0.003 mM) was not significantly different from the control (0.014+/-0.004 mM). A decrease in V(max) values in the presence of oleate indicated that a mixed allosteric inhibition was operating.

Regulation of glucose production by the liver

Glucose is an essential nutrient for the human body. It is the major energy source for many cells, which depend on the bloodstream for a steady supply. Blood glucose levels, therefore, are carefully maintained. The liver plays a central role in this process by balancing the uptake and storage of glucose via glycogenesis and the release of glucose via glycogenolysis and gluconeogenesis. The several substrate cycles in the major metabolic pathways of the liver play key roles in the regulation of glucose production. In this review, we focus on the short- and long-term regulation glucose-6-phosphatase and its substrate cycle counter-part, glucokinase. The substrate cycle enzyme glucose-6-phosphatase catalyzes the terminal step in both the gluconeogenic and glycogenolytic pathways and is opposed by the glycolytic enzyme glucokinase. In addition, we include the regulation of GLUT 2, which facilitates the final step in the transport of glucose out of the liver and into the bloodstream.

Role of acylCoA binding protein in acylCoA transport, metabolism and cell signaling

Long chain acylCoA esters (LCAs) act both as substrates and intermediates in intermediary metabolism and as regulators in various intracellular functions. AcylCoA binding protein (ACBP) binds LCAs with high affinity and is believed to play an important role in intracellular acylCoA transport and pool formation and therefore also for the function of LCAs as metabolites and regulators of cellular functions [1]. The major factors controlling the free concentration of cytosol long chain acylCoA ester (LCA) include ACBP [2], sterol carrier protein 2 (SCP2) [3] and fatty acid binding protein (FABP) [4]. Additional factors affecting the concentration of free LCA include feed back inhibition of the acylCoA synthetase [5], binding to acylCoA receptors (LCA-regulated molecules and enzymes), binding to membranes and the activity of acylCoA hydrolases [6].

Acyl-CoA binding proteins: multiplicity and function

The physiological role of long-chain fatty acyl-CoA is thought to be primarily in intermediary metabolism of fatty acids. However, recent data show that nM to microM levels of these lipophilic molecules are potent regulators of cell functions in vitro. Although long-chain fatty acyl-CoA are present at several hundred microM concentration in the cell, very little long-chain fatty acyl-CoA actually exists as free or unbound molecules, but rather is bound with high affinity to membrane lipids and/or proteins. Recently, there is growing awareness that cytosol contains nonenzymatic proteins also capable of binding long-chain fatty acyl-CoA with high affinity. Although the identity of the cytosolic long-chain fatty acyl-CoA binding protein(s) has been the subject of some controversy, there is growing evidence that several diverse nonenzymatic cytosolic proteins will bind long-chain fatty acyl-CoA. Not only does acyl-CoA binding protein specifically bind medium and long-chain fatty acyl-CoA (LCFA-CoA), but ubiquitous proteins with multiple ligand specificities such as the fatty acid binding proteins and sterol carrier protein-2 also bind LCFA-CoA with high affinity. The potential of these acyl-CoA binding proteins to influence the level of free LCFA-CoA and thereby the amount of LCFA-CoA bound to regulatory sites in proteins and enzymes is only now being examined in detail. The purpose of this article is to explore the identity, nature, function, and pathobiology of these fascinating newly discovered long-chain fatty acyl-CoA binding proteins. The relative contributions of these three different protein families to LCFA-CoA utilization and/or regulation of cellular activities are the focus of new directions in this field.

Activation of the ATP-sensitive K+ channel by long chain acyl-CoA. A role in modulation of pancreatic beta-cell glucose sensitivity

Long-term exposure to elevated levels of long chain free fatty acids decreases glucose-induced insulin secretion from pancreatic islets and clonal pancreatic beta-cells. The mechanism for this loss of glucose sensitivity is at present not known. In this study, we evaluated the possibility that increases in long chain acyl-CoA esters (LC-CoA), the metabolically active form of free fatty acids, might mediate the loss of glucose sensitivity. We observed that cellular levels of LC-CoA increased more than 100% in response to overnight incubation with 0.5 mM palmitic acid complexed to albumin. In the same studies, the total CoA pool increased by about 40%. Patch-clamp studies demonstrated that saturated and unsaturated LC-CoA, but not malonyl-CoA or free CoASH, induced a rapid and slowly reversible opening of ATP-sensitive K+ channels. The effect was concentration-dependent between 10 nM and 1 microM. These findings indicate that the ATP-regulated K/ channels is a sensitive target for LC-CoA and suggest that high levels of LC-CoA, which accumulate in response to hyperglycemia or prolonged exposure to free fatty acids, may prevent channel closure and contribute to the development of beta-cell glucose insensitivity.

