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Biomedical subjects

E Van Schaftingen

Publications and source records attributed to E Van Schaftingen.

At least 19 recordsLinked to original sources
PubMed2006

Variability in erythrocyte fructosamine 3-kinase activity in humans correlates with polymorphisms in the FN3K gene and impacts on haemoglobin glycation at specific sites.

BACKGROUND: Part of the fructosamines that are bound to intracellular proteins are repaired by fructosamine 3-kinase (FN3K). Because subject-to-subject variations in erythrocyte FN3K activity could affect the level of glycated haemoglobin independently of differences in blood glucose level, we explored if such variability existed, if it was genetically determined by the FN3K locus on 17q25 and if the FN3K activity correlated inversely with the level of glycated haemoglobin. RESULTS: The mean erythrocyte FN3K activity did not differ between normoglycaemic subjects (n = 26) and type 1 diabetic patients (n = 31), but there was a wide interindividual variability in both groups (from about 1 to 4 mU/g haemoglobin). This variability was stable with time and associated (P < 0.0001) with two single nucleotide polymorphisms in the promoter region and exon 6 of the FN3K gene. There was no significant correlation between FN3K activity and the levels of HbA1c, total glycated haemoglobin (GHb) and haemoglobin fructoselysine residues, either in the normoglycaemic or diabetic group. However, detailed analysis of the glycation level at various sites in haemoglobin indicated that the glycation level of Lys-B-144 was about twice as high in normoglycaemic subjects with the lowest FN3K activities as compared to those with the highest FN3K activities. CONCLUSION: Interindividual variability of FN3K activity is substantial and impacts on the glycation level at specific sites of haemoglobin, but does not detectably affect the level of HbA1c or GHb. As FN3K opposes one of the chemical effects of hyperglycaemia, it would be of interest to test whether hypoactivity of this enzyme favours the development of diabetic complications.

Adult↗
PubMed2005

The gene mutated in l-2-hydroxyglutaric aciduria encodes l-2-hydroxyglutarate dehydrogenase.

The biochemical defect in L-2-hydroxyglutaric aciduria is still unknown, but the mutated gene has recently been identified on chromosome 14q22. Transfection of human embryonic kidney (HEK) cells with a cDNA encoding the product of the human gene led to a>15-fold increase in L-2-hydroxyglutarate dehydrogenase activity. The overexpressed enzyme had similar biochemical characteristics (including sensitivity to FAD and association with membranes) as the rat liver enzyme. Western blot analysis indicated that it is processed through the removal of a N-terminal approximately 4 kDa fragment, in agreement with a mitochondrial localization. Transfection experiments indicated that the mutations (K81E, E176D, Delta-exon9) found in patients with L-2-hydroxyglutaric aciduria suppressed L-2-hydroxyglutarate dehydrogenase activity. Western blot analysis showed that the three mutated proteins were expressed to various degrees in HEK cells, but were abnormally processed. Taken together, these data indicate that L-2-hydroxyglutaric aciduria is due to a deficiency in L-2-hydroxyglutarate dehydrogenase.

Alcohol Oxidoreductases↗
PubMed2004

[Fructosamine 3-kinase and protein repair].

It was thought until recently that fructosamines are not metabolized in mammalian cells. Quite to the contrary, a mammalian enzyme, fructosamine 3-kinase, has recently been identified, which phosphorylates fructosamines on their third carbon, making them unstable and leading to their shedding from proteins. Fructosamine 3-kinase is therefore responsible for protein deglycation, one of the few protein repair mechanisms that have been identified.

Animals↗
PubMed2003

Mutations in the glucokinase regulatory protein gene in 2p23 in obese French caucasians.

