Identification, purification and genetic deficiencies of the glucose-6-phosphatase system transport proteins

Demonstration of glycated insulin in human diabetic plasma and decreased biological activity assessed by euglycemic-hyperinsulinemic clamp technique in humans

The presence and biological significance of circulating glycated insulin has been evaluated by high-pressure liquid chromatography (HPLC), electrospray ionization mass spectrometry (ESI-MS), radioimmunoassay (RIA), receptor binding, and hyperinsulinemic-euglycemic clamp techniques. ESI-MS analysis of an HPLC-purified plasma pool from four male type 2 diabetic subjects (HbA(1c) 8.1 +/- 0.2%, plasma glucose 8.7 +/- 1.3 mmol/l [means +/- SE]) revealed two major insulin-like peaks with retention times of 14-16 min. After spectral averaging, the peak with retention time of 14.32 min exhibited a prominent triply charged (M+3H)(3+) species at 1,991.1 m/z, representing monoglycated insulin with an intact M(r) of 5,970.3 Da. The second peak (retention time 15.70 min) corresponded to native insulin (M(r) 5,807.6 Da), with the difference between the two peptides (162.7 Da) representing a single glucitol adduct (theoretical 164 Da). Measurement of glycated insulin in plasma of type 2 diabetic subjects by specific RIA gave circulating levels of 10.1 +/- 2.3 pmol/l, corresponding to approximately 9% total insulin. Biological activity of pure synthetic monoglycated insulin (insulin B-chain Phe(1)-glucitol adduct) was evaluated in seven overnight-fasted healthy nonobese male volunteers using two-step euglycemic-hyperinsulinemic clamps (2 h at 16.6 micro g x kg(-1) x min(-1), followed by 2 h at 83.0 micro g x kg(-1) x min(-1); corresponding to 0.4 and 2.0 mU x kg(-1) x min(-1)). At the lower dose, the exogenous glucose infusion rates required to maintain euglycemia during steady state were significantly lower with glycated insulin (P < 0.01) and approximately 70% more glycated insulin was required to induce a similar rate of insulin-mediated glucose uptake. Maximal responses at the higher rates of infusion were similar for glycated and control insulin. Inhibitory effects on endogenous glucose production, insulin secretion, and lipolysis, as indicated by measurements of C-peptide, nonesterified free fatty acids, and glycerol, were also similar. Receptor binding to CHO-T cells transfected with human insulin receptor and in vivo metabolic clearance revealed no differences between glycated and native insulin, suggesting that impaired biological activity is due to a postreceptor effect. The present demonstration of glycated insulin in human plasma and related impairment of physiological insulin-mediated glucose uptake suggests a role for glycated insulin in glucose toxicity and impaired insulin action in type 2 diabetes.

Glycogen storage disease type I: diagnosis, management, clinical course and outcome. Results of the European Study on Glycogen Storage Disease Type I (ESGSD I)

Glycogen storage disease type I (GSD I) is a relatively rare metabolic disease and therefore, no metabolic centre has experience of large numbers of patients. To document outcome, to develop guidelines about (long-term) management and follow-up, and to develop therapeutic strategies, the collaborative European Study on GSD I (ESGSD I) was initiated. This paper is a descriptive analysis of data obtained from the retrospective part of the ESGSD I. Included were 231 GSD Ia and 57 GSD Ib patients. Median age of data collection was 10.4 years (range 0.4-45.4 years) for Ia and 7.1 years (0.4-30.6 years) for Ib patients. Data on dietary treatment, pharmacological treatment, and outcome including mental development, hyperlipidaemia and its complications, hyperuricaemia and its complications, bleeding tendency, anaemia, osteopenia, hepatomegaly, liver adenomas and carcinomas, progressive renal disease, height and adult height, pubertal development and bone maturation, school type, employment, and pregnancies are presented. Data on neutropenia, neutrophil dysfunction, infections, inflammatory bowel disease, and the use of granulocyte colony-stimulating factor are presented elsewhere (Visser et al. 2000, J Pediatr 137:187-191; Visser et al. 2002, Eur J Pediatr DOI 10.1007/s00431-002-1010-0).

CONCLUSION

there is still wide variation in methods of dietary and pharmacological treatment of glycogen storage disease type I. Intensive dietary treatment will improve, but not correct completely, clinical and biochemical status and fewer patients will die as a direct consequence of acute metabolic derangement. With ageing, more and more complications will develop of which progressive renal disease and the complications related to liver adenomas are likely to be two major causes of morbidity and mortality.

Insulin receptors and insulin action in the brain: review and clinical implications

Insulin receptors are known to be located on nerve cells in mammalian brain. The binding of insulin to dimerized receptors stimulates specialized transporter proteins that mediate the facilitated influx of glucose. However, neurons possess other mechanisms by which they obtain glucose, including transporters that are not insulin-dependent. Further, insulin receptors are unevenly distributed throughout the brain (with particularly high density in choroid plexus, olfactory bulb and regions of the striatum and cerebral cortex). Such factors imply that insulin, and insulin receptors, might have functions within the central nervous system in addition to those related to the supply of glucose. Indeed, invertebrate insulin-related peptides are synthesized in brain and serve as neurotransmitters or neuromodulators. The present review summarizes the structure, distribution and function of mammalian brain insulin receptors and the possible implications for central nervous system disorders. It is proposed that this is an under-studied subject of investigation.