Mechanisms by which fatty-acyl-CoA esters inhibit or activate glucose-6-phosphatase in intact and detergent-treated rat liver microsomes

We have studied the effects of fatty-acyl-CoA esters on the activity of glucose-6-phosphatase (Glc6Pase) in untreated and detergent-treated liver microsomes. Fatty-acyl-CoA esters with chain lengths less than or equal to nine carbons do not inhibit Glc6Pase. Medium-chain fatty-acyl-CoA esters (10-14 carbons) inhibit Glc6Pase of untreated microsomes in a dose-dependent manner in the range 1-20 microM. The inhibitory effect is also dependent on the acyl-chain length. The higher the chain length, the stronger the inhibitory effect. It is also dependent on the microsomal protein concentration. The higher the protein concentration, the lower the inhibitory effect. Fatty-acyl-CoA esters with longer chain length (equal to or higher than 16 carbons) inhibit Glc6Pase of untreated microsomes within the range 1-2 microM. However, the inhibitory effect is either partially or totally cancelled, or even changed into an activation effect at higher concentrations. This is due to the release of mannose-6-phosphatase latency. The inhibition is fully reversible in the presence of bovine serum albumin. The mechanism of the Glc6Pase inhibition in untreated microsomes is uncompetitive (Ki for myristoyl-CoA = 1.2 +/- 0.3 microM, mean +/- SD, n = 3). Glc6Pase of detergent-treated microsomes is also inhibited by fatty-acyl-CoA esters, albeit less efficiently. In this case, the mechanism is non-competitive (Ki for myristoyl-CoA = 29 +/- 3 microM).

Characterization of microsomal long-chain acyl-CoA hydrolase activity in the rat submandibular gland

1. Long-chain fatty acyl-CoA hydrolase in submandibular gland microsomes was characterized and compared to that in liver ones. 2. In rat submandibular gland, the microsomal long-chain acyl-CoA hydrolase showed a higher relative activity for polyunsaturated fatty acyl-CoAs than that of liver microsomes. 3. It was suggested that the hydrolase in rat submandibular gland microsomes may play a role in modulation in the acyl-CoA pool size.

Competitive inhibition of liver glucokinase by its regulatory protein

The regulatory protein of rat liver glucokinase (hexokinase IV or D) behaved as a fully competitive inhibitor of this enzyme when glucose was the variable substrate, i.e. it increased the half-saturating concentration of glucose as a linear function of its concentration without affecting V (velocity at infinite concentration of substrate). The inhibition by the regulatory protein and that by palmitoyl-CoA were synergistic with that by N-acetyl-glucosamine, indicating that the two former inhibitors bind to a site distinct from the catalytic site. In contrast, the effects of the regulatory protein and palmitoyl-CoA were competitive with each other, indicating that these two inhibitors bind to the same site. The regulatory protein exerted a non-competitive inhibition with respect to Mg-ATP at concentrations of this nucleotide less than 0.5 mM. At higher concentrations, the latter antagonized the inhibition by the regulatory protein partly by decreasing the apparent affinity for fructose 6-phosphate. The following anions inhibited glucokinase non-competitively with respect to glucose: Pi, sulfate, I-, Br-, No3-, Cl-, F- and acetate. Pi and sulfate, at concentrations in the millimolar range, decreased the inhibition by the regulatory protein by competing with fructose 6-phosphate. Monovalent anions also antagonized the inhibition by the regulatory protein with the following order of potency: I- greater than Br- greater than NO3- greater than Cl- greater than F- greater than acetate and their effect was non-competitive with respect to fructose 6-phosphate. Glucokinase from Buffo marinus and pig liver were, like the rat liver enzyme, inhibited by the regulatory protein, as well as by palmitoyl-CoA at micromolar concentrations. In contrast, neither compound inhibited hexokinases from rat brain, beef heart or yeast, or the low-Km specific glucokinase from Bacillus stearothermophilus.