AIMS/HYPOTHESIS: Glucokinase regulatory protein (GKRP) controls the activity of glucokinase in liver but possibly also in some areas of the central nervous system, suggesting that it could play a role in body mass control. Its gene is located in a region (2p21-23) linked to serum leptin levels. Our goal was to investigate whether mutations in the GKRP gene were associated with obesity. METHODS: Mutations were sought in the GKRP gene of 57 patients from the families of the French genome-wide scan for obesity that contributed most to the positive LOD score with 2p21-23. The identified mutations were further sought in 720 unrelated obese individuals and 384 individuals of normal weight and their effect on the properties of recombinant GKRP were investigated. RESULTS: The most frequent mutation (Pro446Leu) had a similar allele frequency in the obese (0.63) and normal weight (0.64) subjects and did not affect the properties of GKRP. Similarly, no effect on the properties of GKRP was observed with Arg590Tyr, found in 10 out of 720 obese subjects and in 2 out of 384 control subjects (p=0.18). Mutation Arg227Stop was found in one obese family and in 1 out of 384 control subjects and led to an insoluble protein. Mutation Arg518Gln, replacing a conserved residue, led to a marked decrease in the affinity of GKRP for both fructose 6-phosphate and fructose 1-phosphate and to a destabilization of GKRP. However, this mutation did not co-segregate with obesity in the single family in which it was found. CONCLUSIONS/INTERPRETATION: Mutations that affect the properties of GKRP are found in the French population, but they do not seem to account for the linkage between the 2p23 locus and quantitative markers of obesity.

Adaptor Proteins, Signal Transducing↗
PubMed2003

Fructosamine 3-kinase, an enzyme involved in protein deglycation.

The in vivo formation of fructosamines following non-enzymatic reaction of proteins with glucose (i.e. glycation) was first described almost 30 years ago. Until recently, the only known fate of fructosamines in mammalian cells was their spontaneous conversion into advanced glycation end products. The identification in human erythrocytes of a new enzyme, fructosamine 3-kinase, disclosed the existence of a so-far unsuspected intracellular metabolism of these compounds. Fructosamine 3-kinase phosphorylates with high affinity both low-molecular-mass and protein-bound fructosamines on the third carbon of their deoxyfructose moiety, leading to the formation of fructosamine 3-phosphates. The latter are unstable and spontaneously decompose into inorganic phosphate and 3-deoxyglucosone, with concomitant regeneration of the unglycated amine. The presence of proteins related to fructosamine 3-kinase in many prokaryotic and eukaryotic genomes suggests that this 'deglycation' process is not restricted to mammals.

Amino Acid Sequence↗
PubMed2001

The 2,3-bisphosphoglycerate-independent phosphoglycerate mutase from Trypanosoma brucei: metal-ion dependency and phosphoenzyme formation.

Recombinant cofactor-independent phosphoglycerate mutase from Trypanosoma brucei was inactivated by EDTA, and reactivated by Co(2+) much more than by Mn(2+) or Fe(2+). It displayed a minor phosphoglycerate phosphatase activity, which was stimulated by Mn(2+) more than by Co(2+). Upon incubation with [(32)P]phosphoglycerate, radioactivity was incorporated into the enzyme, most particularly in the presence of Mn(2+) or Fe(2+). The phosphorylated residue was identified by tandem mass spectrometry as Ser74, a residue homologous to the phosphorylated serine in alkaline phosphatase. However, the rates of formation and of disappearance of this phosphoenzyme were quite low compared to the mutase reaction. This and other properties indicated that the observed phosphoenzyme is an intermediate in the minor phosphatase activity rather than in the phosphomutase reaction.

2,3-Diphosphoglycerate↗
PubMed2001

High residual activity of PMM2 in patients' fibroblasts: possible pitfall in the diagnosis of CDG-Ia (phosphomannomutase deficiency).