Hepatocellular expression of glucose-6-phosphatase is unaltered during hepatic regeneration

Gluconeogenesis and glycogenolysis are essential hepatic functions required for glucose homeostasis. During the initial phase of hepatic regeneration, the immediate-early genes (IEG) are rapidly expressed, and the IEG RL-1 encodes for glucose-6-phosphatase (G-6-Pase). G-6-Pase is a microsomal enzyme essential for gluconeogenesis and glycogenolysis. This study employs a partial-hepatectomy model to examine the expression and activity of G-6-Pase. After partial hepatectomy, rat hepatic G-6-Pase gene expression is transcriptionally regulated, and mRNA levels are increased approximately 30-fold. However, in contrast to this rapid gene induction, microsomal enzyme activity is unchanged after partial hepatectomy. Western blotting demonstrates that microsomal G-6-Pase protein expression is also unchanged after partial hepatectomy, and similar results are also noted in whole liver homogenate. Thus, despite marked induction in gene expression of the IEG G-6-Pase after partial hepatectomy, protein expression and enzyme activity remain unchanged. These data indicate that, although this hepatocyte IEG is transcriptionally regulated, the physiologically important level of regulation is posttranscriptional. This highlights the importance of correlating gene expression of IEG with protein expression and physiological function.

Genetic analysis of the glucose-6-phosphatase mutation of type 1a glycogen storage disease in a Chinese family

Type 1a glycogen storage disease (GSD) is an autosomal recessive metabolic disorder caused by a deficiency in glucose-6-phosphatase (G6Pase). Polymerase chain reaction (PCR) and nucleotide sequence analysis were used to identify the location and nature of mutations at the G6Pase locus in two siblings affected with type 1a GSD. Both patients are compound heterozygotes with two different single nucleotide substitutions in the two G6Pase alleles. A guanine to adenine transition was identified at base position 327 in the exon 2, converting an arginine to a histidine at codon 83. The second substitution was a thymine to adenine transversion at base position 1101 in the exon 5, converting an isoleucine to an asparagine at codon 341. Family study reveals that both parents are heterozygous carriers: the father with a mutant G6Pase allele at exon 2, the mother with another mutant G6Pase allele at exon 5. This is the first family study in Taiwan on type 1a GSD identified by molecular analysis. The mutations identified herein are novel substitutions in the G6Pase gene. In addition, an adenine to guanine substitution was observed at base position 653 in the exon 5 of G6Pase gene in both sibling patients and their parents, as well as in 15 normal Chinese subjects and three normal Caucasian subjects.

Identification of the site of glycation of human insulin

This study evaluates the nature of glycated human insulin formed following exposure to hyperglycemic conditions in vitro. Glycated insulin was purified by RP-HPLC and its molecular mass (5971.3 Da) determined by plasma desorption mass spectrometry (MS). The difference in mass (163.7 Da) from nonglycated insulin (5807.6 Da) corresponds to a single reduced glucose (glucitol) residue. Following reduction of insulin disulfide bridges, MS confirmed that the B-chain was glycated. Enzymatic digestions with trypsin, endoproteinase Glu-C, and thermolysin, followed by RP-HPLC and identification of fragments by MS, localized glycation to the B-chain (1-5) region. Electrospray tandem MS identified the site of glycation as the B-chain NH2-terminal Phe1 residue. This was confirmed by automated Edman degradation with glycated human insulin.

Rapid substrate translocation by the multisubunit, erythroid glucose transporter requires subunit associations but not cooperative ligand binding

The human erythroid glucose transporter is a GLUT1 homotetramer whose structure and function are stabilized by noncovalent, cooperative subunit interactions. The present study demonstrates that exofacial tryptic digestion of GLUT1 abolishes cooperative interactions between substrate binding sites on adjacent subunits under circumstances where subunit associations and high catalytic turnover are maintained. Extracellular trypsin produces rapid, quantitative cleavage of the human red cell-resident sugar transport protein, GLUT1. One major carboxyl-terminal peptide of M(r)(app) 25,000 is detected by immunoblot analysis. Endofacial tryptic digestion of GLUT1 results in the complete loss of GLUT1 carboxyl-terminal structure. GLUT1-mediated erythrocyte sugar uptake, transport inhibition by cytochalasin B, and GLUT1 oligomeric structure are unaffected by exofacial GLUT1 proteolysis. In contrast, the cytochalasin B binding capacity of GLUT1 and the Kd(app) for cytochalasin B binding to the transporter are doubled following exofacial tryptic digestion of GLUT1. Photoaffinity labeling experiments show that increased cytochalasin B binding results from increased ligand binding to the 25 kDa carboxyl-terminal GLUT1 peptide. Proteolysis abolishes allosteric interactions between sugar import (maltose binding) and sugar export (cytochalasin B binding) sites that normally exist on adjacent subunits within the transporter complex, but interact with negative cooperativity. Following exofacial proteolysis, these sites become mutually exclusive. Dithiothreitol disrupts GLUT1 quaternary structure, inhibits 3-O-methylglucose transport, and abolishes cooperative interactions between sugar import and export sites in control cells. Studies with reconstituted purified GLUT1 confirm that the action of trypsin on cytochalasin B binding is direct, show that proteolysis increases the apparent affinity of the sugar efflux site for transported sugars, and suggest that the membrane bilayer stabilizes GLUT1 noncovalent structure and catalytic function following GLUT1 proteolysis. Collectively, these findings demonstrate that GLUT1 does not require an intact polypeptide backbone for catalytic function. They show that the multisite sugar transporter mechanism is converted to a simple ping-pong carrier mechanism following exofacial GLUT1 proteolysis. They reveal that subunit cooperativity can be lost under circumstances where cohesive structural interactions between transporter subunits are maintained. They also refute the hypothesis [Hebert, D. N., & Carruthers, A. (1992) J. Biol. Chem. 267, 23829-23838] that rapid substrate translocation by the multisubunit erythroid glucose transporter requires cooperative interactions between subunit ligand binding sites.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.