Effects of cholesterol loading of mouse macrophages on carnitine palmitoyltransferase activity and sensitivity to inhibition by malonyl-CoA

Scavenger receptor-mediated uptake of acetylated low density lipoprotein-derived cholesterol by peritoneal mouse macrophages resulted in decreased activity of the malonyl-CoA inhibitable carnitine palmitoyltransferase and a decrease in the sensitivity of this enzyme to inhibition by malonyl-CoA.

Stoichiometry of slow binding of palmitoyl-CoA to liver glucokinase

The interaction of palmitoyl-CoA with porcine glucokinase was studied by the gel permeation technique. The finding that glucokinase "bound" up to 60 molecules was unexpected from the specific inhibition of rat glucokinase by long chain acyl-CoA (Tippett & Neet, J. Biol. Chem. (1982) 287, 12839-12845). Sephacryl S-200 gel filtration in the presence of palmitoyl-CoA demonstrated a protein peak without enzyme activity that was eluted earlier than the active enzyme peak, indicating a large molecular weight shift for the inactivated enzyme form and confirming a large number (greater than or equal to 30) of associated palmitoyl-CoA molecules. The binding was also verified by analyzing the absorption characteristics of the inactivated enzyme peak. In the presence of glycerol, the size of the inactivated peak greatly decreased, but the separation between the two peaks remained unchanged. Therefore, the amphiphile bound predominantly to the inactive enzyme and not to the active form, suggesting that the rapid inhibitory interactions between palmitoyl-CoA and glucokinase previously observed are specific. Parallel enzyme activity studies showed that in the time range of the column experiments (4-20 h), both the rat and pig enzyme were greatly inactivated (greater than 90%) in the presence of palmitoyl-CoA (15 microM) in the absence of glycerol. This slow inactivation is different from the immediate specific inhibition previously reported and depends on both enzyme and palmitoyl-CoA concentrations. The presence of up to 20% glycerol slowed this inactivation process. These results demonstrated that even below the critical micelle concentration, partial inactivation of glucokinase occurs in the presence of palmitoyl-CoA over a long period of time.

Stimulation of glucose phosphorylation by fructose in isolated rat hepatocytes

The phosphorylation of glucose was measured by the formation of [3H]H2O from [2-3H]glucose in suspensions of freshly isolated rat hepatocytes. Fructose (0.2 mM) stimulated 2-4-fold the rate of phosphorylation of 5 mM glucose although not of 40 mM glucose, thus increasing the apparent affinity of the glucose phosphorylating system. A half-maximal stimulatory effect was observed at about 50 microM fructose. Stimulation was maximal 5 min after addition of the ketose and was stable for at least 40 min, during which period 60% of the fructose was consumed. The effect of fructose was reversible upon removal of the ketose. Sorbitol and tagatose were as potent as fructose in stimulating the phosphorylation of 5 mM glucose. D-Glyceraldehyde also had a stimulatory effect but at tenfold higher concentrations. In contrast, dihydroxyacetone had no significant effect and glycerol inhibited the detritiation of glucose. Oleate did not affect the phosphorylation of glucose, even in the presence of fructose, although it stimulated the formation of ketone bodies severalfold, indicating that it was converted to its acyl-CoA derivative. These results allow the conclusion that fructose stimulates glucokinase in the intact hepatocyte. They also suggest that this effect is mediated through the formation of fructose 1-phosphate, which presumably interacts with a competitive inhibitor of glucokinase other than long-chain acyl-CoAs.