Congenital disorders of glycosylation (CDGs) are a rapidly enlarging group of inherited diseases with abnormal N-glycosylation of glycoconjugates. Most patients have CDG-Ia, which is due to a phosphomannomutase (PMM) deficiency. In this article, we report that a significant portion (9 of 54) of patients with CDG-Ia had a rather high residual PMM activity in fibroblasts included in the normal range (means of the controls +/- 2 SD) and amounting to 35%-70% of the mean control value. The clinical diagnosis of CDG-Ia was made difficult by the fact that most (6 of 9) of these patients belong to a subgroup characterized by a phenotype that is milder than classical CDG-Ia. These patients lack some of the symptoms that are suggestive for the diagnosis, such as inverted nipples and abnormal fat deposition, and, as a mean, had higher residual PMM activities in fibroblasts (2.05+/-0.61 mU/mg protein, n=9; vs. controls 5.34+/-1.74 mU/mg protein, n=22), compared with patients with moderate (1.32+/-0.86 mU/mg protein, n=18) or severe (0.63+/-0.56 mU/mg protein, n=27, P<.001) cases. Yet they all showed mild mental retardation, hypotonia, cerebellar hypoplasia, and strabismus. All of them had an abnormal serum transferrin pattern and a significantly reduced PMM activity in leukocytes. Six of the nine patients with mild presentations were compound heterozygotes for the C241S mutation, which is known to reduce PMM activity by only approximately 2-fold. Our results indicate that intermediate PMM values in fibroblasts may mask the diagnosis of CDG-Ia, which is better accomplished by measurement of PMM activity in leukocytes and mutation search in the PMM2 gene. They also indicate that there is some degree of correlation between the residual activity in fibroblasts and the clinical phenotype.

Amino Acid Sequence↗
PubMed2001

Novel arguments in favor of the substrate-transport model of glucose-6-phosphatase.

The purpose of this work was to discriminate between two models for glucose-6-phosphatase: one in which the enzyme has its catalytic site oriented toward the lumen of the endoplasmic reticulum, requiring transporters for glucose-6-phosphate, inorganic phosphate (Pi), and glucose (substrate-transport model), and a second one in which the hydrolysis of glucose-6-phosphate occurs inside the membrane (conformational model). We show that microsomes preloaded with yeast phosphoglucose isomerase catalyzed the detritiation of [2-(3)H]glucose-6-phosphate and that this reaction was inhibited by up to 90% by S3483, a compound known to inhibit glucose-6-phosphate hydrolysis in intact but not in detergent-treated microsomes. These results indicate that glucose-6-phosphate is transported to the lumen of the microsomes in an S3483-sensitive manner. Detritiation by intramicrosomal phosphoglucose isomerase was stimulated twofold by 1 mmol/l vanadate, a phosphatase inhibitor, indicating that glucose-6-phosphatase and the isomerase compete for the same intravesicular pool of glucose-6-phosphate. To investigate the site of release of Pi from glucose-6-phosphate, we incubated microsomes with Pb(2+), which forms an insoluble complex with Pi, preventing its rapid exit from the microsomes. Under these conditions, approximately 80% of the Pi that was formed after 5 min was intramicrosomal, compared with <10% in the absence of Pb(2+). We also show that, when incubated with glucose-6-phosphate and mannitol, glucose-6-phosphatase formed mannitol-1-phosphate and that this nonphysiological product was initially present within the microsomes before being released to the medium. These results indicate that the primary site of product release by glucose-6-phosphatase is the lumen of the endoplasmic reticulum.

Animals↗
PubMed2000

Conversion of a synthetic fructosamine into its 3-phospho derivative in human erythrocytes.

Intact human erythrocytes catalyse the conversion of fructose into fructose 3-phosphate with an apparent K(m) of 30 mM [Petersen, Kappler, Szwergold and Brown (1992) Biochem. J. 284, 363-366]. The physiological significance of this process is still unknown. In the present study we report that the formation of fructose 3-phosphate from 50 mM fructose in intact erythrocytes is inhibited by 1-deoxy-1-morpholinofructose (DMF), a synthetic fructosamine, with an apparent K(i) of 100 microM. (31)P NMR analysis of cell extracts incubated with DMF indicated the presence of an additional phosphorylated compound, which was partially purified and shown to be DMF 3-phosphate by tandem MS. Radiolabelled DMF was phosphorylated by intact erythrocytes with an apparent K(m) ( approximately 100 microM) approx. 300-fold lower than the value reported for fructose phosphorylation on its third carbon. These results indicate that the physiological function of the enzyme that is able to convert fructose into fructose 3-phosphate in intact erythrocytes is probably to phosphorylate fructosamines. This suggests that fructosamines, which are produced non-enzymically from glucose and amino compounds, may be metabolized in human erythrocytes.

Erythrocytes↗
PubMed2000

Analysis of the cooperativity of human beta-cell glucokinase through the stimulatory effect of glucose on fructose phosphorylation.

Using overexpressed Escherichia coli sorbitol-6-phosphate dehydrogenase to monitor fructose 6-phosphate formation, we found that the stimulation of fructose phosphorylation by glucose was reduced in two human beta-cell glucokinase mutants with a low Hill coefficient or when the activity of wild type glucokinase was decreased by replacing ATP with poorer nucleotide substrates. Mutation of two other residues, neighboring glucose-binding residues in the catalytic site, also reduced the affinity for glucose as a stimulator of fructose phosphorylation. Among a series of glucose analogs, only 3, all substrates of glucokinase, stimulated fructose phosphorylation; other analogs were either inactive or inhibited glucokinase. Glucose increased the apparent affinity for inhibitors that are glucose analogs but not for the glucokinase regulatory protein or palmitoyl-CoA. These data indicate that the stimulatory effect of glucose on fructose phosphorylation reflects the positive cooperativity for glucose and is mediated by binding of glucose to the catalytic site. They support models involving the existence of two slowly interconverting conformations of glucokinase that differ through their affinity for glucose and for glucose analogs. We show by computer simulation that such a model can account for the kinetic properties of glucokinase, including the differential ability of mannoheptulose and N-acetylglucosamine to suppress cooperativity (Agius, L., and Stubbs, M. (2000) Biochem. J. 346, 413-421).

Catalytic Domain↗
PubMed2000

Overexpression and purification of fructose-1-phosphate kinase from Escherichia coli: application to the assay of fructose 1-phosphate.

Fructose 1-phosphate is a metabolite that plays a regulatory role in liver and is best measured using an assay based on its conversion to fructose 1,6-bisphosphate by a bacterial fructose-1-phosphate kinase (Fru1PK). The open reading frame encoding Escherichia coli Fru1PK has been introduced in an expression plasmid (pET3a) based on the T7 promoter-driven system, which was used to overexpress the enzyme. The conditions for the production of soluble Fru1PK were optimized. The purification procedure used involved ammonium sulfate precipitation and chromatography on DEAE-Sepharose and was aimed mostly at stabilizing the enzyme and at freeing Fru1PK from bacterial contaminants that could interfere in the fructose 1-phosphate assay. From a 1-liter culture, more than 50 mg protein is obtained. This preparation can be used in an enzymatic assay that measures specifically fructose 1-phosphate in tissue extracts.

Animals↗
PubMed2000

How many forms of glycogen storage disease type I?

UNLABELLED: Glucose-6-phosphatase is a multicomponent enzymatic system of the endoplasmic reticulum, which catalyses the terminal steps of gluconeogenesis and glycogenolysis by converting glucose-6-phosphate to glucose and inorganic phosphate. Glycogen storage diseases type I (GSD I) are a group of metabolic disorders arising from a defect in a component of this enzymatic system, i.e. the glucose-6-phosphate hydrolase (GSD Ia), the glucose-6-phosphate translocase (GSD Ib) and possibly also the translocases for inorganic phosphate (GSD Ic) or glucose (GSD Id). The genes encoding the glucose-6-phosphate hydrolase and the glucose-6-phosphate translocase have both been cloned and assigned to human chromosomes 17q21 and 11q23, respectively. Investigation of patients with GSD I shows that those with GSD Ia are mutated in the glucose-6-phosphate hydrolase gene, whereas those diagnosed as GSD Ib, GSD Ic or GSD Id are mutated in the glucose-6-phosphate translocase gene, and are therefore GSD Ib patients, in agreement with the fact that they all have neutropenia or neutrophil dysfunction. This suggests that the biochemical assays used to differentiate GSD Ic and GSD Id from GSD Ib are not reliable. CONCLUSION: In practice therefore appears to be only two types of GSD I (Ia and Ib), which can be differentiated by (1) measurement of glucose-6-phosphatase activity in fresh and detergent-treated homogenates and (2) by mutation search in the genes encoding the glucose-6-phosphate hydrolase and the glucose-6-phosphate translocase.