Purification of long-chain acyl-CoA hydrolase from bovine heart microsomes and regulation of activity by lysophospholipids

Long-chain acyl-CoA hydrolase (EC 3.1.2.2) has been purified 12,000-fold from bovine heart muscle microsomes by extraction with Miranol detergent, followed by column chromatography on Reactive Blue agarose and DEAE-cellulose. The purified enzyme was nearly homogeneous on polyacrylamide gel electrophoresis and had a molecular weight of 41,000 in the presence of dodecyl sulfate. The specificity and kinetic properties of the enzyme were studied using several acyl-CoA derivatives as potential substrates. The enzyme showed a wide degree of specificity with little dependence on either the fatty acyl chain length or the degree of unsaturation of the acyl group. The kinetic properties were in accord with the Michaelis-Menten equation under most conditions, although high concentrations of substrates generally inhibited the enzyme. Arachidonoyl-CoA, which was the most effective substrate, had a Km value of 0.4 microM and a Vmax value of 6.0 mumol min-1 mg-1. The enzyme was strongly and specifically inhibited by constants of 16 and 30 nM, respectively. Other lysolipids and detergents such as deoxycholate and Triton X-100 were weak inhibitors. These properties and others distinguish this enzyme from other acyl-CoA hydrolases and support the idea that lysophospholipids may be important in vivo in the regulation of lipid metabolism.

Effect of glycerol on glucokinase activity: loss of cooperative behavior with respect to glucose

Glucose phosphorylation catalyzed by rat liver glucokinase measured at saturating concentrations of MgATP2- shows a cooperative response with respect to glucose in the concentration range 0.25-5 mM with a Hill coefficient of 1.6. In this range of glucose concentrations, the degree of cooperativity was dependent on the presence of glycerol in the assay mixture, and it decreased progressively and disappeared completely as the glycerol concentration reached about 20% (v/v) glycerol. If attention was confined to concentrations above 5 mM, no cooperativity could be detected either in the absence or in the presence of glycerol. The limiting velocity of the glucokinase reaction (measured at saturating concentrations of glucose and MgATP2-), and the half-saturation concentration for glucose and MgATP2- were all decreased by about 50-60% as the glycerol concentration was raised from zero to 30% (v/v). The presence of glycerol had no effect on the qualitative inhibition patterns of MgADP2-, glucose 6-phosphate, or N-acetylglucosamine, and only slight effects on the quantitative half-saturation values and inhibition constants. All of these effects caused by glycerol were fully reversible by decreasing the concentration of glycerol by dilution. Simulation studies based on the "mnemonical" model of glucokinase action proposed earlier [A. C. Storer and A. Cornish-Bowden (1977) Biochem. J. 165, 61-69] show that the effects of glycerol on glucokinase-catalyzed glucose phosphorylation can simply be explained assuming the glycerol favors the existence of the conformation of the enzyme with a higher affinity for glucose and thus supports the model.

Suppression of kinetic cooperativity of hexokinase D (glucokinase) by competitive inhibitors. A slow transition model

Hexokinase D ('glucokinase') displays positive cooperativity with mannose with the same h values (1.5-1.6) as with glucose but with higher K0.5 values (8 mM at pH 8.0 and 12 mM at pH 7.5). In contrast, fructose and 2-deoxyglucose exhibit Michaelian kinetics [Cárdenas, M. L., Rabajille, E., and Niemeyer, H. (1979) Arch. Biol. Med. Exp. 12, 571-580; Cárdenas, M. L., Rabajille, E., and Niemeyer, H. (1984) Biochem. J. 222, 363-370]. Mannose, fructose, 2-deoxyglucose and N-acetylglucosamine acted as competitive inhibitors of glucose phosphorylation and decreased the cooperativity with glucose. Their relative efficiency for reducing the value of h to 1.0 was: fructose greater than mannose greater than 2-deoxyglucose greater than N-acetylglucosamine. Galactose, which is not a substrate nor an inhibitor, was unable to change the cooperativity. The competitive inhibition of glucose phosphorylation by N-acetylglucosamine or mannose was cooperative at very low glucose concentrations (less than 0.5 K0.5), suggesting the interaction of the inhibitors with more than one enzyme form. These and previously reported results are discussed on the basis of a slow transition model, which assumes that hexokinase D exists mainly in one conformation state (E1) in the absence of ligands and that the binding of glucose (or mannose) induces a conformational transition to EII. This new conformation would have a higher affinity for the sugar substrates and a higher catalytic activity than EI. Cooperativity would emerge from shifts of the steady-state distribution between the two enzyme forms as the sugar concentration increase. The inhibitors would suppress cooperativity with glucose by inducing or trapping the EII conformation. In addition, the model postulates that the different kinetic behaviour of hexokinase D with the different sugar substrates, cooperative with glucose and mannose and Michaelian with 2-deoxyglucose and fructose, is the consequence of differences in the velocities of the conformational transitions induced by the sugar substrates.