Antiporters↗
PubMed2000

3-phosphoglycerate dehydrogenase deficiency in a patient with West syndrome.

3-phosphoglycerate dehydrogenase deficiency is a severe but treatable disorder of serine synthesis, first described in 1996 (Jaeken et al. 1996a). The patient presented with West syndrome, severe psychomotor delay, failure to thrive, microcephaly, atypical ocular movements, and pyramidal signs. Treatment with oral L-serine abolished seizures and improved psychomotor development, hyperexcitability, head growth, cortical and subcortical hypotrophy, and hypomyelination of the brain on MRI scans. 3-phosphoglycerate dehydrogenase deficiency is a treatable congenital error that probably leads to West syndrome.

Brain↗
PubMed2000

Glucose-6-phosphatase mutation G188R confers an atypical glycogen storage disease type 1b phenotype.

Glycogen storage disease type 1a (GSD 1a) is caused by a deficiency in microsomal glucose-6-phosphatase (G6Pase). A variant (GSD 1b) is caused by a defect in the transport of glucose-6-phosphate (G6P) into the microsome and is associated with chronic neutropenia and neutrophil dysfunction. Mutually exclusive mutations in the G6Pase gene and the G6P transport gene establish GSD la and GSD 1b as independent molecular processes and are consistent with a multicomponent translocase catalytic model. A modified translocase/catalytic unit model based on biochemical data in a G6Pase knockout mouse has also been proposed for G6Pase catalysis. This model suggests coupling of G6Pase activity and G6P transport. A 5-mo-old girl with hypoglycemia, hepatomegaly, and lactic acidemia was diagnosed with GSD 1a. She also developed neutropenia, neutrophil dysfunction, and recurrent infections characteristic of GSD 1b. Homozygous G188R mutations of the G6Pase gene were identified, but no mutations in the G6P translocase gene were found. We have subsequently identified a sibling and two unrelated patients with similar genotypic/phenotypic characteristics. The unusual association of neutrophil abnormalities in patients with homozygous G188R mutations in the G6Pase gene supports a modified translocase/catalytic unit model.

Animals↗
PubMed2000

Identification, cloning, and heterologous expression of a mammalian fructosamine-3-kinase.

Fructosamines are thought to play an important role in the development of diabetic complications. Little is known about reactions that could metabolize these compounds in mammalian tissues, except for recent indications that they can be converted to fructosamine 3-phosphates. The purpose of the present work was to identify and characterize the enzyme responsible for this conversion. Erythrocyte extracts were found to catalyze the ATP-dependent phosphorylation of 1-deoxy-1-morpholinofructose (DMF), a synthetic fructosamine. The enzyme responsible for this conversion was purified approximately 2,500-fold by chromatography on Blue Sepharose, Q Sepharose, and Sephacryl S-200 and shown to copurify with a 35,000-M(r) protein. Partial sequences of tryptic peptides were derived from the protein by nanoelectrospray-ionization mass spectrometry, which allowed for the identification of the corresponding human and mouse cDNAs. Both cDNAs encode proteins of 309 amino acids, showing 89% identity with each other and homologous to proteins of unknown function predicted from the sequences of several bacterial genomes. Both proteins were expressed in Escherichia coli and purified. They were shown to catalyze the phosphorylation of DMF, fructoselysine, fructoseglycine, and fructose in order of decreasing affinity. They also phosphorylated glycated lysozyme, though not unmodified lysozyme. Nuclear magnetic resonance analysis of phosphorylated DMF and phosphorylated fructoseglycine showed that the phosphate was bound to the third carbon of the 1-deoxyfructose moiety. The physiological function of fructosamine-3-kinase may be to initiate a process leading to the deglycation of fructoselysine and of glycated proteins.

Adenosine Triphosphate↗