Effects of streptozotocin-induced diabetes on microsomal long-chain fatty acyl-CoA synthetase and hydrolase

Streptozotocin-induced diabetes significantly decreased rat liver microsomal long-chain fatty acyl-CoA (LCA-CoA) hydrolase. The decrease was observed using either palmitoyl-CoA (35 per cent, p less than 0.01) or oleoyl-CoA (23 per cent, p less than 0.01) as the substrate for the enzyme. Under the same conditions, diabetes did not significantly alter activity of LCA-CoA synthetase. Daily subcutaneous injections of protamine zinc insulin (10-12 units/day) into the diabetic rats returned their blood glucose to normal but only partially corrected the LCA-CoA hydrolase activity and did not effect LCA-CoA synthetase activity. The decreased LCA-CoA hydrolase and the unchanged LCA-CoA synthetase activities in the diabetic rat liver were interpreted as factors that may contribute to elevation of fatty acyl-CoA levels in the diabetic liver.

Inhibition of the hormone-sensitive lipase in adipose tissue by long-chain fatty acyl coenzyme A

The effects of free fatty acids and fatty acyl esters of coenzyme A and carnitine on the activity of a hormone-sensitive lipase preparation made from pigeon adipose tissue were determined. Oleic acid (100 microM) resulted in a 40% inhibition of lipase activity. A more potent inhibition of lipase activity was seen with long-chain fatty acyl CoA compounds. The concentration required for half-maximal inhibition with oleoyl CoA and palmitoyl CoA was 25-40 microM, whereas palmitoyl carnitine stimulated lipase activity. Activated lipase preparations (preincubated with Mg2+, ATP, cyclic AMP and protein kinase) were 4-6 times more sensitive to inhibition by oleoyl CoA than were nonactivated preparations. An increase in cellular levels of fatty acyl coenzyme A could, therefore, contribute to the feedback inhibition of lipolysis in adipose tissue.

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.

Interconversions between different sulfhydryl-related kinetic states in glucokinase

Rat liver glucokinase (EC 2.7.1.2) undergoes two distinct sulfhydryl-related reversible kinetic transitions. During normal assays in the presence of both substrates but without added reducing agents, the activity decays ("kappa" decay) over time to a new steady-state rate. The half-time for this decay is essentially constant at glucose levels from 2 to 200 mM and averages 6.2 +/- 2 min. Glucokinase in this kappa steady state displays an increased Km for glucose but has the same Vmax as normal, sulfhydryl-activated glucokinase. The kappa form does not itself exhibit kinetic cooperativity with glucose. In contrast, glucokinase incubated with neither glucose nor sulfhydryl reagents decays (mu decay) to a form whose Vmax is near zero. The t 1/2 for this transition is about 0.5 min at 0 or very low (0.5 mM) glucose concentrations. For both decays, incubations of enzyme with intermediate levels of reducing agents give steady-state mixtures of activated and either kappa and/or mu forms, depending on conditions during the decay. Enzyme at intermediate stages of the kappa decay displays an unchanged Vmax, intermediate (increased relative to activated enzyme) glucose S0.5 values, and diminished glucose cooperativity. In contrast, enzyme at intermediate steady-state mixtures of activated and mu forms has a normal glucose S0.5 and cooperativity but a diminished Vmax from the activated states. The enzyme at any stage of each decay may be fully reactivated by the addition of sulfhydryl reducing agents such as dithiothreitol, dithioerythritol, glutathione, or mercaptoethanol. A model is proposed to account for this complex behavior in glucokinase kinetics which proposes different enzymatic states (kappa and mu) locked in by sulfhydryl oxidation of different conformations dictated by glucose concentration. These sulfhydryl-related transitions may be important in regulation of glucokinase activity, since glucokinase is very sensitive (at least 20-fold differential activity) to concentrations of glutathione within the physiological range, perhaps allowing the normally variable glutathione levels or cytosolic redox potential to modify the rate of uptake and storage of blood glucose through control of glucokinase activity.